Average daily temperatures could reach 40°C in some parts of the world, especially in the tropics. The most recent forecast indicates that the weather will be so hot in most of the UK and other countries, which has caused concern among individuals and especially among construction workers who carry out their work in open spaces.
Heat poses a major risk to the health and safety of workers conducting their duties because a sizable portion of the construction crew works outside. A recent tool station poll found that over half of the tradespeople are concerned about getting burned while working outside, and more than a fifth admit to forgoing sun protection when working on a project. There have been discussions about the legal obligations employers have to protect their workers from the effects of excessive heat.
What is the Maximum Temperature in the Workplace?
The Health and Safety Executive (HSE) states that there are no set regulatory requirements for the minimum or maximum temperature in the workplace. However, in order to safeguard workers, the Workplace (Health, Safety and Welfare) Regulations of 1992 mandate that firms maintain a “reasonable temperature.” However, the type of job being done and the surrounding factors at the workplace will determine whether the temperature is “appropriate.”
Minimum Workplace Temperature
According to the Approved Code of Practice, a workplace’s minimum temperature should typically be at least 16 degrees Celsius. When physically demanding labour is being done, the temperature should be at least 13 degrees Celsius. The employer has a responsibility to establish what will be an acceptable level of comfort in the specific circumstances; these temperatures are not absolute legal requirements.
Higher Temperatures at Work
Due to the high temperatures present in places like glass factories and foundries, for example, an accurate figure cannot be provided at the higher end of the scale. As long as the proper safeguards are in place, it is still possible to work safely in these conditions. Other factors, such as radiant temperature, humidity, and air velocity, become more important, and the relationship between them becomes more complex with rising temperatures.
What is Thermal Comfort and how is it Measured?
To determine whether the temperature that people are working in is appropriate, thermal comfort can be employed. It relates to a worker’s mental state, including whether they feel overheated, and takes into account a variety of environmental, occupational, and psychological aspects.
Thermal comfort is influenced by six environmental and individual factors. The thermal comfort of an employee is influenced by a number of factors, some of which may be independent of one another.
The environmental factors are:
Air temperature
Radiant temperature
Air velocity
Humidity
The Personal factors include:
Clothing Insulation
Metabolic heat
Factors affecting thermal comfort
According to the HSE, “Thermal comfort is measured by the number of employees reporting of thermal discomfort, not by the temperature of the room.” According to the regulator, heat discomfort can have an impact on morale and productivity and put people’s health and safety in danger. For instance, persons who are working in uncomfortable heat are more likely to act unsafely since it impairs their judgment. The capacity of an individual to perform manual work can also decline.
How can Employers Safeguard Employees who are Outside?
Although companies have little control over the outdoor temperature, the HSE offers numerous suggestions for how they can safeguard workers. These consist of:
Bringing shade to rest spots and scheduling more regular breaks.
The addition of shade in areas where people are working.
Providing free access to chilled water for drinking.
Encouraging workers to remove personal protective equipment (PPE) while they are resting to aid in heat loss; and
Instructing them on how to spot the early signs of heat exhaustion.
What is the Recommended PPE Usage in Scorching Heat?
Because of discomfort, workers may not properly use PPE in high temperatures, according to the HSE. Employers are urged to take this danger into account because it makes working on a construction site risky. In order to make employees more comfortable, managers can ensure that they are not wearing any more PPE than is necessary for their jobs.
Alternative uniform designs made of lighter materials can also be taken into account. Workers may find it useful to modify their work attire according to their personal comfort if it has numerous layers. The HSE advises using high-factor sunscreen with at least SPF 15 on exposed skin.
How should Workers Express Concerns?
According to the HSE, employees can discuss concerns with their manager, supervisor, or union representative. It states that companies should conduct a risk assessment and take appropriate action if a considerable number of workers express complaints about thermal discomfort.
According to the regulation, in addition to Workplace Regulations, employers must analyze the risks to their employees’ health and safety and take appropriate measures when necessary and when reasonably practical. One of the potential dangers that firms should address to comply with legal requirements is the temperature of the workplace. Employers should speak with staff members or their representatives to build practical coping mechanisms.
The employees are also free to do a variety of activities that will enhance their workplace’s thermal comfort. They comprise;
Depending on how hot or chilly it is, add or remove layers of clothing.
Utilize a desk or pedestal fan to improve airflow.
If window coverings are available, use them to block the sun’s heating influence.
Drink a lot of water in hot weather (avoid caffeinated or carbonated drinks)
Work away from sources of radiant heat or direct sunshine if at all possible.
Periodically pause to cool off in hot settings and warm up in cold ones.
If possible, bring up the matter with your managers, your union, or other workplace officials.
Drink a lot of water in hot weather
Even though any of the aforementioned measures might help in some small way to lessen your thermal discomfort, there are a number of additional things that your manager or company could do to assist. Discuss the following with your manager, supervisor, union official, or employee representative:
Introduce work systems to limit exposure, such as flexible hours or early/late starts to help avoid the worst effects of working in high Temperatures, by, whenever possible, ensuring windows are open, fans that promote local cooling, radiators that can be switched off or air conditioning units that are maintained.
Easing up on formal attire
Insulating hot equipment or pipes, relocating workstations out of direct sunlight or away from hot equipment, and include thermal Risk assessments in workplace risk assessments.
Given that thermal stress can have a detrimental effect on an employee’s health, employers should consider and pay attention to their employees’ worries and provide the necessary support.
Jacketing of reinforced concrete columns is normally done to reduce the lateral deformation of columns, and ultimately increase the strength and load-carrying capacity of the column. This activity is usually carried out on columns that are failing as a result of overload, material deterioration, poor maintenance, inadequate construction practices, etc. Furthermore, when the load-carrying capacity of a column is to be increased as a result of construction modification or change of usage of the building, jacketing of the column can also be done.
Materials such as concrete, steel, or laminates made of a fibre-reinforced polymer may be used for column jacketing, and as the stiffness of the jacket in the transverse direction increases, the resistance to lateral deformation of the column increases.
A column member is in a state of uniaxial deformation if the lateral expansion is totally restrained, i.e., if no lateral deformation is permitted to occur. Columns subjected to uniaxial compression fail as a result of lateral tensile strains caused by the Poisson effect, which causes lateral expansion (Abdelrahman, 2023).
Figure 1: Concrete columns under uniaxial load (Abdelrahman, 2023)
The Poisson effect is the term used to describe a material’s expansion or contraction in directions perpendicular to the direction of loading. If lateral expansion occurs freely, the member is under uniaxial stress, and tensile-splitting cracks parallel to the direction of loadings are the resultant cracks. Typically, uniaxially loaded columns with low or normal compressive strengths exhibit this form of failure.
However, since the stiffness of jackets can never be infinite, it is impractical to achieve a uniaxial stress state in jacketed columns. As a result, the triaxial stress state and deformation are always present in jacketed columns.
High tensile stresses are induced in the jacket as its stiffness increases, and vice versa, the original column section experiences high confining pressure and hence experiences less lateral displacement. It should be noted that as the cracking of concrete increases, the concrete Poisson ratio increases as well, allowing for greater confinement before failure.
The Behaviour of Jacketed Square Columns
When a square column is jacketed, the jacket is subjected to high internal pressure, causing tensile stresses in the transverse direction of the column jacket. Different deformations are seen along the jacket wall, as illustrated in Figure 2b, indicating that the distribution of confining stresses is not uniform (Abdelrahman, 2023).
The tensile stress causes elongation of the jacket at the corners; however, extra flexural deformation is also seen at the mid-wall width (δsquare in Fig. 2b); as a result, the confining effect is diminished in the mid-wall zones. As the dimensions of the square column increase, less confinement is developed at the mid-wall width and the overall confinement of the column decreases.
Figure 2: Confinement of the columns induced by the jacket (Abdelrahman, 2023)
However, as seen in Figure 2d, jacketing does not properly confine rectangular columns, especially those with a large aspect ratio. This is mostly caused by the initial column section expanding, which causes a significant outward distortion of the long side of the column (the δrectangle in Figure 2d).
The behaviour of Jacketed Circular Columns
When jacketing is applied to circular columns, uniform internal pressure develops in the jacket, producing uniform tension and equal lateral displacement in the jacket wall. In this instance, the original concrete section is confined as shown in Figure 2c, and the maximum benefit of the jacket is achieved since it undergoes minimal deformation (see the δcircle in Figure 2c).
Figure 3: Jacketing of circular columns
Jacketing with Reinforced Concrete
Reinforced conncrete jacketing can be applied to columns by increasing the size of the column and adding more longitudinal and transverse steel reinforcement. The essential factor in determining whether the concrete strengthening operation is successful is the composite action between the old and new concrete.
Concrete jackets are used to increase the axial-moment capacity, shear capacity, or stiffness of columns. The lateral deformation of a building can be improved by making the columns stiffer, but this will also result in higher straining actions because of the increased lateral load demand. Concrete jackets can also be used to fix the buckling issues of slender columns.
The use of concrete jacketing in the strengthening of RC columns has its drawbacks, despite being a very popular alternative. For instance, it is adverse to architects and building owners since the size of the finished concrete section after restoration increases and the amount of open space in the building decreases.
Care should be used when applying the jacket because it necessitates extensive drilling into the original column section, which may already be weak, as well as in the slabs/beams and footings. The procedure of applying concrete jackets takes time, and accuracy is necessary when casting the concrete monolithically, especially for the upper portions of the jackets.
The strength of the repaired column member following the addition of the concrete jacket mostly depends on the strength of the jacket as well as the increased strength of the original member following the jacket’s confinement (Abdelrahman, 2023). As a result, it is ideal to have the jacket on all four sides of the column; however, this is not always possible, especially for the columns that are located at the edge or corners of structures.
Procedure for the Construction of Concrete Jacket
The following steps can be adopted in the construction of column jacketing;
(1) Since the column capacity is greatly reduced during the application of the jacket, propping of the slabs surrounding the column and the slabs at other floors must be provided to establish an alternative load path to the foundations.
Figure 4: Propping of slab and beams is important before the commencement of column jacketing
(2) If there is no steel corrosion, the concrete cover is removed until sound concrete is reached in the original section. Since pneumatic hammering may cause the substrate to micro-crack, sandblasting is advised to roughen the concrete surface. If the original section’s reinforcement has corroded, the concrete cover should be completely removed so that the longitudinal steel bars are visible. If the level of corrosion is severe, the steel bars are cleaned, painted with anticorrosion paint, or replaced with new steel bars.
Figure 5: Removal of concrete cover prior to jacketing
(3) To mechanically join the new concrete portion to the old concrete section, steel dowels are placed along the column all around the cross-section. The foundation must receive the longitudinal steel bars of the jacket, and holes in the slabs must be constructed to allow the longitudinal bars to pass the floor slabs.
Figure 6: Profile of dowels anchored to original column and reinforced concrete jacket
(4) The specified longitudinal and transverse steel reinforcement for the jacket is set in place, and it is advised that the steel dowels that were planted be fastened to the longitudinal bars. The hooks will stop the longitudinal steel bars from early buckling and improve the confinement level of the column cross-section.
Figure 7: Installation of longitudinal and transverse steel reinforcement
(5) The jacket should be cast using a material that is compatible with the original concrete section’s properties. In order to reduce the strain on the steel dowels and the likelihood of fresh concrete cracking, the shrinkage of the jacket material should be kept to a minimum. If the original column has been provided with both dowels and surface roughening, monolithic behaviour—where there is no separation at the joint between the old and new concrete—can be anticipated.
Figure 8: Summary of steps for the construction of column jacket (Abdelrahman, 2023)
Material Specifications for Concrete Jacketing
The design mix used to cast the concrete jacket should have physical properties that are comparable to or compatible with those of the concrete used in the original section. Compressive strength, elastic modulus, coefficient of thermal expansion, and creep are some of these properties. Unless it can be demonstrated through testing that similar physical properties are attainable, the type of aggregate used in the mix should be comparable to that of the original concrete.
If the properties of the two materials differ, a vertical separation between the original section and the jacket can be anticipated over time. In this regard, grout or mortar used in the jacket shouldn’t have long-term behaviour that differs from that of concrete. The tensile properties of the jacket may be improved by adding fibres or shrinkage-compensating material to the mixture.
Prior to casting, a bonding agent may be applied to the old concrete as long as the manufacturer’s material specifications are rigorously followed. In order to prevent honeycombs or voids from appearing in the jacket, it is recommended to utilize the form-and-pump approach during concreting.
Reference Abdelrahman A. (2023): Strengthening of Concrete Structures, Springer https://doi.org/10.1007/978-981-19-8076-3_1
Planning is the key to the successful execution of any construction project. A project take-off plan can be categorized under project initiation and planning management. This is a deliberate and measurable plan set up to strategize, manage and ensure the efficient delivery of the project.
A project take-off plan is set up to review and analyse the ten (10) project management body of knowledge (PMBOK) areas as it relates to the project being undertaken. The project take-off committee are required to evaluate and oversee all the phases of the project end-to-end from initiation to execution and close out of the project.
Portfolio contractors, new companies, or site engineers with no prior experience on how to set up or mobilize to the site after the awarding of a contract always find it challenging, thereby wasting useful project time. The aim of this article is to give some inputs, tips and tools needed to set up or mobilize to site.
Tips for Initiating a Project Take-off Plan
The initial useful steps that are required in the preparation of a project take-off plan are provided below;
Project feasibility study Many projects fail to achieve proper success because of a lack of feasibility study and not being able to forecast ahead as to why and how a project might succeed or fail. This study includes but is not limited to awareness of the project team to the following important indicators;
Socio-economic factors or conditions
Demography
Labour rate and material rate in the construction area
Availability of materials within the construction area
Environmental conditions in the dry and rainy season
Topography of the site,
The security situation in the area
Behavioural characteristics of the locals
Availability of important machines, tools, personnel,
Availability and rate of rent for personnel during the project, etc
At the end of these studies, the contractor will be equipped with many intelligence data gathering to position the project for success.
Appointment of a project team This includes Engineers, Quantity surveyors, architects, Project Managers, etc. The team are appointed and their roles and responsibilities are defined as well as the organograms to be used on the project site.
Sourcing of Artisans and Craftsmen: It is also important to source artisans and craftsmen like iron benders, carpenters, masons, and labourers who are very key to the success of any project.
Review and analysis of the Bill of Engineering Measurement and Evaluation (BEME), Bill of Quantity (BOQ), drawings, notes and schedule: This activity should be carried out in order to come up with different documents like Project deliverables, labour and material bills, the scope of work, work breakdown, work plan, and programme of work of different items of work towards achieving a fast track and value engineering approach to the project.
Setting out details and bar bending schedule: For clarity in the arrangement of the elements of the construction, and not to waste time during the substructure stage or at any setting out stages, setting out details in the form of a drawing should be done. Also, before reinforcement cutting, bending, fixing, and placement, a bar bending schedule should be prepared. It is important that the bending schedule take into account offcuts and wastes so that procurement will not be done twice.
Design of program of work with respect to contract duration: Project time starts counting immediately after taking possession of the site. Many projects encounter difficulties because no timelines were established that guide the construction operations. This can be avoided by creating a program of work at the planning stage of the project. A program of work assigns a timeline to all project activities/tasks to ensure project deadlines are met.
Informing the consultant/client of award and mobilization to site: This activity falls under project stakeholder management. All stakeholders should be informed prior to taking possession of the site to avoid any conflict from the stakeholders. All required contract documents and approvals should be obtained. Thereafter, official possession of the site by the contractor is established.
Site safety planning: The site layout, convenience points, muster points, safety signs and symbols, and personal protective equipment (PPE) should be procured and commissioned where necessary.
Procurement of Project Vehicle: A project vehicle always comes in handy to help project logistics and procurement management.
Appointment of guards and a storekeeper: The issue of project site security is a very important aspect that must be on all project manager’s watch lists. Inventories and materials must be properly safeguarded. How and when materials come into the site, the quantity delivered, and how they are utilised should be properly recorded by the storekeeper.
Site clearing: Depending on site condition, site clearing is done manually or by mechanical means using machines. E.g. JCB backhoe or Bulldozer.
Construction of site office, store, cafeteria and temporary perimeter fencing: Depending on the anticipated duration of the project, site offices, store, and cafeteria can be constructed using different materials. Some of the popular materials are timber, roofing zinc; sandcrete blocks; polystyrene; or steel. There are also portable cabins and containers that can be refurbished and converted to a site office.
Delivery of a container site office to site
Project billboard: Project billboards give more branding and visibility to the project site. It contains important information like the project title, project address, project number (contract number), project approval details, name of the contractor, name of consultants, name of client, etc.
Borehole construction and water storage tank: The availability of potable water is essential for all sites. The project team should make concerted efforts to ensure that potable water is available for running the site. Water is required for construction works, cleaning, and human consumption. Essentially, all wet works require water at one stage or the other.
Office and store administration: Provision for tools to be used in the administrative aspect of the project site includes- pen; pencil; masking tape; chairs; tables; generator; fan; board (for pasting pictures, drawings, schedules, instructions and program of work); laptop; calendar; notepad; keys; chains; coffee; refrigerator; stapler; pin; calculator; envelopes; instruction booklet; weather booklet; visitors’ booklet; requisition/purchase order booklets, etc.
Procurement: At this stage, the major stockpiling of site inventories starts in earnest. This procurement is done at different stages of the project depending on the scale and scope of the projects.
For a building project, some of the necessary materials include but are not limited to materials and equipment like;
cement
sand
gravel
reinforcement bars
binding wire
marine board
planks
1″ x 3″, 2″ x 3″ and 2″ x 4″ timber
Nails of different sizes and types
A toolbox of pliers, screwdrivers, spanners, cutters, wrenches, etc
buckets and head pans
water hose
chisels and hammers
paint and brush
concrete mixer
shovel and spade
poker vibrator
wheelbarrow
pumping machine
generator and fuel can
acrow props or bamboo
perry beam
spirit and water levels
lines
Digger
plumb bob
rammer
mechanical and electrical materials (e.g extension cables and boxes)
slump cone and mould for concrete test
caution tape
blocks
a blue line and blue
dumpy level
laser light equipment for horizontal and vertical control
crane
Damp-proof membrane (DPM)
hardcore
fabrics (BRC wire mesh)
setting out materials and tools (builders rope, measuring tape (10m and 50m iron tape), 2×3 wooden and 12mm iron peg, 1 x 2 profile board, builder square, spirit level, markers, masking tape, 2’’ and 3’’ nail, hammer etc.)
camera for taking site pics for project progress and presentation
safety gadgets (first aid box, safety boots, jacket, helmets and signboard).
Note that some of these items must be tested and approved before any bulk purchases.
Conclusion
From the foregoing, the contractor or site engineer will increase in confidence and knowledge of some of the things requires during a project take-off plan. This plan is not a one size fit all approach and it is to be used where it is applicable. It is intended to familiarize the project team with the preparation of a basic take-off plan for a building site in Nigeria and also, to serve as a checklists before mobilization to the site. The plan should be documented in a report-like format for proper review and reference in the future.
Airfield pavements are designed and constructed to provide durable all-weather travel surfaces for the safe landing, takeoff, and quick movement of passengers and aircraft while maintaining an appropriate level of comfort for users. During the design and construction phases, considerable consideration is given to the following factors in order to meet these functional requirements for pavements:
(a) selection of pavement type, (b) selection of materials to be used for various pavement layers and treatment of subgrade soils, (c) structural thickness design for pavement layers, (d) subsurface drainage design for the pavement system, (e) surface drainage and geometric design, and (f) rideability of the pavement surface.
Figure 1: Typical airfield pavement
Realistic methods for evaluating the loading characteristics of aircraft and the structural reaction of the pavement are necessary for the design of an airfield’s pavement. It has long been understood that the gross weights of the aircraft utilizing the pavement, as well as the arrangement, spacing, and tyre pressures of their undercarriage wheels, determine the severity of load-induced stresses in a pavement and subgrade.
Due to changes in traffic operations and functional applications of the pavements, the traffic loading analysis process for airport pavements differs slightly from that for highway pavements. The steps in the traffic loading analysis of airfield pavements include;
Estimation of expected initial year traffic volume
Estimation of expected annual traffic growth rate
Estimation of traffic stream composition
Computation of traffic loading
Estimation of design traffic loading for different functional areas
The information on the first two steps may be obtained from the planning forecast of the relevant airport authority.
Traffic Stream Composition
The nose gear and main landing gear of an aircraft transfer the weight of the aircraft to the pavement. The typical wheel arrangements on the main landing gear legs of civil aircraft are shown in Figure 2. For the purpose of pavement design, it is necessary to identify the types of aircraft, landing gear details, and their relative frequencies of arrival. This is because different aircraft have varied gross weights and wheel configurations.
Figure 2: Typical wheel configurations of a main leg of the aircraft landing gear.
Computation of Traffic Loading
The maximum takeoff weights of the aircraft are typically taken into account when designing pavement. It is also typical to assume that the main landing gears carry 95% of the aircraft’s gross weight and the nose gear carries 5%. Both the equivalent load idea and Miner’s hypothesis have been used in the analysis of mixed traffic loading.
Figure 3: Typical landing gear of an aircraft
For instance, the FAA method [Federal Aviation Administration, 1978] uses the variables in Table 1 to convert the annual departure of all aircraft into the equivalent departures of a selected design aircraft. In thi case, the designer should multiply the annual departures of a given aircraft type by the conversion factor to obtain annual departures in design aircraft landing gear.
Aircraft Type
Design Aircraft
Conversion Factor F
Single-wheel
Dual-wheel
0.8
Single-wheel
Dual-tandem
0.5
Dual-wheel
Dual-tandem
0.6
Double dual-tandem
Dual-tandem
1.0
Dual-tandem
Single-wheel
2.0
Dual-tandem
Dual-wheel
1.7
Dual-wheel
Single wheel
1.3
Double dual-tandem
Dual-wheel
1.7
Table 1: Conversion factor for computing annual departures
The FAA uses the equivalent single-wheel load (ESWL) concept to create the thickness design curves for flexible airfield pavements. When designing airfield pavement, the concept of ESWL is frequently used to evaluate the impact of multiple-wheel landing gears.
The control response chosen for ESWL computation, pavement thickness, and the relative stiffness of pavement layers all affect how much ESWL a particular landing gear has. ESWL calculations based on equal deflection (at the surface or at the pavement-subgrade interface) or equal stress (at the bottom face of the bound layer) are frequently used for airport pavement design.
Equal Stress ESWL
A detailed method for calculating the ESWL would require both a correct analytical solution for the required stress produced by the target wheel assembly and a process of trial-and-error to determine the magnitude of the single wheel that will produce the same value of stress. Because of how time-consuming this process is, simplified approaches have been used in practice.
A simplified method for calculating the equal subgrade stress ESWL of a pair of dual wheels for flexible pavement design is shown in Figure 4.
Figure 4: Equal subgrade stress ESWL of dual wheels for flexible pavement design
If the pavement thickness is less than or equal to d/2, where d is the lowest edge-to-edge distance between the tire imprints of the dual wheels, the ESWL is equal to one wheel load P with the specified 45° spread of applied pressure.
The method further assumes that ESWL = 2P for any pavement equal to or thicker than 2S, where S is the center-to-center spacing of the dual wheels. For pavement thicknesses between d/2 and 2S, ESWL is determined, as shown in Figure 4, by assuming a linear log–log relationship between ESWL and pavement thickness. Note that d is equal to (S – 2a), where a is the radius of the tyre imprint given by;
a = √(P/πp)
where p is the tyre pressure.
This simplified procedure provides an approximate ESWL estimation for flexible pavements.
In the case of rigid pavements, computation of stresses for equal stress ESWL should be based on rigid slab analysis such as the well-known Westergaard formulas, which give the maximum bending stress σmax and the maximum deflection δmax.
Figure 5: An aircraft landing on an airfield pavement
Solved Example
The total load on a set of dual wheels is 45,000 lb. The tire pressure of the wheels is 185 psi. The centre-to-centre spacing of the wheels is 34 in. Calculate the equal stress ESWL if the thickness of the pavement structure is (a) h = 30 in. and (b) h = 70 in.
Solution Load per wheel = 22,500 lb., radius of tire imprint a = √(22500/185π) = 6.22 in., S = 34 in., and d = (S – 2a) = 21.56.
By means of a log–log plot as shown in Figure 4, ESWL is determined to be 33,070 lb. for h = 30 in. For h = 70 in., since h > 2S, ESWL = 2(22,500) = 45,000 in.
Airfield Pavement Loading in the UK
In the UK, Aircraft Classification Number (ACN) and Pavement Classification Number (PCN) are the two parameters often used in airfield pavement design.
The ACN of an aircraft describes the relative severity of its loading on a pavement supported by a given subgrade. Two mathematical models—one for rigid pavements and the other for flexible pavements—are used to compute ACNs.
For a given limiting stress or the number of load repetitions, the ACN of an aircraft is mathematically defined as twice the single wheel load (expressed in thousands of kilograms) at a standard tyre pressure of 1.25 MPa, which requires the same pavement thickness as the actual main wheel gear of the aircraft. The standard thickness is the pavement’s thickness.
ACN Calculation for Rigid Pavement
The method of calculating ACNs for aircraft on rigid pavements is set out below with reference to Figure 1:
Figure 1: ACN Rigid pavement model
(i) Calculate the reference thickness (tc), the thickness of concrete slab which when loaded at the centre by one main wheel gear of the actual aircraft gives a maximum flexural stress of 2.75 N/mm2 (fct) on a subgrade whose Modulus of Subgrade Reaction (k) is one of the standard values (see (iv)). The mathematical model for the stress calculation is the Westergaard solution for an elastic slab on a dense liquid subgrade (Winkler Foundation). The modulus of elasticity for concrete is taken as 27.6 x 103 MN/m2 and Poisson’s ratio as 0.15. (ii) Calculate the single wheel load (WR) which at a tyre pressure of 1.25 MPa induces a flexural stress of 2.75N/mm2, in slab of thickness tc. (iii) The ACN = 2 x (WR/1000) = WR/500 where WR is in kgs. (iv) Calculate ACNs for each aircraft for the following four categories of subgrade characterised in terms of a standard k.
Subgrade Category
k
High
150 MN/m2/m
Medium
80 MN/m2/m
Low
40 MN/m2/m
Ultra-low
20 MN/m2/m
ACN Calculation for Flexible Pavement
The method of calculating ACNs for aircraft on flexible pavements is set out below with reference to Figure 2:
(i) Calculate the reference thickness (tf), the thickness of conventional flexible pavement which allows 10,000* load repetitions by one main wheel gear of the actual aircraft on a subgrade whose CBR is one of the standard values (see (iv)). The method of calculation is based on the CBR Equation and Boussinesq deflection factors. (ii) Calculate the single wheel load (WF) which at a tyre pressure of 1.25 MPa allows the same 10,000 load repetitions on a flexible pavement of total thickness tf. The calculation is carried out using the following formula: WF = 0.005tf2/(0.878/CBR – 0.01249) (iii) The ACN=2 x WF/1000 = WF/500 where WF is in kgs. (iv) Calculate ACNs for each aircraft for the following four categories of subgrade characterised in terms of a standard CBR.
Subgrade Category
CBR
High
15%
Medium
10%
Low
6%
Ultra-low
3%
Pavement Classification Number (PCN)
By the definition of the ACN-PCN method, the PCN is the ACN of the aircraft which imposes a severity of loading equal to the maximum permitted on the pavement of unrestricted use.
PCNs are reported as a five-part code as follows:
Part i: The PCN Number: The highest permitted ACN at the appropriate subgrade category.
Part ii: The type of pavement: R = rigid, F = flexible. If the actual pavement is of mixed construction the engineer will need to decide whether the behaviour and mode of failure of the pavement are likely to be those of a rigid or flexible one, then classify accordingly.
Part iii: The pavement subgrade category: A = High B = Medium C = Low D = Ultra Low The ranges of subgrade strength covered by these categories are shown in the Tables above. Note that these strength ranges are not equivalent for rigid and flexible pavements.
Part iv: The maximum tyre pressure authorised for the pavement: W = High, no limit. X = Medium, limited to 1.5 MPa (217 psi) Y = Low, limited to 1.0 MPa (145 psi) Z = Very low, limited to 0.5 MPa (73 psi)
Part v: Pavement design/evaluation method: T = Technical design or evaluation U = By experience of aircraft actually using the pavement
The Design ACN
The design ACN is based on the Design Aircraft; which is normally the aircraft with the highest ACN on the actual subgrade.
The actual weight of aircraft when using the pavement must be considered in determining the design ACN. The Maximum All-Up Weight figure will normally be used, but lighter weights are appropriate where:
(i) the runway length imposes restrictions on the operating weights, (ii) the pavement is only used by landing aircraft (e.g. fast turn offs) and (iii) the pavement is only used by unladen aircraft (e.g. the accesses to maintenance hangars). To compute an ACN at a weight between the published values it is assumed that ACNs vary linearly with weight.
The design ACN should also relate the actual value of the subgrade under a pavement. The ACNs listed in B are for four standard subgrade categories. If the value of actual subgrade is not the same as that of a standard subgrade, the design ACNs are to be calculated by linear interpolation or extrapolation of ACNs for the standard subgrades.
The high category subgrade for flexible pavements is for CBR 15%. When designing pavements for subgrades with CBR greater than 15% the following rules may be applied:
(i) Single and Dual Main Wheel Gears Take the ACN for CBR >15% to be the same as the ACN for CBR 15%.
(ii) Dual-Tandem Main Wheel Gears Take the ACN for CBR ≥ 20% as equal to 0.95 x the ACN for CBR 15%. Values for CBRs between 15% and 20% can be obtained by linear interpolation e.g. ACN for CBR 17% = 0.98 x the ACN for CBR 15%.
(iii) Tridem Main Wheel Gears Take the ACN for CBR ≥ 20% as equal to 0.97 x the ACN for CBR 15%. Values for CBRs between 15% and 20% can be obtained by linear interpolation.
For rigid pavements, the effect of the higher subgrade values is less significant and it is, therefore, acceptable to assume that: ACN for k >150 MN/m2/m = ACN for k of 150 MN/m2/m.
For pavements which would subsequently be difficult to strengthen, it may be appropriate to design for a higher ACN e.g. for aprons adjacent to hangars and terminal buildings. Where a design ACN of less than 10 is being considered a check should be made to ensure that the pavement is strong enough for the expected use by aircraft servicing vehicles.
Provided the PCN for a pavement is equal to or greater than the ACN of the aircraft and the operating tyre pressure does not exceed the PCN limitation, unrestricted use of the pavement by that aircraft (or those with lower ACNs) is permitted. The term ‘unrestricted use’ of a pavement is not specifically defined. However, it is a pavement design parameter which should reflect current and forecast use over an appropriate design life before major maintenance is required.
Axially loaded timber members are usually employed in building construction. Sometimes, these members are also subjected to simultaneous axial compression and bending. Some of these members include timber columns or beams, truss members, vertical wall studs, and bracing components. The design of timber columns involves the selection of adequate timber grade and section that will safely carry the anticipated load without undergoing excessive deflection or failure by squashing, buckling, or shear.
The limit states associated with the primary design effects for axially loaded members must also comply with the pertinent design rules and requirements of EN 1995-1-1:2004 (EC5). The equilibrium states and strength conditions are ultimate limit states because they are related to failure scenarios. The displacement condition pertains to situations encountered in typical usage, but EC5 offers no guidance on the state’s limiting criteria.
Figure 1: Old timber column structure
Where lateral instability of a member may occur, Section 10 of EN 1995-1-1 sets limitations on the maximum permissible deviation from straightness. For strength-related conditions, the design stress is calculated and compared with the design strength, which is modified as necessary by strength factors. When using the partial factor technique, the design stress cannot be greater than the design strength in order to comply with code reliability requirements.
When members or structures are subjected to combined stresses, such as those caused by the combined effects of axial and bending actions, extra design effects will need to be considered.
In this article, we are going to present an example of how we can design axially loaded timber columns according to the requirements of Eurocode 5.
Timber Columns Subjected to Axial Compression
These are members that are subjected to a compressive axial force acting perpendicular to the grain and along the member’s centroidal x-x plane. Such members serve as posts, columns, wall studs, or struts in pin-jointed trusses.
Figure 2: Axial Compression
For timber sections subjected to axial compression, the failure strength is dependent on several factors such as:
strength/stiffness – compressive strength and modulus of elasticity of the timber;
the geometry of the member – cross-sectional sizes and length;
support condition – the amount of lateral support and fixity at its ends;
geometric imperfections – deviations from nominal sizes, initial curvature and inclination;
material variations and imperfections – density, the effect of knots, the effect of compression wood and moisture content.
When a timber column is subjected to an axial load, there is a tendency for it to displace laterally and to eventually fail by buckling because of imperfections in the geometry of the member or variations in its properties, or a combination of both, as the slenderness ratio, λ, of the member increases.
Figure 3: Buckling of timber columns
The slenderness ratio is defined as the effective length of the member, Le, divided by its radius of gyration, i,;
λ = Le/i
where the radius of gyration about an axis i = √I/A, I is the second moment of area about the axis, and A is the cross-sectional area of the member. For any member, there will be a slenderness ratio, λy, about the y–y axis and, λz, about the z–z axis and when using a rectangular section, as shown in section A–A in Figure 3.
DesignExample
Verify the capacity of 200 mm x 100 mm timber section of strength class C18 to satisfy ultimate limit state requirements under the following conditions:
Service Condition: Class 2 Characteristic permanent action Gk = 10 kN Variable medium-term action Qk = 25 kN Height of column = 3.0m Support condition: Effectively held in position but not in direction in both axes.
Solution Geometric properties Depth of member h = 200 mm Width of member b = 100 mm
Area of member A = bh = 200 x 100 = 20000 mm2 Second moment of area about the y-y axis Iy = bh3/12 = (100 x 2003)/12 = 6.667 x 107 mm4 Radius of gyration about the y-y axis iy = √(Iy/A) = √(6.667 x 107/20000 ) = 57.74 mm
Second moment of area about the z-z axis Iz = hb3/12 = (200 x 1003)/12 = 1.667 x 107 mm4 Radius of gyration about the z-z axis iz = √(Iz/A) = √(1.667 x 107/20000 ) = 28.86 mm
Effective length about the y-y axis LE,y = Effective length about the z-z axis LE,z = 1.0 L = 3000 mm
Therefore slenderness ratio about the y-y axis; λy = LE,y/iy =3000/57.74 = 51.957
The slenderness ratio about the z-z axis; λz = LE,z/iz =3000/28.86 = 103.95
Timber Properties (Table 1 BS EN 338:2003(E)) For strength class C18; Characteristic compression strength parallel to the grain fc.0.k = 18 N/mm2 Fifth percentile modulus of the elasticity parallel to the grain E0.05 = 6.0 kN/mm2
Partial safety factors UKNA to BS EN 1990:2002, Table NA.A1.2(B)) for the ULS Permanent actions, γG = 1.35 Variable actions, γQ = 1.5
Material factor for solid timber, γM = 1.3 (UKNA to EC5, Table NA.3)
Design Action Design Action at ultimate limit state NEd = 1.35Gk + 1.5Qk NEd = 1.35(10) + 1.5(25) = 51 kN = 51000 N
Modification factors Factor for medium duration loading and service class 2, kmod.med = 0.8 (EC5, Table 3.1) System strength factor, (not relevant) ksys = 1.0
Fixed offshore platforms are used by the petroleum industry for a variety of operations, including drilling, production, housing, and storage. These constructions are situated offshore but closer to the coast than regular offshore platforms as a result of the recent growth in demand for LNG terminals. With regard to loading and foundation design, LNG terminals and offshore platforms are very similar, and the majority of what is covered here also applies to LNG terminals.
Steel constructions that are supported by piles or suction foundations or gravity platforms (GBS = “Gravity Based Structure”) that rest on the ocean floor can be used for permanent, fixed offshore platforms. Typically, open-ended tubular piles are driven for this purpose. Instead of driven piles, a few steel platforms are supported by steel suction caissons. The gravity platforms are made of steel or concrete and have skirts that reach the seafloor.
Gravity-based offshore platform
The Ekofisk Oil Storage Tank, which is situated in approximately 60 meters of water, has short skirts that only penetrate about 0.6 meters into extremely dense sand, in contrast to the tallest GBS to date, which is located in the Troll field in the North Sea in about 330 meters of water. All of this serves to illustrate how important a site’s geotechnical properties are to the design of any structure.
Offshore platforms frequently have large deck loads and, in exposed places, will be subject to powerful storm loads. High dynamic loads are imposed by earthquakes in seismic regions, which may weaken the soil. Because of the loads that the foundation must support, foundation engineering is an important step in the design process.
Jack-up platforms can only be used in shallow waters, are often only installed for a short time (months rather than years), and are usually supported by spudcans.
Jack-up offshore platform
The installation of various kinds of big floating structures is one of the most recent technologies for oil and gas exploration in the deep sea. These floating structures must be properly anchored.
Most offshore oil and gas fields have a small number of buildings. This is in contrast to an offshore wind farm, which is made up of numerous independently created wind turbines placed throughout the wind farm region. Suction caissons, monopiles, jacket piles, tripods, and gravity-type foundations are available for wind energy towers.
Foundation Design Issues for Offshore Platforms
For offshore platforms, some significant foundation design concerns include:
Bearing Capacity
Regardless of the structure, the subsoil needs to be able to support the static and cyclic loads with a safe enough margin against excessive displacements or failure. Potentially distinct from the bearing capacity under monotonic loading is the capacity under cyclic loading. Design considerations such as the soil’s bearing capacity under cyclic loads are also very important.
The parameters required for the evaluation of the bearing capacity are;
− Monotonic shear strengths under different stress paths − Cyclic shear strength under combined average and cyclic shear stresses for triaxial and simple shear stress paths. − Undrained angle of shearing resistance (sand).
Permanent Displacement (e.g. settlement)
Static actions will result in initial settlements as well as settlements in the soil outside and beneath the platform as a result of consolidation and creep. Due to increasing shear stresses and the dissipation of cyclically produced pore pressure, the cyclic loading from waves and/or earthquakes will result in extra permanent displacements.
The permanent displacements will decrease the freeboard—the distance between the deck of the platform and the water’s surface—and may put mechanical connections to things like oil wells, risers, and pipelines attached to the platform under stress.
The parameters required for the evaluation of the permanent displacement are;
− Compressibility − Permeability − Permanent shear strain and pore pressure under combined average and cyclic shear stresses for triaxial and simple shear stress paths − Compressibility after cyclic loading
Cyclic displacements
The soil and the platform are displaced cyclically as a result of cyclic loads. Cyclic displacements of the platform could make it difficult for the people and equipment on the platform to function optimally. Cyclical displacements, like permanent displacements, can lead to stresses in the structural components and any connections to the structure.
The parameters required for the evaluation of the cyclic displacements are;
− Cyclic shear strain as a function of cyclic shear stress under combined average and cyclic shear stresses for triaxial and simple shear stress paths. − Initial shear modulus.
Foundation Stiffness
For structural dynamic assessments under wave and/or earthquake loads, equivalent soil spring stiffness data are required. For platforms in deep water, the resonance period may be close to the period of the wave load, resulting in a large amplification of the wave load. Therefore, it’s crucial to either calculate and design for the wave load amplification factor or make sure the resonance period is sufficiently far from the wave load period.
The parameters required for the evaluation of the foundation stiffness are;
− Cyclic shear strain as a function of cyclic shear stress under combined average and cyclic shear stresses for triaxial and simple shear stress paths − Initial shear modulus − Damping
Reaction stresses in the soil
GBS constructions need to be built to withstand the stresses that the soil will experience from both static and cyclic loads. The soil reaction stresses may be particularly severe in some areas of an uneven seabed that is made up primarily of dense sand. Due to creep and the cyclical erosion of the soil modulus, the soil response stresses may also redistribute over time.
The parameters required for the evaluation of the reaction stresses are;
− Monotonic and cyclic shear strengths. − Compressibility under virgin loading and reloading. − Cyclic and permanent shear strains and permanent pore pressure under combined average and cyclic shear stresses for triaxial and simple shear stress paths. − Seabed topography, objects on the seafloor.
Skirt penetration
To ensure that GBSs with skirts will reach the intended penetration depth, the skirt penetration resistance must be computed.
The parameters required for the evaluation of the skirt penetration are;
− Undrained anisotropic monotonic shear strengths. − Remoulded shear strength (or sensitivity). − Drained angle of shearing resistance (sand). − Residual interface angle of shearing resistance (sand). − CPT resistance (sand). − Seabed topography and objects on the seafloor. − Boulders in the soil within the skirt penetration depth.
Pile driveability
The selection of pile driving equipment must be carefully considered in order to guarantee that the pile can withstand the driving stresses and that an acceptable penetration depth is achieved.
The parameters required are;
− Axial and lateral response. − Shear strength. − Soil modulus or strain at 50% ultimate strength − CPT cone resistance
Punch-through
The legs of a jack-up rig are often preloaded with ballast water before the footings (spudcans) are placed. Due to the rising ballast burden, these will pierce the seabed. In soils with a limited increase in soil strength with depth and in soils where a strong layer of limited thickness lays on top of a weaker layer, rapid, uncontrolled penetration may happen.
Analyses of liquefaction potential
This is necessary for locations with sands or silty sands, as well as seismically active regions and areas with the potential for high wave loading.
Liquefaction potential can be assessed using the following parameters;
Around offshore sites, erosion and scour may be brought on by waves and currents. In comparison to clay, sand has a higher propensity for scouring, which normally rises as water depth falls. A graded gravel fill may be necessary around the perimeter of gravity platforms constructed on sand in order to prevent erosion. Skirts around the edge will assist stop the base from scouring. Scour can be accommodated in the case of piles by including a specific scour depth in the design.
Permeability tests can be used to assess erosion and scour potential.
Soil and Rock Parameters Required
To address these design issues highlighted for offshore platforms, geotechnical and geological information needs to be collected. The sections below list the basic and specific soil and rock parameters needed for a typical offshore platform’s foundation design.
Clays
− General description − Layering − Grain size distribution − Water content − Total unit weight − Atterberg (plastic and liquid) limits − Indicative shear strength (miniature vane, torvane, pocket penetrometer, fall cone, UU, etc.) − Remoulded shear strength − Sensitivity − Soil stress history and overconsolidation ratio − Organic material content
Sand, silt, or gravel
− General description − Layering − Grain size distribution − Water content (silt) − Maximum and minimum densities − Relative density − Drained angle of shearing resistance − Soil stress history and overconsolidation ratio − Angularity − Carbonate content − Organic material content
Rock
− General description − Rock Quality Designation (RQD) − Water absorption − Total unit weight − The unit weight of solid blocks − Unconfined compression strength − Mineralogy − Carbonate content
Typical Scope of Geophysical Investigations
The following are the typical scope of work for the geophysical investigation.
Seabed Topography Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc. Means of survey: Swath bathymetry, preferably multibeam
Seabed features Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc. Means of survey: Sidescan sonar, line spacing 100-200 m depending on water depth, with sonar range set to provide 200 % coverage from line overlap.
Subsurface Information Minimum survey area: Usually 1 km x 1 km in shallow water, 2 km x 2 km in deep water. Possible extension to 5 km x 5 km in areas with geohazards to incorporate possible platform location shifts etc. Means of survey: High-resolution / ultra high-resolution seismic survey for shallow geology and fault offset analysis. Line spacing: 100 m to 200 m depending on water depth. May be performed simultaneously with sidescan sonar. Also, 3D exploration seismic data (where available) to approximately 1.5 milliseconds for regional geohazard analysis and drilling hazard analysis to approximately 1000 m depth.
Typical Scope of Geotechnical Investigation
Gravity Platforms
The typical scope of work for the geotechnical survey of a gravity platform includes the following;
(1) 1 no. BH with continuous sampling down to 15 m, thereafter sampling with less than 0.5 m gaps to 0.5 – 0.7 times the platform diameter, followed by alternate sampling and CPT (preferably (P)CPT) with less than 0.5 m gaps. The minimum depth of penetration should be 1.5 times the platform diameter.
(2) 3 nos. BHs with continuous sampling to 15 m, thereafter sampling with less than 0.5 m gaps. The minimum depth of penetration should be 50 m.
(3) 10 nos. continuous (P)CPTs. The minimum depth of penetration should be 50 m or 1.5 times the platform diameter.
Sample testing
The samples retrieved from the subsurface exploration should be subjected to the following engineering tests;
Index testing,
Triaxial tests
Oedometer tests.
Permeability tests.
Simple shear tests and anisotropically consolidated compression and extension triaxial tests, monotonic and cyclic.
Shear wave velocity measurements by bender elements to determine initial shear modulus.
Resonant column tests.
X-ray photographs to determine soil layering within the tube, inclusions, sample quality.
Radioactive core logging (optional).
Piled Platform
The typical scope of work for the geotechnical survey of a piled offshore platform includes the following;
(1) 1 no. BH with samples every metre down to 15 m, thereafter sampling with less than 0.5 m gaps to 30 m, followed by alternate sampling and CPT (preferably (P)CPT) with less than 0.5 m gaps or 2 nos. BHs: one with sampling only and one with near-continuous CPT. The minimum penetration should be 4 pile diameters or pile penetration plus pile group, diameter, whichever is greater.
(2) Continuous (P)CPTs at a location 5-10 m from the main borehole. The penetration depth should be 30m.
Sample Testing
Index testing,
Testing for pile capacity and drivability, and for (static mudmat) bearing capacity
Jack-up Rig Platform
Jack-up rig platform
The typical scope of work for the geotechnical survey of a jack-up offshore platform includes the following;
(1) 1 no. BH with samples at every metre down to 15 m, and thereafter gaps less than 0.5 m. The minimum depth of penetration should be 30 m or anticipated spudcan penetration + 1.5 times spudcan diameter, whichever is deeper.
(2) 1 no. Continuous (P)CPT at a location 5 to 10 m from the main borehole and/or at each leg location. The minimum depth of penetration should be 20 m.
Sample Testing
Index testing
Testing for static bearing capacity
The scopes of work listed above are representative of common platforms and uniform soil types. The size of the platform and the soil’s heterogeneity over the platform’s footprint determine how many boreholes are needed. Additionally, one should account for potential adjustments to the footprint’s size or precise placement.
Once the design is underway, it’s also feasible that the platform’s type will be changed. The intended use of the proposed platform and the implications of risk analysis for potential subpar performance of the structure or the soil also influence the number of test sites.
The number of borings, (P)CPTs, and other in-situ tests are also based on the site’s ground variability as identified by preliminary geophysical investigations. Additional borings or (P)CPTs are especially required close to geophysical anomalies or close to discontinuities in the layers of the soil or rock.
Therefore, depending on size and soil variability, a GBS may require one or two deep boreholes in addition to shallow boreholes/CPTs. For a larger structure, more shallow boreholes are likely to be required. Depending on the size and soil variability, between one and four deep boreholes may be needed for piled construction.
For the measurement of horizontal soil resistance on piles and the design of mudmats, further CPTs or sample borings may be necessary. For axial pile design at sand sites utilizing existing CPT-based design approaches, near-continuous CPT profiles are necessary. It has been recognized that deeper penetration of piles leads to an increase in the axial capacity for piles, hence it’s important to characterize these deeper soils properly. For instance, one wouldn’t want to overlook clay bands, at the intended point elevation in a deep sand.
Conclusion
Site investigations for offshore platforms should ideally be carried out in stages. The number of in-situ testing may be limited in the early stages of a project to the data required to assess the viability of various construction forms. In this case, extra testing will be needed before moving on to the detailed design step.
The chance that the platform location will change due to changes in the field layout, the platform’s qualities, and the mooring leg properties should be taken into account throughout the site investigation. A more trustworthy evaluation of in-situ strength and stiffness parameters would include penetration tests (cone, T-bar, or ball), vane shear tests, and pressuremeter testing in addition to sample borings.
The removal and transportation of soil particles as a result of water, wind, or other physical disturbances is known as soil erosion. Water erosion tends to be more significant and problematic than wind erosion and usually occurs as sheet erosion, which is the periodic removal of thin sheets of soil over an area, or as gully erosion that forms incised channels.
Significant loss of soil due to erosion can create geohazards and affect civil engineering structures and infrastructures if not properly controlled or managed.
Gully erosion geohazard
Furthermore, cropland productivity and agriculture can be harmed by soil surface erosion, and eroded soil sediments can negatively affect streams, lakes, and reservoirs. The erosion of roadside embankments can result in rills and gullies on the embankments, increased surface runoff, more soil erosion, and ultimately slope failure.
The US Department of Agriculture (USDA) developed the universal soil loss equation (USLE) to predict the soil loss caused by surface erosion. This equation has been widely used to predict the average annual rate of sheet and rill erosion in agricultural fields. The equation, which takes into account terrain, crop system, rainfall pattern, and management techniques, is written as follows:
A = R × K × (LS) × C × P
where: A = average annual soil loss; it is conventionally expressed in tons/ac/yr, R = rainfall and runoff factor; it depends on the rainfall intensity and duration, K = soil erodibility factor; it represents a soil’s ability to resist erosion and is determined by the soil texture, soil structure, organic matter content, and soil permeability, L = slope length, S = steepness factor, C = cover and management factor; it is the ratio of soil loss in an area with specified cover and management to the corresponding soil loss in a clean-tilled and continuously fallow condition. For bare ground, C = 1.0, P = support practice factor; it is the ratio of soil loss with a support practice such as contouring, strip-cropping, or implementing terraces compared to up-and-down-the-slope cultivation. For construction sites such as roadside embankment, P is not used in the equation.
The USDA published the Revised Universal Soil Loss Equation (RUSLE) in the year 1996 to replace the original USLE. The RUSLE still uses the same form and factors as the original USLE, but additional data were added and the factor assessment technique was changed.
The factor K was changed to be time-varying to reflect freeze-thaw conditions and consolidations; the topographic factors for slope length and steepness, LS, were changed to reflect the ratio of rill to interrill erosion; the cover-management factor C was changed from the seasonal soil loss ratios to a continuous function that is the product of four subfactors; and the factor P was expanded to consider environmental factors.
Control of Surface Erosion
In field practice, various surface erosion control techniques are used. Innovative, cost-effective, and sustainable techniques are always emerging as research and technology progresses.
Several of the most widely used surface erosion management techniques are described in the sections below.
Riprap
Riprap is “a flexible channel or bank lining or facing consisting of a well-graded mixture of rock, broken concrete, or other material, usually dumped or hand-placed, which provides protection from erosion,” according to the US Federal Highway Administration (1989).
Riprap is used for erosion control
Riprap is frequently used to protect and stabilize embankments, slope drains, storm drains, and side slopes of rivers, channels, lakes, and dams. Rock riprap, wire-enclosed rock, grouted rock, precast concrete block revetments, and paved lining are some examples of riprap revetments.
Riprap in river bank erosion control
The most popular surface erosion prevention technique for river and channel banks is rock riprap. The figure above provides an illustration. The individual stones for riprap are preferably angular in shape and well-graded to allow for interlocking. Together with the weight of the stone, this interlocking property creates a strong stone layer that can withstand erosion.
On steep slopes, rock riprap can be unstable, particularly when rounded rock is used. Other materials, such as geosynthetics matting, should be taken into consideration for slopes steeper than 2:1. The following factors may be taken into account while designing rock riprap:
The following design recommendations can be adopted for rock riprap;
• Gradation: Rather than using one consistent size of rock, use a well-graded combination of rock sizes. • Stone quality: Use hard riprap material (most igneous stones, including granite, have appropriate durability) so that freeze-thaw cycles do not quickly disintegrate it. • Riprap depth: The thickness of the riprap layer should be double or more than the largest stone diameter. • Filter material: Before laying the riprap, apply a filter material, often a layer of gravel or geosynthetic fabric. This keeps the earth beneath the riprap from penetrating it. • Riprap limits: Place riprap so that it reaches the maximum depth of the flow or until enough vegetation has grown to effectively control erosion. • Curved flow channels: Ensure that riprap reaches upstream and downstream of the beginning and end of the curve as well as the entire curved section to five times the bottom width. • Riprap size: The riprap material’s size varies from an average of 5 cm to 60 cm in diameter depending on the shear stress of the flows to which it will be subjected.
Compost
Composting can slow erosion and hasten the development of vegetation in highly erodible environments. During composting, microorganisms transform organic matter into a stable amendment using an aerobic process. Composting prevents waste from going to landfills and transforms waste into valuable resources with significant economic and environmental advantages.
Composts of many kinds have been used to reduce surface erosion on embankments and naturally occurring slopes. Some of them are;
(1) organic stuff composted from grass clippings and agricultural waste (2) manure compost made from animal and plant waste, (3) co-composting material made from biosolids and green waste, and (4) composts made from wood chips and forestry waste.
Composts made from municipal solid waste and leftover food have both been used to control erosion. Compost’s characteristics, including pH, soluble salts, moisture content, organic matter content, maturity, stability, particle size, pathogens, physical contaminants, and other factors, can differ greatly depending on where it comes from and how it is made.
Various organizations may specify compost that is appropriate for technical uses. To assure the quality of the finished dry compost, the USDA and the United States Compost Council (USCC) have established the “Test Methods for the Examination of Composting and Compost.”
Vegetation Cover
Another popular erosion control strategy is vegetation. It protects the soil surface from raindrop impact, shields the soil surface from overland flow’s scouring effect, and reduces the velocity of the flowing water to decrease its erosive capacity. In addition to strengthening the soil’s ability to withstand erosion and enhance the rate of infiltration, vegetation roots can also reduce runoff.
Vegetative cover is more aesthetically pleasing and is relatively easy to achieve and maintain. It is frequently combined with other erosion control techniques such as mulches, geosynthetic coverings, and blankets made of compost.
The following elements should be taken into account while designing and installing vegetative covers:
• Acidity, moisture retention, drainage, texture, organic matter, and fertility of the soil. • The state of the site, specifically its slopes and vegetative cover. • Climate factors like wind, precipitation, and temperature. • Species choice, which is influenced by the local climate, planting seasons, water demand, soil preparation, weed management, post-construction land usage, and anticipated maintenance requirements, such as irrigation and cost. • Methods of the establishment. • The maintenance techniques.
Geosynthetics
Geosynthetics can be conveniently employed for slope stabilization, erosion control, and sediment management. Geosynthetics can be used in many different ways, and new ways are always being developed. Examples include hard armors, nondegradable RECPs, and degradable rolling erosion control products (RECPs).
On rehabilitated lakeshores, riverbanks, and alongside recently built roadsides, degradable materials can be utilized to improve the development of flora. When vegetation alone can adequately protect a site after an erosion control material has deteriorated, then degradable materials can be utilized.
Geosynthetics in slope erosion protection
However, products that are not biodegradable offer vegetation long-term reinforcement. They are used in more difficult erosion control situations when immediate, high-performance erosion protection is required. By permanently bolstering the vegetative root system, the materials increase the erosion resistance of soil, rock, and other materials.
The use of geosynthetic erosion control measures frequently performs multiple functions, such as collecting and draining surface runoff, filtration, separating, strengthening, and establishing and maintaining vegetation coverings.
Soil Binders
Soil binders (materials with cementation properties) can be used to increase the resistance of a soil matrix to water and pressure. Soil binders are used to hold soil particles together and can be used for soil stabilisation and erosion control. The usage of stabilized soil and the local environmental conditions have a big impact on how well conventional soil binder applications work.
Cement is a common binder used in construction despite its several environmental disadvantages. Quicklime, hydrated lime, and lime slurry are the compounds used as lime-soil binders. Due to its weak resistance to the abrasive effect of traffic, fly ash is primarily employed to stabilize subbase or subgrade; it is not among the soil binder products suited for surface stabilisation.
Erosion Control Structures
Numerous structural measures can be used as either temporary or permanent installations for erosion control. There are many different forms of grade control structures, such as diversions, grassed or paved streams, buried pipe outlets, bench terraces or berms, retarding structures, debris basins, and many others. In the sections that follow, it will be explained where and how some of these measurements might be employed, as well as the placement and design criteria;
Diversions
Diversions are constructed over a slope in the form of graded channels with a supporting ridge on the lower side. They provide the function of collecting surface or subsurface water and directing it to a discharge point where it can be safely disposed of. These structures can be level or graded, temporary or permanent. While level terraces have closed ends and hold runoff, graded terraces transport water in a predetermined direction at a non-erosive velocity.
Water diversion structure
Diversions are helpful above borrow zones, gully heads, and cut slopes. They can be used to divert runoff water away from crucial construction sites and can be built across cut slopes to reduce slope length into non-erosive segments. They could be constructed on-site or off-site.
Water should empty from diversion points into prepared individual outlets, established disposal zones, or natural outlets. Individual outlets could be constructed as chutes, paved or grassed streams, or underground pipes. Diversion should be designed to handle rainwater with a return period of 10 years.
Berms
Steps or benches found on steep slopes are called berms. They are a variation of diversions and accomplish essentially the same thing. They shorten slope lengths and divide runoff water volume into manageable slugs when placed and designed properly.
Berms on slopes
Grading the berm in such a way that the outside edge is higher than the edge next to the cut slope gives it the capacity to carry the volume of water. By burying a pipe or using paved canals, runoff water from the berm can be removed.
Waterways
Waterways act as outlets for berms, diversions, and other constructions. They can be constructed or naturally occurring, shaped to the necessary size, and covered in vegetation or asphalt to collect runoff water. They are often built into one of the following common cross sections: rectangular, parabolic, trapezoidal, or V-shaped. Parabolic waterways are most frequently used in areas that will be vegetated and it is the shape that commonly occurs in nature.
Constructed rectangular waterway
Protection from erosion is essential for the efficient operation of a waterway. This can be achieved by paving with concrete or rock or by establishing flow velocities that are benign for the grass that will be employed. Waterways ought to be built at least to handle the runoff from a rain event with a 10-year frequency.
Debris Basin
A debris basin is used to temporarily hold runoff water that contains substantial amounts of silt, sand, or gravel. Typically, it consists of a primary spillway made of perforated pipe and an earth embankment. Upon entering the impoundment, runoff water slows. The water is automatically pushed out through the main spillway as a substantial portion of the debris settles out.
Debris basin
Measures should be provided for periodic cleanup of the drainage basin, or the structure should be built to store the anticipated debris yield over the duration of the construction. There are both permanent and temporary debris basins. Those used to contain construction-related debris are often temporary and are demobilised when the project is finished and the region has stabilized.
Retarding Structures
A retarding structure is made up of earth embankment, a main spillway, and an emergency spillway structure. They could be built on-site or off-site. Their function is to temporarily retain runoff and then slowly release it to protect the land below. They can lower runoff peaks, allowing for the use of smaller bridges and culverts.
It is possible to do away with bridges and possibly save money if retarding structures are built on-site and the roadbed is filled. Cooperation from locals and groups like soil and water conservation districts will be required because any one of the structures has a variety of community benefits, including flood mitigation, grade control, and water for cattle, recreation, and irrigation. This kind of collaboration is encouraged by memorandums of understanding that a number of state transportation departments have with respective conservation organizations.
The main spillway typically comprises an earth fill exit and a horizontal pipe or monolithic concrete outflow made of reinforced concrete. The retarding storage space is typically intended to be empty in 10 days or fewer. The emergency spillway should, at the very least, be able to manage the routed runoff from a storm with a 25-year frequency or from a storm with a frequency equal to the structure’s design life, whichever is higher.
The runoff anticipated to occur at a frequency consistent with the level of protection to be offered should be able to be contained by the retarding storage.
Grade Stabilisation Structures
Structures for grade stabilization maintain grade or manage head cutting in both natural and man-made channels. They literally step runoff water down a hill at a regulated rate while lowering channel grade and flow velocity.
Grade stabilisation structure
Earth embankments with pipe spillways or mechanical structures made of concrete, masonry, steel, aluminium, or treated wood are examples of grade stabilization systems. Although the design is dependent on site conditions, these structures should typically be able to manage storm runoff with a 25-year return period.
Gabions
Gabions are riprap (natural stones) contained in cages or cylinders made of galvanized steel-wire mesh. These are employed to prevent erosion on slopes, stream banks, or shorelines. Instead of being stacked vertically, they are typically positioned on slopes at an angle—either damaged or stepped back. The wireframe (cage) of a gabion determines how long it will last, and quality gabions have a fifty-year structural stability guarantee.
Gabion wall
Conclusion
Erosion is the loss of soil due to the dislodgement and transportation of soil from one site to another. As soil erodes, soil nutrients are lost, rivers become clogged with dirt, and finally, the area becomes a desert or in the case of deep gullies, the area becomes uninhabitable.
Though erosion occurs naturally, human activity has the potential to significantly worsen it. Land that was once healthy and alive can become parched and lifeless due to erosion, which can also lead to mudslides and landslides.
When the proper techniques, equipment, and approaches are applied at the appropriate times, erosion can be easily managed on a construction site. Planting vegetation is the most natural and efficient technique to control erosion. Plant roots, particularly those of trees, cling to the earth and can effectively stop excessive soil movement.
A precast prestressed girder is an alternative economical solution to the construction of long-span bridges. This solution allows for quick bridge construction by using girders that are fabricated off-site, transported to the job site, and then installed into place.
In a construction site where efficiency or minimal traffic interruption is necessary, prestressed bridge girders are recommended because they require little to no falsework. Prestressed girders are very cost-effective and can be used for spans of up to 50 metres (166 ft).
A 164 feet long precast prestressed girder
In prestressed concrete design, structures are generally designed at the serviceability limit state (SLS). However, it is equally important to ensure that the designed structure has the sufficient moment and shear capacity at the ultimate limit state (ULS). In this article, checking for bending capacity at the ULS is discussed.
In order to determine the ultimate moment capacity of a given section which has been designed to the serviceability limit state of bending, it is necessary to determine the position of the neutral axis at the ultimate limit state. This usually involves a trial-and-error calculation procedure.
On-site installation of a prestressed girder
Initially, a value for the neutral axis depth x is assumed. For the assumed depth, compressive forces in various parts of concrete and tensile force in all steel reinforcement are calculated. For equilibrium, the algebraic sum of total tensile and compressive forces must be zero. If the sum is not zero, calculations are repeated for a new value of neural axis depth and the calculations are continued till the correct value of x is found.
Typical construction details of a prestressed girder
Ultimate Bending Strength Calculation of a Prestressed Girder
Calculate the ultimate bending strength of a bridge beam girder shown below. The calculation is adapted from Bhatt (2011).
The details of material properties, prestress details, etc. are follows. Concrete: Concrete grade 40/50, fck = 40 MPa, material safety factor, γm = 1.50, fcd = fck/γm = 26.7 MPa Using a rectangular stress block: λ = 0.8, η = 1.0, fck ≤ 50 MPa, ηfcd = 26.7 MPa Maximum strain in compression εcu3 = 3.5 × 10-3; σc ≤ 50 MPa
Steel: 12.7 mm nominal diameter 7-wire strand with an effective cross-sectional area of 112 mm2. fpk = 1860 MPa, fp0.1k ≈ 0.88 × fpk = 1637 MPa, γm = 1.15 fpd = fp0.1k /γm = 1424 MPa, Es = 195 GPa
Prestressing details: 20 strands are located at the following distances from the soffit: six each at 50, 100 and 200 mm and two at 1100 mm from the soffit. The cables are spaced horizontally at 50 mm c/c.
Total prestress at service Ps = 2037 kN Stress σpe due to prestress force in the strand = Ps/Area of 20 stressed strands = 2037 × 103/ (20 × 112) = 909 MPa Prestrain εpe = σpe/Es = 909 / (195 × 103) = 4.66 × 10-3
The calculations consist of the following steps.
Step 1: Estimate a value for neutral axis depth: i. Total tensile force: Assume all 18 strands in the bottom three rows only ‘yield’ i.e. stress is a maximum of fpd equal to 1424 MPa. The area of each strand = 112 mm2. Therefore total tensile force T = 18 × 112 × 1424 × 10-3 = 2871 kN
ii. Total compressive force: Assuming a rectangular stress block, the uniform compressive stress = ηfcd = 26.7 MPa, λ = 0.8 (a). Total compressive force in top flange = 26.7 × 200 × 400 × 10-3 = 2136 kN (b). Total compressive force in the top trapezium = 26.7 × 0.5 × (200 + 400) × 120 × 10-3 = 961.2 kN Total compressive force C = 2136 + 961.2 = 3097.2 kN C > T. Therefore stress block λx < (200 + 120) or x < 400 mm
Step 2: For an assumed value of neutral axis depth x, calculate the total tensile and compressive forces and check for equilibrium.
Iteration 1: Assume x = 350 mm. Rectangular stress block depth s = λx = 0.8 × 350 = 280 mm. The uniform compressive stress = ηfcd = 26.7 MPa
ii. Total compressive force: (a) Compressive force in top flange = 2136.0 kN (b) Total compressive force in part of the trapezium:
Depth dx of stress block inside the trapezium = s – depth of top flange = 280 – 200 = 80 mm Width Bx of the trapezium at the bottom edge of the stress block: Bx = width of web + (width of top flange – width of web) × (1 − dx /depth of trapezium) Bx = 200 + (400 – 200) × (1− 80/120) = 267 mm Compressive force in the trapezium = 26.7 × 0.5 × (400 + 267) × 80 × 10-3 = 712.4 kN Total compressive force C = 2136.0 + 712.4 = 2848.4 kN
iii. Total tensile force: Bending strain εb in strand at d from the compression face = εcu3 × (d – x)/x Calculations are shown in the Table below.
Distance from soffit (mm)
d (mm)
εb × 103
εs = (εb + εpe) × 103
σs, MPa
No. of strands
T (kN)
50
1150
8.0
12.66
1424
6
957
100
1100
7.5
12.16
1424
6
957
200
1000
6.5
11.16
1424
6
957
1100
100
-2.5
2.16
421
2
94
Total
20
2965
T = 2965.3 kN The difference between the total tensile force and compressive force = 2965 – 2848 = 117 kN The compressive force is too small, indicating that the neutral axis depth is larger than 350 mm.
Step 3: Assume x = 450 mm. Stress block depth s = λx = 0.8 × 450 = 360 mm
Iteration 2: ii. Total compressive force: (a). Total compressive force in top flange = 26.7 × 200 × 400 × 10-3 = 2136 kN
(b). Total compressive force in the top trapezium: Depth dx of stress block inside the trapezium = s – depth of top flange = 360 – 200 = 160 mm > 120 mm The entire trapezium is in compression. Compressive force in the trapezium = 26.7 × 0.5 × (400 + 200) × 120 × 10-3 = 961.2 kN
(c). Compressive force in the web: Depth of web in compression = s – 200 – 120 = 40 mm Compressive force in web = 26.7 × 40 × 200 × 10-3 = 213.6 kN Total compressive force C = 2136.0 + 961.2 + 213.6 = 3310.8 kN
iii. Total tensile force: Bending strain εb in strand at d from the compression face = εcu3 × (d – x)/x
Calculations are shown in the Table below.
Distance from soffit (mm)
d (mm)
εb × 103
εs = (εb + εpe) × 103
σs, MPa
No. of strands
T (kN)
50
1150
5.44
10.10
1424
6
957
100
1100
5.06
9.72
1424
6
957
200
1000
4.28
8.94
1424
6
957
1100
100
-2.72
1.94
378.0
2
85
Total
20
2956
T = 2955.6 kN
The difference between the total tensile force and compressive force = 2956 – 3311 = −355 kN
Step 4: Interpolation From the two values of x and the corresponding difference between the total tensile and compressive forces, the value of x for which the difference is zero can be determined by linear interpolation.
x
T – C
350
117
450
-355
x = 350 + (450 – 350) × 117/ [117 − (− 355)] = 375 mm
Step 5: Final calculation Assume x = 375 mm. Stress block depth s = λx = 0.8 × 375 = 300 mm
ii. Total compressive force: (a). Total compressive force in top flange = 26.7 × 200 × 400 × 10-3 = 2136 kN
(b). Total compressive force in the top trapezium: Depth dx of stress block inside the trapezium = s – depth of top flange = 300 – 200 = 100 mm < 120 mm Width Bx of the trapezium at the bottom edge of stress block = width of web + (width of top flange – width of web) × (1−dx /depth of trapezium) = 200 + (400 – 200) × (1 − 100 /120) = 233 mm Compressive force in the trapezium = 26.7 × 0.5 × (400 + 233) × 100 × 10-3 = 845.5 kN Total compressive force C = 2136.0 + 845.5 = 2981.5 kN
iii. Total tensile force: Calculations are shown in the Table below;
Distance from soffit (mm)
d (mm)
εb × 103
εs = (εb + εpe) × 103
σs, MPa
No. of strands
T (kN)
50
1150
7.23
11.89
1424
6
957
100
1100
6.77
11.43
1424
6
957
200
1000
5.83
10.49
1424
6
957
1100
100
-2.57
2.09
408.0
2
91
Total
20
2962
T = 2962 kN
The difference between the total tensile force and compressive force = 2962 – 2982 = −20 kN This is small enough to be ignored. In fact the correct value of x = 372 mm. The ‘error’ is because of linear interpolation.
Step 6: Ultimate moment: The ultimate moment is obtained by calculating the moment of all forces, tensile as well as compressive about the soffit.
(a). Compressive force in top flange = 2136 kN Lever arm from the soffit = 1200 – 200/2 = 1100 mm
(b). Compressive force in the top trapezium = 845.5 kN Depth of stress block inside the trapezium dx = 100 mm Width of the trapezium at the bottom edge of stress block Bx = 233 mm Lever arm = 1200 – 200 – (dx/3) × [1 + Bx / (400 + Bx)] = 954 mm iii. Tensile force forces at various level act at their position from the soffit as shown in the Table above.
The ultimate bending moment Mu is given by Mu = [(2136 × 1100) + (845.5 × 954) – (957 × 50) – (957 × 100) − (957 × 200) – (91.4 × 1100)] × 10-3 = 2721.1 kNm
Calculation adapted from: Bhatt P. (2011): Prestressed Concrete design to Eurocodes. Spon Press, Taylor and Francis. ISBN: 978-0-203-84725-1
Geosynthetics are planar products manufactured from a polymeric material (polypropylene, polyester, polyethene, polyamide, PVC, etc) or natural materials and are used with soil, rock, earth, or other geomaterials as an integral part of a civil engineering project, structure, or system.
Geonaturals are products made from natural fibres (such as jute, coir, cotton, wool, etc.) that are utilized mostly in temporary civil engineering applications. Due to the fact that these materials biodegrade quickly when utilized with earth materials, they don’t have as many field applications as geosynthetics do.
During the production of geosynthetics, planar textile structures are created by combining elements such as fibres or yarns. Depending on their characteristics and purposes, many of such products fall under the general term ”geosynthetics”.
For instance, a permeable geosynthetic made of textiles is called a geotextile. Geogrids are a regular network of tensile elements with holes large enough to interlock with surrounding fill material and are generally utilized for soil reinforcement.
Figure 1: Geogrid as sol reinforcement in road construction
Low permeability geosynthetics called geomembranes are employed as fluid barriers in engineering constructions such as sanitary landfills. To benefit from the properties of each component, geotextiles and related products like nets and grids can be mixed with geomembranes and other synthetic materials.
Geotextile-geonet composites, geotextile-geomembrane composites, geotextile-geomembrane-grid composites, geotextile-polymeric core composites, and even three-dimensional polymeric cell structures can be combined to create these products, which are known as geocomposites.
The process utilized to assemble the filaments or tapes into the planar textile structure determines the type of geotextile. Geotextiles fall into one of two categories: woven or nonwoven. Polyolefin sheets are extruded and oriented to create rigid geogrids with integral joints. Flexible geogrids are constructed from polyester strands that have been knit or weaved together at their crossover points and then coated with a polymer.
The use of geosynthetics to replace customary construction methods has a wide range of applications. Subgrade stabilization, ground improvement, slope stabilization, reinforced earth, a system for lining and cover of landfills, retaining walls, and erosion and scour protection measures for river banks are some of the potential applications. The materials have been around for thirty years, but they haven’t been extensively utilised in all the possible areas of civil engineering construction.
The article discusses the current state of geosynthetics application in various civil engineering projects as well as its potential in the future. It also discusses the challenges of using this material, along with suggestions for how to get over them.
Applications and Functions of Geosynthetics
Geosynthetics have the following primary functions in civil engineering construction works:
Separation
Filtration
Drainage
Reinforcement
Barrier
Protection
Separation
A geosynthetic material is referred to as performing a “separation function” if it must prevent the mixing of adjacent dissimilar soils and/or fill materials during construction and over the anticipated service life of the application under consideration. Synthetic geotextiles are utilized for this. Depending on the manufacturing process, the following categories of geotextiles are currently available:
Woven Geotextile
Nonwoven Geotextile
Knitted Geotextile
Stitched Geotextile
Monofilament, multifilament, or fibrillated yarns, as well as slit films and tapes, are used to create woven geotextiles. Although nonwoven textile production is a recent industrial development, the weaving method has been around for quite some time. Continuously extruded synthetic polymer fibres or filaments are spun, blown, or otherwise placed onto a moving belt.
The mass of filaments or fibres is then either heat bonded, in which the fibres are welded together by heat and/or pressure at their sites of contact in the nonwoven mass, or needle punched, in which the filaments are mechanically entangled by a succession of small needles.
Instead of using a weaving loom, a knitting machine can be used to interloop one or more yarns (or other components) to create a knitted geotextile. Geotextiles that have been stitched together or sewn together using fibres, yarns, or both. Figure 2 illustrates how the geotextile layer keeps the structural integrity and functionality of both materials intact by preventing the mixing of soft soil and granular fill.
Figure 2: Geotextile as a separator
A geosynthetic layer is typically placed at the interface between soft foundation soil and the overlaying granular layer in many geosynthetic applications, particularly in roadways, rail tracks, shallow foundations, and embankments. In this case, it becomes challenging to pinpoint if the primary role of the geosynthetic is reinforcement and/or separation.
When the ratio of the applied stress (s) on the subgrade soil to the shear strength (cu) of the subgrade soil has a low value (less than 8), separation can be a dominant function over reinforcing, and it is largely independent of the settling of the reinforced soil system.
Filtration
A geosynthetic may serve as a filter that allows for enough fluid flow with minimal soil particle migration across its plane during the course of the application’s anticipated service life. Figure 3 demonstrates how a geosynthetic allows water to escape from a soil mass while limiting uncontrolled soil particle mobility. When a geosynthetic filter is positioned next to base soil (the soil to be filtered), a gap forms between the geosynthetic’s structure and the original soil structure.
Figure 3: Use of geosynthetics as filter fabric in a retaining wall
Certain soil particles, especially those nearest to the geosynthetic filter and with diameters smaller than the filter opening size, can go through the geosynthetic under the influence of seepage flow thanks to this discontinuity. The retention of the base material and permeability are the two parameters on which the design of the geotextile filter is based.
Soil Retention Criterion O95 < αDw O95 = Apparent opening size of geotextile α = 0.5 to 5.0 (a constant) Dw = Representative soil particle size = D90 for unidirectional flow = D15 for cyclic flow Permeability Criterion
Permeability criterion • Less critical applications and less severe conditions Kgt > Ksoil • Critical applications and severe conditions Kgt > 10Ksoil
Traditional granular filters can be replaced by geosynthetic filters since they are less prone to clog up regularly over the course of a structure’s service life. Table 1 provides a comparison of granular and geotextile filters. The geotextile filters finds application in earth dams, embankments, engineered landfill etc.
Granular Filter
Geotextile Filter
Quality control can be difficult
Quality control can be easily assured
Installation is more complex
Installation is easy
There can be discontinuities in the filter media
The filter media is continuous
Clogging can be frequent
Clogging takes a very long time
Intimate contact with surface
Surface preparation is required
Large volume of granular material
Conserves granular materials
Table 1: Comparison between granular filter and geotextile filter
Drainage
A geosynthetic is said to perform the drainage function if it permits adequate fluid flow with little soil particle movement within its plane from the surrounding soil mass to different outlets. Figure 4 demonstrates how water from the pavement surface is captured and carried to the embankment drainage built by the geosynthetic layer placed in the pavement.
Figure 4: Drainage function of geosynthetics
It should be mentioned that a geosynthetic dissipates the surplus pore water pressure while carrying out the filtering and drainage tasks by allowing water to flow in plane and across its plane. This can be accomplished by combining the geocomposite with geotextile (for filtering action) and geonet (for drainage function). It also considers load distribution.
Reinforcement
A geosynthetic serves as reinforcement by increasing the mechanical characteristics a soil mass as a result of its incorporation. A composite material known as “reinforced soil” is created when soil and geosynthetic reinforcement are mixed. This material has strong compressive and tensile strengths and is conceptually comparable to reinforced concrete.
In actuality, the primary function of any geosynthetic used as reinforcement in geotechnical structures is to withstand applied stresses or prevent unacceptable deformations. By acting as a tensioned part in this process and being joined to the soil/fill material via friction, adhesion, interlocking, or confinement, the geosynthetic preserves and enhances the stability of the soil mass.
The soil is reinforced using geogrid. It is a planar polymeric product made of crossing tensile-resistant ribs that are integrally joined at the joints to form a mesh-like or net-like regular open network. Extruded geogrid, bonded geogrid, and woven geogrid are the three different terms for the geogrid that results from joining the ribs together.
A regularly punctured polymer sheet is stretched longitudinally to create an extruded geogrid, which has a substantially higher tensile strength in the longitudinal direction than the transverse direction. a geogrid created by stretching a polymer sheet that has been punctured regularly in both the longitudinal and transverse directions; as a result, it has an identical tensile strength in both directions.
When building embankments with a constrained width and requiring that the face of the wall stay nearly vertical, reinforced earth (RE) techniques are typically used. The use of expensive retaining walls is completely eliminated by the RE wall technique. When ground conditions are unfavorable and adequate backfill materials are locally accessible, savings of between 25 and 50 percent have been documented. Figure 5 shows the various parts of the RE wall.
Figure 5: Components of a reinforced earth wall
The proper establishment of the friction connection between the soil and the reinforcement, which will depend on the composition, gradation, density, and properties of the fill material and geosynthetics, is essential for the stability of RE structures. RE structures are best suited to cohesionless soil that has been compacted to densities that cause volumetric expansion during shear.
Conversely, cohesive fill material produces short-term structural instability as a result of pore water pressure buildup, creep behavior, and frost actio. Early in the 1970s, because to worries about potential corrosion of metallic reinforcement, geotextiles were first used as reinforcing materials. While steel-reinforced walls have been built to heights exceeding 40 m, geosynthetic reinforced walls have been built to maximum heights of less than 20 m.
Barrier
In the lining and cover of an engineered landfill, geomembrane is employed as an impermeable barrier. It is a planar, comparatively impermeable synthetic sheet used as a barrier or liner in construction projects to restrict fluid flow. The material could be polymeric, asphaltic, or a combination of both. For leachate migration under subsoil conditions, natural or compacted clay liners are insufficient to provide the necessary level of environmental protection or necessitate additional thickness.
Figure 6: Geoynthetics as a liner in a landfill
The use of geomembrane as a suplimentary barrier provides an economical approach to achieve the regulatory standard. Depending on the need, it can be employed in a variety of ways, including GCL (geosynthetic clay liner), HDPE (high density poly ethylene), and LDPE (low density poly ethylene). An engineered landfill offers the chance to use geosynthetics for all of their purposes in various locations. Figure 6 illustrates a typical application of geosynthetics in constructed landfills in all of its forms and purposes.
Protection
When a geosynthetic is applied on a soil surface, it stabilizes the surface by preventing soil particles from moving and dispersing due to wind and rain erosion, frequently while permitting or encouraging the growth of flora. Due to the hydraulic pressure of the soil, the conventional design of cementing the banks is not an inexpensive solution. To prevent erosion of river banks and seashores, geotextile gabions, tubes, and bags are utilized.
Figure 7: Geosynthetics in slope protection
Conclusion
The usage of geosynthetic materials in civil engineering projects has a lot of promise. Although the use of geosynthetic materials in the building of landfills and embankments has gained widespread recognition, additional applications are still lacking. By providing the necessary knowledge at the undergraduate level and scheduling short-term courses for field engineers, certain confidence-building tactics might be implemented. The creation of a uniform code of conduct for use in India is essential, and testing facilities may be improved to make them easily accessible to all users within appropriate travel distances.
In today’s construction industry, high strength concrete is becoming more popular. Using high-strength concrete has a number of advantages, including reducing the volume of concrete sections required in structural elements, increasing building occupancy rates, and extending the building’s service life (Papadakis, 2000).
When high-strength concrete (HSC) is employed in building construction, there is a greater chance of having smaller concrete sections compared to when normal-strength concrete (NSC) is used. However, will the use of less volume of HSC translate to a reduction in CO2 emission for which the construction industry is famed for? Well, the answer depends.
Due to their inherent performance qualities, high strength concrete (50–100 MPa) and ultra high strength concrete (UHSC) (>100 MPa) are being utilized more frequently in the construction industry. As a result of their significant global usage, these concrete mixes have a higher carbon footprint. It is therefore important to take into account the embodied carbon of the HSC and UHSC.
Figure 1: High strength concrete is usually adopted in highrise building development
When compared to normal strength concrete, high-strength concrete has a lower water-to-binder ratio. As a result of this, a lot of cement or binder is needed and a lot of CO2 is emitted during the production of high strength concrete.
However, as stated above, the volume of concrete structures can be considerably reduced by adopting concrete systems with high mechanical properties. Because less material is required for the same structural performance, this “performance strategy,” which includes the use of high-performance concrete, reduces carbon dioxide emissions (Fantilli et al, 2019).
Summarily, high concrete strength has a higher carbon footprint, especially during the production stage (Habert and Roussel, 2009), because more binder (and sometimes fibres) is required. As a result, material reduction may not always be sufficient to offset the rise in CO2 emissions caused by higher concrete classes.
Studies on the Environmental Impacts of High Strength Concrete
The Global Warming Potential (GWP) of ultra-high performance concrete and normal-strength concrete was compared quantitatively for bridge structures at the construction stage by various researchers (Bertola et al., 2021; Dong, 2018; Rangelov et al., 2018; Sameer et al., 2019).
According to a comparison made by Aqib and Ma (2022), it was observed that the global warming potential of ultra-high-performance concrete was higher than that of normal-strength concrete by more than 60%. This can be attributed to the increased cement usage in ultra-high-performance concrete and the presence of steel fibres. As the cement content is much lower in conventional concrete, the GWP is also lower.
Reference
GWP of normal-strength concrete (kgCO2eq)
GWP of ultra high-strength concrete (kgCO2eq)
Percentage difference (%)
Dong, 2018
348
877
60%
Bertola et. al 2021
7217
10914
66%
Sameer et al., 2019
390
1700
77%
Rangelov et al., 2018
434
1930
78%
However, at the maintenance stage or during the service life of the structure, it was observed that ultra-high-performance concrete had less GWP compared to normal-strength concrete. Due to its superior durability and high strength, the maintenance needs of ultra-high-performance concrete are very low as compared to conventional concrete and hence, the life cycle GWP of ultra-high-performance concrete is very low as compared to normal strength concrete (Aqib and Ma, 2022).
In another study by Larsen et al (2017), a 40m long pedestrian bridge was designed with normal strength concrete (fck = 30 MPa) and ultra high performance concrete (fck = 150 MPa). The bearing structure is divided into two spans of 20 m, and consists of two simply supported T-beams as shown below.
Figure 2. Cross-sections for normal strength concrete and ultra high performance concrete bridges (left side) and the longitudinal structural model (right side). (Larsen et al, 2017)
Using life cycle analysis (LCA), the authors determined the environmental impact of each alternative. It is important to note that due to the fact that ultra high strength concrete contains about two to three times the cement content of normal strength concrete, it was important to reduce the volume of the section made with UHSC. After the design, the total amount of concrete used in the UHPC alternative was reduced by 36.7% compared to the NSC alternative.
Figure 3. Contribution analysis to overall global warming potential (GWP), expressed in tons CO2 equivalents (Larsen et al, 2017)
The contribution of the major life cycle phases to the total GWP in tons CO2 equivalent across the lifetime of each alternative is shown in Figure 3. The NSC alternative has lifetime CO2 equivalent emissions of 81.7 tons, compared to 68.6 tons for the UHPC option.
Material manufacturing is the major source of CO2 equivalent emissions for both alternatives at 58% of the overall emissions for the NSC alternative and 50% of the total emissions for the UHPC alternative.
The major findings of the study suggest that using UHPC in pedestrian bridges may have some environmental and design viability. Over a 200-year period, the UHPC alternative had overall lower consequences than the NSC alternative, which were much less. This suggests that the UHPC alternative is better in terms of the environment.
However, the results of this study need to be critically analyzed in light of methodological decisions made regarding the selection of functional units and life cycle phases, the uncertainty surrounding future emissions from material production, and the restrictions of the common T-beam bridge design in a comparative analysis.
In another study, Fantilli et al, (2019) evaluated three RC buildings with 14, 30, and 60 floors, respectively, whose structures are developed using finite element software for four different concrete classes (C25, C40, C60 and C80). The entire amount of concrete and reinforcement required to meet static and dynamic requirements was determined in this manner. When these quantities are multiplied by the materials’ unit carbon dioxide emissions, the global warming impact is calculated.
Figure 4: The RC structures of the existing building meshed with CDM Dolmen (Fantilli et al, 2019)
The parametric CO2 content per cubic metre of the concrete classes are provided in the Table below;
Material
Parametric CO2 amount(kg/m3)
C25
215
C40
272
C60
350
C80
394
Indicators that link the carbon footprint of concrete to both the cement volume per unit and the compressive strength can be found in the literature. To estimate the unit CO2 emissions as a function of the cylindrical compressive strength of concrete, Habert and Roussel (2009) presented the following empirical relationship:
kg of CO2 per cubic meter of concrete = δ√Strength of concrete
Where δ = 46.5 kgCO2
Figure. 5: Carbon footprint of steel and concrete versus concrete strength in the case of: a) structure #1; b) structure #2 and c) structure #3. (Fantilli et al, 2019)
From the study, it was observed that Building #1, which has the lowest number of storeys, exhibited a gradual increase in CO2 with concrete class (Fig. 5a). In contrast, the carbon footprint in the 60-story skyscraper (i.e., Structure #3 – Fig. 5c) reduces as concrete strength increases. At Structure #2, which has 30 stories (see Fig. 5b), emissions increase from classes C25 to C40 but fall for higher classes (especially from C60 to C80). As would be expected, each structure uses less structural material as concrete strength rises.
From the study, the lowest effect of CO2 emission may be ensured for low-rise buildings (14 storeys) by employing normal strength concrete (e.g., C25). In rare circumstances, even when low-strength concrete is present, the theoretical cross-sectional area can be less than the minimum. As a result, the use of high strength concrete in these elements does not rule out the possibility of using less concrete and steel rebar. In comparison to normal strength concrete, it causes an increase in CO2 emissions, particularly in low-rise structures.
The use of high-strength concrete, on the other hand, reduces the environmental impact of high-rise buildings (60 floors). As a result, in tall buildings, the use of the performance strategy to reduce the environmental impact of concrete structures becomes particularly successful, because the volume of structural elements can be significantly reduced.
References
Aqib S. M. and Ma Z. J. (2022): A Review on Carbon Emissions of Ultra-High-Performance Fiber Reinforced Concrete as a Building Construction Material. International High Performance Buildings Conference. Paper 426. https://docs.lib.purdue.edu/ihpbc/426
Bertola, N., Küpfer, C., Kälin, E., & Brühwiler, E. (2021). Assessment of the environmental impacts of bridge designs involving UHPFRC. Sustainability (Switzerland), 13(22), 1–19. https://doi.org/10.3390/su132212399
Dong, Y. (2018). Performance assessment and design of ultra-high performance concrete (UHPC) structures incorporating life-cycle cost and environmental impacts. Construction and Building Materials, 167, 414–425. https://doi.org/10.1016/j.conbuildmat.2018.02.037
Fantilli A. P., Mancinelli O. And Chiaia B. (2019): The carbon footprint of normal and high-strength concrete used in low-rise and high-rise buildings, Case Studies in Construction Materials, 2019(11)e00296, https://doi.org/10.1016/j.cscm.2019.e00296.
Habert G. and N. Roussel (2009): Study of two concrete mix-design strategies to reach carbon mitigation objectives, Cem. Concr. Compos. 31 (6) (2009) 397–402.
IngLarsen I. L., Aasbakken I. G., O’Born, Vertes K. and Thorstensen R. T. (2017): Determining the Environmental Benefits of Ultra High Performance Concrete as a Bridge Construction Material. IOP Conf. Series: Materials Science and Engineering 245 (2017) 052096 doi:10.1088/1757-899X/245/5/052096
Papadakis V. G. (2000): Effect of supplementary cementing materials on concrete resistance against carbonation and chloride ingress, Cement and Concrete Research, 30(2):291–299
Rangelov, M., Spragg, R. P., Haber, Z. B., & H, D. (2018). Life-cycle assessment of ultra-high performance concrete bridge deck overlays (1st Edition).
Sameer, H., Weber, V., Mostert, C., Bringezu, S., Fehling, E., & Wetzel, A. (2019). Environmental assessment of ultra-high-performance concrete using carbon, material, and water footprint. Materials, 16(6). https://doi.org/10.3390/ma12060851
