Is there a better way to show and view a project than by digital twin? For example, suppose you plan to stay true to the design of a project during construction. Is there a better way to achieve this than by digital twin? Perhaps what comes to your mind when you hear “digital twin” is BIM. But, then, you may be surprised to know that digital twin goes beyond BIM.
For instance, BIM focuses on the system and sub-systems that ensure a building is constructed correctly. However, digital twin can be a combination of a building and a computational model to monitor, manage, and optimize the building. Thus, digital twin concentrates on the operation, performance, and functionality of the building, including use patterns, occupant behaviours, and space utilization. Nevertheless, both tools are vital in this age of intelligent building design, operation, maintenance, and management.
This article covers everything about digital twin in the architecture, engineering, and construction (AEC) industry, including the purpose, types, examples, and benefits of digital twin in construction. By reading this article, you will discover why digital twin is an essential tool in the construction industry and built environment and why you should now adopt and invest in digital twin.
Purpose of Digital Twin
At present, digital twin is growing in importance in the AEC industry because it is used to integrate real-time data from an existing asset with its digital representation to provide insights across the asset’s lifecycle. Digital twin is a proven game-changer in the AEC industry because it solves significant asset design, construction, and operation challenges. Adopting digital twins is also the best way to improve asset performance.
When you think of digital twin, virtual creations or exact digital replicas of an asset should come to your mind. For example, digital twin is like the 3D interior renderings of a building, and it is a tool that helps to utilize, manage, and optimize the spaces in the building. Thus, with the digital twin, you can easily make simulations and predictions based on real-world data and conditions to make informed decisions and develop failure-proof working methods.
You may ask where BIM comes in or how digital twin uses BIM. The fact is that BIM remains the most efficient path to creating an accurate, high-value digital twin. Usually, BIM uses the data created during the planning and design of a project. Likewise, digital twin uses this same data and the data created during the construction and operation of the completed project. Thus, this data can also be used to make calculated and informed decisions for future projects.
Types of Digital Twin
Depending on the degree of maturity and digital transformation you desire, the types of digital twin are briefly discussed below.
Descriptive twin
3D architectural models and renderings and BIM comprise descriptive twin. Descriptive twin provides data and editable visual and photorealistic representations of a project for scheduling construction processes and actual construction.
Informative twin
When you need to make informed decisions about facility management operation and estimation, you need information twin. Information twin involves using BIM with sensors, simulations, and operations data to draw insights at any given time within an asset’s lifecycle.
Comprehensive twin involves leveraging BIM and AI for data-based advanced modelling and simulation to carry out prescriptive analysis and make recommendations to aid decision-making in future situations.
Autonomous twin
An autonomous twin is the ideal digital twin. Unlike comprehensive twin, where professionals still make final decisions, autonomous twin goes further to make decisions through AI-driven advanced simulation and 3D visualization algorithms.
Examples of digital twin
A digital twin is several things to the AEC industry, mainly because it is linked to assets, the spaces in assets, and the people who use these assets and spaces. For instance, digital twin is a tool for safety planning, asset management, space optimization, and improving occupant or employee experience. Examples of digital twin in the AEC industry are highlighted below.
• Developing new building construction models
There are several designs that architects conceive, but that never come to reality. This happens because architects sometimes need clarification on the practicality of such designs and if these designs meet all stability and safety requirements. To make matters worse, unfortunately, it is almost impossible to test such designs in the real world to see if they will work or not because of the capital, time, and other resources involved. However, a digital twin can solve this challenge.
With a digital twin, architects and other professionals in the AEC industry can test a design idea in reality-based simulations that would use real-world data and involve all the necessary real-world factors. Similarly, they can test such design’s stability, safety, sustainability, and practicality and get accurate feedback as if the design was executed in the real world.
• Reduction of construction and operating costs during building renovations
One significant advantage of digital twin is that it makes coordinating teams and construction processes easier. In addition, one can run several sequencing and logistics scenarios in advance. Thus, giving you an idea of what you are to do and how you are to do them during the construction of a project. Also, it is possible to feed back real-world data to the digital twin in cases where situations and events change.
• “What if” analysis of safety systems and emergency action plans
With “what if” analysis, you can simulate a scenario to know how a particular asset is performing and will perform under specific conditions. For example, a digital twin can be used to understand how a rail line was designed and constructed, how it functions, and its limitations. Similarly, a digital twin can provide workers with real-time tracking and alerts of work areas. Thus, reducing risks to workers, making construction attractive to people, and increasing overall efficiency and work quality on construction sites.
• Productivity and collaboration improvements through space delegation
Important information about an existing asset can be stored, analyzed, and updated throughout the asset’s lifetime. This information becomes helpful in making informed and calculated decisions when embarking on future projects. This information can also assist with de-risking the execution of the project.
• Asset performance and sustainability optimization through use monitoring
Professionals can improve the performance and sustainability of an asset from the insights obtained from a digital twin. For example, a digital twin can be used to predict maintenance events that may arise and plan the required maintenance activities. Consequently, unexpected circumstances and maintenance costs will be reduced throughout the asset’s lifetime. Furthermore, a digital twin can detect a fault or abnormality in a building and automatically dispatch a worker to rectify such issues without delay.
Benefits of digital twin
BIM is known to be very analytical. However, digital twin improves the digital capabilities of BIM and makes it easier for asset owners, operators and managers to visualize the real-time status, working conditions, and position of physical assets. Some benefits of digital twin for firms, owners and managers in the AEC industry are discussed below.
• Competitive edge If you own a firm, digital twin can differentiate you from your competitors and put you ahead of them. In addition, with digital twin, firms can prove to their clients that they can provide them value and a broader range of services by leveraging technology to harness and utilize real-time data.
• Savings on money Digital twin allows firms, asset owners, operators and managers to make informed and calculated decisions, thus helping to save cost and extending the value and lifecycle of assets.
• Savings on time Usually, there are conversations between firms and stakeholders before a project kicks off. These conversations and collaborations help to prioritize the use and performance requirements of such projects or assets. Therefore, it becomes easier to plan to achieve the project goals and eliminate time wastage. Similarly, asset owners and operators can seamlessly proceed with operating the asset without having to bother about misplaced operation or maintenance documents.
• Streamlined facility management Digital twin usually offloads most of the burdens of asset management. For instance, a digital twin of an asset can be used to spot the problem areas and provide potential solutions to the problems that technicians can work with.
Conclusion
A digital twin is simply an exact, digital, and dynamic replica of an asset after its design and construction, for example, a building. Thus, the digital twin of the building can change with its use, just like the physical building, because it is responsive to data input. Its use will also change as more data is supplied from sensors, IoT, and AI.
Just like BIM technology has become indispensable to the AEC industry, so is digital twin technology, especially for facility management. However, models and data from BIM are of little or no use to asset owners, operators, and managers once a project is completed. Therefore, a digital twin is vital for asset owners, operators and managers, and professionals in the AEC industry because it provides insights to optimize assets, systems, and workplaces.
References
[1] Deng, M., Menassa, C. C., and Kamat, V. R. (2021). “From BIM to Digital Twins: a Systematic Review of the Evolution of Intelligent Building Representations in the AEC-FM Industry”. ITcon 26, 58–83. Doi:10.36680/j.itcon.2021.005
[2] Autodesk, Inc. (n.d.). Retrieved from https://www.autodesk.com/solutions/digital-twin/architecture-engineering-construction
A biaxial eccentrically loaded pad footing occurs when the column transmitting load to the foundation is subjected to compressive axial force and bending moment in the two principal axes. As a result of the biaxial bending, two eccentricities ex and ey of the axial load occur on the pad footing, thereby leading to non-uniform pressure distribution on the foundation. The effects of such non-uniform pressure distribution must be accounted for during the geotechnical and structural design of the pad foundation.
This article will consider the geotechnical and structural design of a biaxial eccentrically loaded pad footing. The footing for a single column may be made square in plan, but where there is a large moment acting about only one axis it may be more economical to have a rectangular base.
Different pressure distribution of pad footings
When a bending moment M and axial force N are acting on a pad foundation, the pressures are given by the equation for axial load plus bending. This condition is valid provided there is positive contact between the pad base and the ground along the complete length D of the footing so that;
p = N/BD ± My/I
where I is the second-moment area of the base about the axis of bending and y is the distance from the axis to where the pressure is being calculated.
Substituting for I = BD3/12 and y = D/2, the maximum pressure is;
pmax = N/BD + 6M/BD2
and the minimum pressure is;
pmin = N/BD – 6M/BD2
For biaxially loaded footings, this pressure must be verified in both directions, and the maximum pressure should not exceed the allowable bearing capacity of the soil. Furthermore, the reinforcement design must also be carried out in both directions.
Design Example of Biaxial Eccentrically Loaded Pad Footing
A 1500 x 1500mm pad foundation is subjected to the following loads from a 250 mm x 250 mm square column;
Permanent load axial load; FGz1 = 650.0 kN Variable laxial load; FQz1 = 135.0 kN Permanent moment in x; MGx1 = 25.0 kNm Permanent moment in y; MGy1 = 21.0 kNm Variable moment in x; MQx1 = 13.0 kNm Variable moment in y; MQy1 = 11.0 kNm
The design is to be done in accordance with EN1997-1:2004 incorporating Corrigendum dated February 2009 and the UK National Annex incorporating Corrigendum No.1
Pad foundation details
Length of foundation; Lx = 1500 mm Width of foundation; Ly = 1500 mm Foundation area; A = Lx × Ly = 2.250 m2 Depth of foundation (thickness of footing); h = 500 mm Depth of soil over foundation; hsoil = 600 mm Level of water; hwater = 0 mm Density of water; γwater = 9.8 kN/m3 Density of concrete; γconc = 25.0 kN/m3
Column details Length of column; lx1 = 250 mm Width of column; ly1 = 250 mm position in x-axis; x1 = 750 mm position in y-axis; y1 = 750 mm
Column loads Permanent load in z; FGz1 = 650.0 kN Variable load in z; FQz1 = 135.0 kN Permanent moment in x; MGx1 = 25.0 kNm Permanent moment in y; MGy1 = 21.0 kNm Variable moment in x; MQx1 = 13.0 kNm Variable moment in y; MQy1 = 11.0 kNm
Recently, there have been discussions on the choice of piping methods for plumbing systems in building structures. These discussions have arisen due to the implications of some piping methods on the structural system, durability, performance, and maintenance of building structures.
To worsen the case, the use of substandard construction materials such as low-quality pipes which deteriorates quickly has not helped. As a result, a lot of leakages occur in the plumbing system of such defective houses, soaking the walls, and making the building uninhabitable. Thus, this article discusses piping methods and recommends the best approach to help reduce building maintenance costs and prevent deterioration and eventual building collapse.
Piping system deterioration in a building
Piping Methods
Piping is primarily used in building structures for plumbing purposes. Building plumbing systems consist of piping networks that distribute drinking water and safely dispose of sewage into sewerage systems. Three piping methods are commonly used in building structures. They are;
surface piping
conduit piping, and
duct piping.
These piping methods are discussed in detail below.
Surface Piping
When you see a building with PVC pipes running on the exterior of a building, then know that surface piping is employed for plumbing. This type of piping is the foremost piping method for plumbing works, and the major advantages of surface piping to other piping systems are ease of installation and maintenance.
Surface piping in a building
However, surface piping usually needs to be more aesthetically pleasing and appealing. Thus, running pipes on the surface of buildings are often undesirable because they significantly affect building appearance. However, in instances where pipes or joints leak, they are so easy to detect, repair, or replace without affecting the building or breaking the walls.
Surface pipes can also run through openings created in walls. These openings are of small sizes, generally ranging between ½ to 6 inches. Thus, surface piping involves minor demolition and has negligible impact on the structural system, structural integrity, and performance of a building, especially when it is a frame structure. However, surface pipes deteriorate quickly due to exposure to adverse weather conditions, leading to increased maintenance costs.
Another critical thing to note is that surface pipes can move within openings when fluids flow through them. This is because the fluids’ forces can cause the movement of pipes. Similarly, thermal expansion and contraction are possible, especially in PVC pipes. Thus, openings for surface pipes can be opened up even when they are caulked.
Complex surface piping on a building
Conduit Piping
In construction, almost anything can be concealed in the structural components or members of buildings. So, pipes are included. Conduit piping involves concealing pipes in a building’s walls or other structural members. Unfortunately, conduit piping is mostly not considered during the structural design of building structures. Similarly, there are lots of conduit piping in buildings that occur as a result of afterthoughts.
Pipes concealed in a slab and beam
In most cases, conduit piping involves running pipes across the length of structural members or through them. Consequently, this reduces the load-carrying capacity of such members and the formation of honeycombs, especially around the pipes. Therefore, conduit piping has a higher probability of affecting the structural system, structural integrity, and building performance. However, with BIM and technical coordination, building services can be integrated during the design stage and most potential issues can be identified and resolved.
Building appearance adversely affected by leakage of concealed pipes
Unlike surface piping, conduit piping can lead to significant demolitions, especially in cases of afterthoughts. Furthermore, drilling, cutting or breaking of walls or structural members induces vibrations that can affect such members’ structural stability and integrity. Lastly, it may be challenging to conduct maintenance works on concealed pipes. Thus, leakage is a significant concern because it can result in the corrosion of reinforcements and affect the durability of building components.
Unsightly conduit plumbing construction
Duct Piping
Duct piping is the new normal for modern building constructions. Duct piping is the best practice, and it solves all the challenges with surface and conduit piping methods. For duct piping, an architect working on a building design provides ducts, usually small spaces that run vertically through all the floors, to receive and house MEP services, including plumbing pipes. Thus, the probability of installing pipes due to afterthoughts is significantly reduced.
Duct to house MEP services
The advantages of duct piping cannot be overstated. It is a cost-effective method with less concern for leakage and maintenance. In addition, duct piping encourages durability, as plumbing pipes are shielded from adverse weather conditions, and it also improves the physical appearance of a building.
Best practices for duct piping
It is one thing to provide ducts in building structures, while it is another to do it right. The best practices for duct piping are listed below.
Group like services together. For instance, all water services can be together in a duct, while electrical and air-conditioning services can be put together in a duct.
A duct must be big enough to allow a person to move freely and work inside it, or good enough to allow someone to work from the exterior.
There must be a fixed metal ladder in each duct or provision for opening a duct at every floor level.
Ensure that ducts are rat-proof.
Ensure duct openings are from outside to prevent attending to service lines inside the building.
Keep all ducts locked for safety.
Conclusion
It is time we recognized that the conduit and surface piping methods had done more harm than good to buildings. Arguably, they have also increased the cost of maintenance of buildings. Conduit and surface piping have worked effectively in the past, but it seems that they no longer work in this age because a lot has changed in terms of the quality and durability of building materials.
The duct method of piping is the way forward. It is for our good and that of our clients if we eliminate the methods that affect building structures’ durability, performance, and structural integrity. With duct piping, we can be sure of reducing building maintenance costs and saving buildings from deterioration and eventual collapse.
Groundwater lowering operations may have an impact on water levels in a large area, even some distance away from the actual construction site. Although this usually doesn’t result in problems, there are some situations where unfavourable side effects such as subsidence could occur.
The following effects are covered in this article:
Settlement resulting from the instability of excavations when groundwater is not adequately controlled.
Ground settlements caused by loss of fines.
Ground settlements induced by increases in effective stress, and associated structural damage or distress.
Artificial groundwater recharge systems can be used to mitigate some of the negative consequences of groundwater lowering.
Settlement due to groundwater lowering
Every groundwater reduction operation will inevitably result in ground settlements. Most of the time, the settlements are so small that surrounding buildings show no distortion or damage. On occasion, however, settlements may be significant enough to cause structures to deform or stress in a detrimental way. This can range from modest architectural finish cracking to serious structural damage. In extreme circumstances, these effects have impacted numerous structures and have spread out hundreds of metres from the construction site itself.
A pre-construction building condition survey must be performed whenever there is any possibility that groundwater lowering (or any other construction operation) may cause ground settlements beneath existing structures. The objective of this activity often referred to as a dilapidation study, is to document the existing condition of any structures that may be impacted by settlement.
Groundwater decreasing may result in settlements for a variety of reasons, some of which are simple to avoid and some of which are more difficult to do so:
1 Settlement resulting from the instability of excavations when groundwater is not adequately controlled. 2 Settlement caused by loss of fines. 3 Settlement induced by increases in effective stress.
Settlement due to poorly controlled groundwater
Uncontrolled seepages, groundwater “blow,” and unstable excavations could result from inadequate groundwater lowering control. These issues could be the result of a number of factors, including a failure to recognise the importance of groundwater control, an improper attempt to cut costs by reducing or eliminating groundwater control from temporary works, a lack of standby or backup facilities to prevent pumping interruptions, and ground or groundwater conditions that were not taken into account during the site investigation or design or detected by construction monitoring.
Soil material will be washed into the excavation if there is a sudden “blow” or failure of the excavation. This has the potential to produce expansive settlements near the excavation that are unpredictable and far greater than the stress settlements linked to successful groundwater control. Any structures in the vicinity of the uncontrolled settlements are probably going to sustain significant damage.
Settlement due to loss of fines
A phenomenon known as “loss of fines” can cause settlement if a groundwater lowering system continuously pumps “fines” (particles the size of silt and sand) in the discharge water. In the early phases of pumping, most dewatering systems will pump fines as a more permeable zone forms around the well or sump.
However, if fines are pumped for an extended length of time, the removal of particles would loosen the soil and could result in the formation of subsurface erosion channels (sometimes referred to as “pipes”). Ground movements and settlement may be caused by compaction of the loosening soil or by the collapse of such erosion channels.
Continuous pumping of fines is not normally a problem with wellpoints, deep wells or ejectors, provided that adequate filter packs have been installed and monitored for fines in their discharge. Occasionally, a sand pumping well may be encountered, perhaps caused by a cracked screen or poor installation techniques. Such wells should be taken out of service immediately.
Wellpoint groundwater lowering system
Sump pumping is the technique that results in fines being lost most frequently. This is due to the frequent neglect of installing sufficient filters surrounding sump pumps, which causes fine soil particles to become mobile as groundwater is pulled towards the pump.
Powers (1985) outlines the different types of soil where sump pumping should be avoided. These consist of:
Uniform fine sands
Soft non-cohesive silts and soft clays
Soft rocks where fissures can erode and enlarge due to high water velocities
Rocks where fissures are filled with silt, sand or soft clay, which may be eroded
Sandstone with uncemented layers that may be washed out.
Even the best-engineered sump pumping systems may experience issues with various types of soil. A method of groundwater lowering employing wells (wellpoints, deep wells, or ejectors) with properly designed and installed filters should be seriously considered.
Settlement due to increases in effective stress
As groundwater levels drop, pore water pressures naturally drop as well, increasing effective stress. The soil layer will compress as a result, resulting in ground settlements. The vast majority of the time, however, the effective stress settlements are so negligible that no harm is done to neighbouring structures.
The following variables will affect effective stress settlement:
The presence and thickness of a highly compressible layer of soil below the groundwater level, which will be affected by the pore water pressure reduction. Examples include soft alluvial silts and clays or peat deposits. The softer a soil layer (and the thicker it is), the greater the potential settlement.
The amount of drawdown. The greater the drawdown of the groundwater level, the greater the resulting settlement.
The period of pumping. In general, at a given site, the longer the pumping is continued, the greater the settlement.
Settlements caused by groundwater lowering will generally increase with time and will be greatest at the end of the period of pumping.
Light gauge steel framing is typically based on the use of standard C or Z-shaped steel sections produced by cold rolling from strip steel. In general, hot-rolled steel sections used in fabricated steelwork, such as Universal Beams, are different from cold-formed steel sections. Galvanized steel with a typical thickness of 0.9 to 3.2 mm is used in cold-formed sections to prevent corrosion.
Cold-formed steel sections are widely utilised in many construction industries, including mezzanine floors, commercial, industrial, and hotel buildings. They are also becoming more popular in the residential market. In North America, Australia, and Japan, light steel frame is already well-established in the residential home construction industry. This article provides information on the various types of light gauge steel frame construction methods for residential buildings.
Figure 1: Typical light gauge steel framing
Methods of Constructionusing Light Gauge Steel
Cold-formed sections, which can be prefabricated into panels or modules or constructed on-site using a variety of connecting techniques, are the fundamental building blocks of the light steel gauge frame. The various types of construction are discussed in the sections that follow.
‘Stick-build’ construction
In this construction approach (shown in Figure 2), discrete members are put together on-site to create columns, walls, rafters, beams, and bracing, which are then covered with cladding, internal lining, and other components. Although the elements are typically shipped pre-punched and cut to length, connections are done on-site using bolts, screws, or other suitable site procedures.
The main advantages of ‘stick-build’ construction are:
construction tolerances and modifications can be accommodated on site
connection techniques are relatively simple
manufacturers do not require the workshop facilities associated with the panel or modular construction
large quantities of light steel members can be densely packed and transported in single loads
components can be easily handled on-site.
In comparison to the other methods, “stick-build” construction is typically labour-intensive on-site, but it can be effective in a complex building when prefabrication is difficult. In North America and Australia, where there is a strong infrastructure of trained contractors, this type of building is common. This is a result of the widespread use of power tools in the craft of timber frame construction. Traditional timber contractors have easily transitioned to light gauge steel frames in these nations.
Panel Construction
As shown in Figure 3, wall panels, floor cassettes, and roof trusses can be manufactured in a factory and then erected on-site. Panels are fabricated in specialised jigs for precision. To speed up building on-site, some finishing materials may be used in the factory. Steel pieces alone or with facing materials and insulation placed at the factory can make up panels. The panels are connected on-site utilising customary methods (bolts or self-drilling screws).
Figure 3: Typical panel construction of light gauge steel framing
The main advantages of panel or sub-frame construction are:
speed of erection of the panels or sub-frames
quality control in production
reduced site labour costs
scope for automation in factory production.
The fact that the panels are prefabricated in a manufacturing environment improves their geometrical precision and dependability compared to stick-build construction. To achieve quick panel assembly and to reach the highest level of building efficiency, precise foundation planning and installation are essential.
Modular Construction
In modular construction, units can be brought to the site with all interior finishes, fixtures, and fittings already installed because they are totally constructed in the factory, as shown in Figure 4. To create a stable final construction, units may be piled one above the other or side by side.
Figure 4: Typical modular construction
Where massive production runs for the same basic configuration of modular unit are feasible, modular building is most cost-effective. This is possible because the costs of prototype and setup, which are largely scale-independent, may be distributed over numerous units.
Platform and ‘balloon’ construction
“Stick-build” or panel components can be put together in either “platform” or “balloon” construction. The walls are not physically continuous in platform construction since the floors and walls are created one level at a time. In some types of construction, loads are carried from the walls above to the walls below through the floor joists.
The wall panels used in “balloon” construction are frequently significantly larger and extend over multiple stories. These panels require temporary bracing while the floors are being placed since they are more challenging to assemble than single-storey height panels. The fundamental benefit of this strategy is that loads are carried directly from the walls above to those below. The external cladding or finishes are often installed and affixed to the frames on-site in both types of construction.
In the design of steel structures, it is necessary to determine the section’s dimensional properties under consideration before a member’s resistance to bending, compression or other types of loading can be calculated. According to Eurocode 3, the section properties usually considered are the effective and gross section properties.
According to clause 6.2.2.1 of EN 1993-1-1, the properties of the gross cross-section should be determined using the nominal dimensions. Holes for fasteners need not be deducted, but allowance should be made for larger openings. Splice materials should not be included. When the effects of holes and openings are considered in the analysis, it is referred to as net section properties.
Effective section properties are the characteristics of a fictitious cross-section whose area has been reduced to account for the effects of local buckling. It can also be essential to make additional reductions to account for distortional buckling. The effective properties of the section are always used to compute the bending and compression resistances of light steel members (cold-formed steel sections such as that found in roof purlins).
Light gauge steel sections (cold-formed sections)
Effective Section Properties
According to clause 6.2.2.4 of EN 1993-1-1, when the cross-sections with a class 3 web and class 1 or 2 flanges are classified as effective Class 2 cross-sections, see clause 5.5.2(1) of EN 1993-1-1, the proportion of the web in compression should be replaced by a part of 20εtw adjacent to the compression flange, with another part of 20εtw adjacent to the plastic neutral axis of the effective cross-section in accordance with Figure 6.3 of EN 1993-1-1 (shown below).
Effective class 2 web
However, the effective cross-section properties of Class 4 cross-sections should be based on the effective widths of the compression parts. For cold-formed sections, it should be based on the requirements of EN 1993-1-3.
For light gauge steel sections, there is an implied presumption that the cross-section belongs to class 4 rather than classifying it (although this term is not used in BS EN 1993-1-3). Using the same methods as for class 4 sections to BS EN 1993-1-1, the design process concentrates on calculating the effective section properties after making this assumption. Effective section characteristics are used to reduce the amount of calculation necessary without too conserving on cross-section resistance by simplifying the intricate stress distributions related to local buckling.
The effective width approach substitutes simplified equivalent stresses acting over two equal widths of beff/2 for the actual stress distribution acting across element width b. It is assumed that there is no stress in the centre of the plate, which is the area most susceptible to local buckling, and it is completely disregarded. The end result is a straightforward model where uniform stress assumed to act over a narrower plate is equal to the steel’s yield strength.
The effective width concept
The approach used by BS EN 1993-1-3 applies the above-illustrated effective width idea to the cross-section of a light gauge steel element. Each of the features of the cross-section—flanges, webs, lips, etc.—is treated as the flat plate in the Figure above. Each element that is under compressive stress has its effective width beff determined (either due to applied axial compression or bending).
The effective area of the element Aeff is then obtained by multiplying beffby the section thickness t. Elements not subjected to compressive stress are not susceptible to local buckling, so the full element width b may be used in the calculation of the effective section properties.
Gross Section Properties
The term gross section properties, as the name implies, refers to the entire cross-section without any reduction for local buckling. For the majority of common section shapes, calculating the gross section properties only requires adding up the elemental areas and first and second moments of area (for flanges, web, stiffeners, etc.), determining the major and minor centroidal axes’ locations, and deriving the second moment of area for the entire section from these values. If necessary, a similar procedure can be repeated for additional properties.
Fencing of landed properties is very common in Nigeria. For residential and commercial buildings in rural, semi-urban, and urban areas in Nigeria, it is very common to construct fences using sandcrete block walls around the perimeter of the property. Fencing is done for a lot of reasons such as security, restriction of unguided access, privacy, and protection of the property from encroachment and land grabbers.
Other materials can be used in the construction of fences such as concrete walls, sandcrete block walls, timber/wooden panels, gabion walls, steel plates, wire mesh, bricks etc. However, sandcrete blocks are the most common in Nigeria. This is mainly due to the availability of materials and labour, convenience, flexibility, different options for finishes, stability, low maintenance, and durability of sandcrete block fences.
Fence Construction in Nigeria
The materials that are commonly used for the construction of masonry wall fences are;
Sandcrete blocks
Cement
Sand
Gravel, and
Water
Other ancillary components that are usually found in fences are copings, railings, barbed fence wire, electric fence wires, etc. For beauty, other finishes such as cornices and mouldings of different types may be added. It is also important to note that fences are ultimately provided with access gates which can be constructed of wrought iron, timber, aluminium, or stainless steel.
Typical wire fence
For durability and aesthetics, it is very important that fences constructed with sandcrete masonry walls be plastered on both faces and the top of the wall protected with concrete coping. The coping may be precast or cast in-situ.
For fences that are to be constructed on sites with marginal soils or marshy soils with low bearing capacity, it is very important that reinforced concrete columns (pillars) be provided at intervals of about 3 metres. Normally also, it is not advisable to have a very long stretch of fence walls without introducing separation (open joints) or properly bound block pillars. This adds more stability and beauty to the wall, and prevents progressive failure whenever there is a lateral impact on the wall.
It is also important that the fence be chained (in a manner similar to reinforced concrete lintels) at the foundation level and/or at the mid-level. By so doing, the movements, swelling pressure, and shrinkage-induced stresses from the surrounding soil will not cause cracks or failure of the fence.
The height of the fence wall around a property can be determined by a lot of reasons such as building regulations, cost, client taste/intentions, aesthetic requirements, and security requirements. Typically, the average height of fences constructed using sandcrete blocks is around 2.25 metres (approximately 10 blocks) above the ground level. The height may however vary depending on some of the factors listed above which are explained below;
Building Regulations: In many locations or estates in Nigeria, there are laws or guidelines on how fences should be constructed. Some of these guidelines may include set-back from the road, height, design, colour, etc. The maximum height of a fence may occasionally be specified by a local government ordinance, building control or town planning agencies, etc., in which case all residents building fences in that region must abide.
Clients’ taste/intentions: Where regulations on fencing do not abide, the height and type of fence may also be influenced by individual preferences. Some people like shorter fences so they can display their homes better, while others may prefer a completely concealed compound. Sometimes also, the height of a fence may be influenced by cost, especially when the client is indifferent to security and/or privacy.
Aesthetic requirements: As was previously mentioned, the height of a fence may be influenced by the desire to display the home’s attractiveness. In this situation, people almost always choose short or average fences.
Security requirements: Many homeowners use higher fences to protect their homes from intruders, enhance privacy, and take people’s attention off their property. This is very typical in areas with volatile security issues.
TheProcess of Fencing in Nigeria
When fences are to be constructed on sites with good soil, strip foundations are typically adopted. For sloping terrain, the foundation can be stepped at intervals to prevent failure of the foundation due to scour or loss of bearing capacity due to erosion. The depth of foundation for fences is typically around 2 feet (600 mm), while the thickness of the concrete strip can be between 100 mm to 150 mm of grade 15 concrete, or according to the structural engineer (for challenging soils).
Stepped fence on a sloping terrain
Fences can be constructed using different types of sandcrete blocks such as 9 inches hollow blocks, six inches blocks (hollow or solid), five inches blocks (hollow or solid), etc. The process of constructing fences in Nigeria is therefore as follows;
(1) Excavation of the strip footing to the required level (2) Levelling and compaction of the footing base to receive concrete (3) Establishment of the block pillar or reinforced column locations and installation of appropriate bases and rebars (4) Pouring of the concrete strip footing (5) Laying of blocks and forming of the block pillars (where applicable) (6) Casting of the columns (where applicable) (7) Installation or casting of the copings (8) Installation of the gates and railings (9) Plastering and finishes (10) Installation of barbed wire or electric security wires
Cost of Fencing One Plot of Land in Nigeria
In many states in Nigeria, the size of one plot of land is 450 m2 (5000 ft2), typically comprising a parcel of land measuring 30 m x 15 m (100 feet x 50 feet). This dimension will be used in estimating the cost of fencing a plot of land in Nigeria. Furthermore, the size of the opening for gates is usually about (4 to 5 metres). For this calculation, 5 metres will be adopted.
It will be assumed that reinforced concrete columns will be provided at intervals of 3m across the fence line.
Therefore, the perimeter to be fenced = 2(30) + 2(15) – 5 = 85 m Height of fence = 2.25m (above natural ground level) + 0.5m (below ground level) = 2.75m
Cost of foundation excavation
Total number of partitions to be excavated = 24 Cost of excavation per partition = ₦1200 The total cost of excavation = ₦28,800
Cost of Concrete works (foundation and pillars)
The volume of concrete required for the foundation = 6m3 The volume of concrete required for columns = 2.8 m3 Cement required = 40 bags @ ₦4,200 = ₦168,000 Granite required = 15 tonnes = ₦135,000 (depending on location, transportation alone may double this price) Sand required = 10 tonnes = ₦40,000 Labour = ₦72,000 Total cost of concrete works = ₦415,000
Cost of Column formworks
1″x12″ planks = 28 pcs (reuse twice) @ ₦1,500 = ₦42,000 3″ inches nails (allow) = ₦4,000 Labour (allow) = ₦20,000 Total = ₦66,000
Cost of Column Rebars
Y10mm bars = 28 pcs @ ₦3,300 = ₦92,400 R6 mm stirrups = 35 pcs @ ₦1,200 = ₦42,000 Binding wire allow = 1 roll = ₦17,000 Labour allow = ₦25,000 Total = ₦176,400
On-going fence construction work
Cost of block wall
The total area of the block wall = 233.75 m2 Number of blocks required = 2340 (no allowance for wastes has been made) Unit Price of 6 inches blocks = ₦220 Total cost of blocks = ₦514,800
Cement required = 45 bags The unit price of cement = ₦4200 The total cost of cement = ₦189,000
Total cost for the fence walls = ₦514,800 + ₦189,000 +₦70,000 + ₦187,200 = ₦961,000
(Depending on the location, allowance for the cost of water should also be made) Furthermore, allowance for contingencies such as scaffolding should also be made.
The total cost of fencing one plot of land in Nigeria = ₦28,800 + ₦415,000 + ₦66,000 + ₦176,400 + ₦961,000 = ₦1,647,200
Therefore, the cost of fencing one plot of land in Nigeria is about ₦1,647,200, disregarding the contractor’s profit and overhead.
Luis Menard developed dynamic compaction (DC), also known as dynamic deep compaction, in the middle of the 1960s. However, there are claims that the technique was used more than a thousand years before. In this ground improvement technique, a heavy weight is dropped on the ground’s surface to compact soils to depths of up to 40 feet (12.5 metres).
The technique is utilised to reduce settlements in foundations, reduce seismic subsidence and liquefaction potential, permit development on fills, densify refuse dumps, enhance mining spoils, and reduce settlements in collapsible soils.
Applicable Soil Types for Dynamic Compaction
The most suitable types of soil for dynamic compaction are granular, permeable soils. In cohesive soils, the compaction energy tends to be absorbed by fine soil particles, which reduces the technique’s effectiveness. Table 1 shows the anticipated improvement for various soil types.
Soil Description
Expected Improvement
Typical Energy Required (tons ft/cf)
Gravel and sand <10% silt, no clay
Excellent
2.0 – 2.5
Sand with 10—80% silt and <20% clay, PI < 8
Moderate if dry; minimal if moist
2.5-3.5
Finer-grained soil with pI > 8
Not applicable
–
Landfill
Excellent
6-11
Table 1: Expected Improvement and Required Energy with Dynamic Compaction (Hussin, 2006)
Note that for Table 1 above, Energy = (drop height x weight x number of drops)/soil volume to be compacted; 1 ton ft/ft3 = 94.1 kJ/m3.
For the procedure to be successful, the groundwater table must be at least 6 feet (1.8 metres) below the working surface. Sand or stone columns have been built using dynamic compaction in organic soils by repeatedly filling the crater with the material and forcing the column through the organic layer.
Equipment
Although specifically designed rigs have been made, the weight is typically dropped using a cycle-duty crane. Standard cranes are often not designed to handle high-cycle dynamic loading, so in order to maintain a safe working environment, the cranes must be in good condition and constantly maintained and inspected while performing the operation.
Typical weight for dynamic compaction
The crane is often set up with enough boom to drop the weight from heights of 50 to 100 ft (15.4 to 30.8 m) and only one rope to let it almost “free fall,” maximising the force of the weight hitting the soil. The weight to be lowered must be less than the crane’s and cable’s safe single-line capability. Typically weights range from 10 to 30 tons (90 to 270 kN) and are constructed of steel to withstand the repetitive dynamic forces.
Procedure
Lifting and dropping a weight repeatedly till it hits the ground surface is the major technique for dynamic compaction. The primary drop locations are normally laid out in a grid of 10 to 20 feet (3.1 to 6.2 metres), with a secondary pass placed at each of the primary pass’s midpoints. Prior to doing subsequent drops at that place, the crater is filled with granular material once it has reached a depth of 3 to 4 ft (about 1 m).
Large soil vibrations are generated by the operation, which may negatively affect surrounding existing structures. In particular, structures within 500 feet (154 metres) of the locations of the intended drops should be examined for their vibration sensitivity and their antecedent conditions. Monitoring the vibrations when carrying out dynamic compaction is also advisable. If dynamic compaction is intended to be carried out within 200 feet (61.5 metres) of an existing structure, extreme caution and close observation should be made.
Materials
A clean, freely draining granular earth is often used to fill the craters left behind by the operation. Sandy soils can be treated by using a backfill made of sand. Finer-grained soils or landfills are frequently treated with a crushed stone backfill.
Design
An examination of the proposed construction and the current subsurface conditions will serve as the starting point for the design (bearing capacity, settlement, liquefaction, etc.). The minimal values required to deliver the required performance are then determined using the same study with the improved soil characteristics (e.g., SPT N value, etc.). The next step is to calculate the vertical and lateral extent of improved soil required to deliver the desired performance.
The square root of the energy from a single drop (weight times drop height) applied to the ground surface determines the depth of the effect. Dr. Robert Lucas created the correlation shown below using data collected in the field:
D = k(W x H)0.5
where; D is the maximum influence depth in meters beneath the ground surface, W is the weight in metric tons (9 kN) of the object being dropped, and H is the drop height in meters above the ground surface. The constant k varies with soil type and is between 0.3 and 0.7, with lower values for finer-grained soils.
Even though the largest depth of improvement predicted by this method is in the upper two-thirds, the improvement gradually decreases until it reaches zero in the bottom third. The degree of improvement attained within this zone is increased by landing strikes repeatedly at the same spot. However, when the rate of improvement falls, there comes a moment when the benefits become less valuable.
Table 1 lists the anticipated range of unit energy needed to reach this goal. Treatment of landfills is helpful in minimising voids, but it has minimal impact on the biodegradation of components in the future. As a result, treatment of landfills is normally only allowed in areas designated for planned roadways and pavement, not for structures. The soils are loose within 3 to 4 feet (1 m) of the surface once dynamic compaction is finished. A low-energy “ironing pass,” which typically entails dropping the same weight from a height of 10 to 15 feet (3.0 to 4.5 metres) over the entire surface area, is used to compact the surface soils.
Quality control and quality assurance
Penetration testing is used in the majority of applications to evaluate improvement. Penetration testing is challenging in construction waste or landfills, however large-scale load experiments using fill mounds or shear wave velocity tests can be carried out. To measure the progress made and make necessary adjustments, a test area might be treated at the start of the programme. To identify “soft” sections of the site that need more treatment, the depth of the craters can also be measured. When adequate improvement is attained, the decrease in penetration with further drops serves as a signal.
References
Hussin J. D. (2006): ‘Methods of soft ground improvement’ in ‘The Foundation Engineering Handbook’ Edited by Gunaratne M. Taylor and Francis USA.
Soil stabilisation is the alteration of one or more soil properties, by mechanical or chemical means, to create an improved soil material possessing the desired engineering properties. Stabilising soils during road construction can make the pavement more durable and resilient. Furthermore, it can stop erosion and dust production on the surface of the road.
The major objective of soil stabilisation is to develop a soil material or soil system that will endure over the design lifespan of the project under the design use conditions. In the same way that soils vary around the world, so do their engineering properties. Soil testing is therefore essential for the success of soil stabilisation. Prior to construction, ideally before selecting or buying materials, the chosen method of soil stabilisation should be tested in a lab.
The base of highway pavement is the most critical component of a road. As a result, road pavements are susceptible to the performance of the soil supporting them. Finding a method to balance road performance, constrained budgets, and tightening environmental laws is becoming a bigger challenge for road engineers. The cost-effectiveness of treatments to enhance the long-term performance of conventional pavements is declining. In comparison to the work at hand, road budgets, especially for maintenance, appear to be decreasing yearly.
Need for Soil Stabilisation of Roads
The stability of the underlying soils frequently affects the long-term performance of pavement structures. The structural integrity of each layer of pavement must meet minimal requirements in order for the engineering design of these built facilities to support and distribute the superimposed loads. These layers must withstand shear, severe deflections that can cause fatigue cracking in the layers above, and excessive permanent deformation.
In order to transform inexpensive natural earth materials into useful construction materials, it may be necessary to improve their engineering properties as they don’t always satisfy these specifications in their natural state. This is frequently achieved by stabilising or altering these problematic soils physically or chemically. The necessary strength and stability needed to ensure acceptable performance under traffic loading and environmental demands are frequently lacking in in-situ subgrades.
While stabilisation is a viable alternative for enhancing soil characteristics, the engineering properties that result from stabilisation vary greatly due to heterogeneity in soil composition, differences in the micro and macro structure of soils, heterogeneity of geologic deposits, and due to differences in the physical and chemical interactions between the soil and candidate stabilisers. The utilisation of site-specific treatment alternatives is required for stabilisation due to these variances.
On-going soil stabilisation
Advatanges of Soil Stabilisation
The basis of pavement design is the assumption that each layer of material in the pavement system will meet a minimum level of specified structural quality. Each layer must be able to withstand shearing, resist excessive deflections that could lead to fatigue cracking either inside the layer or in layers above it, and avoid excessive permanent deformation. The ability of a soil layer to disperse the load over a larger area often increases with soil quality, allowing for a reduction in the needed thickness of the soil and surface layers.
The following benefits come from stabilising soil for use in road pavements:
(a) Improved engineering characteristics Soil stabilisation improves the engineering properties of the soil, e.g., (i) strength – to increase the strength and bearing capacity, (ii) volume stability – to control the swell-shrink characteristics caused by moisture changes, and (iii) durability – to increase the resistance to erosion, weathering or traffic loading.
(b) Quality improvement The most common benefits obtained through soil stabilisation include improved soil gradation, a decrease in plasticity index or swelling potential, and gains in toughness and durability. Stabilization can also be employed in wet conditions to give construction projects a working surface. The process of improving soil quality in this way is known as soil modification.
(c) Thickness reduction Through the addition of additives, a soil layer’s strength and stiffness can be increased, allowing the stabilised material’s design thickness to be reduced in comparison to an unstabilized or unbound material. If the specific stabilised material achieves the required gradation, the design thickness strength, stability, and durability requirements of a foundation or subbase course can be decreased.
(d) Reduced maintenance requirements Soil Stabilisation can improve soil qualities, cut down on maintenance, and create an all-weather surface. Stabilization can improve the condition of the surface by reducing dust, rutting, potholes, and corrugation.
Strength parameters for mixture designs should be used to determine the ideal binder content. A pavement material should be resistant to abrasion and ravelling brought on by vehicle traffic when it is unsurfaced (i.e., no wearing course). Soil stabilisation can be utilised to lower dust, improve skid resistance, and decrease ravelling. However, regular grading and periodic reshaping cannot be used to maintain pavements that have been stabilised by a cementing operation. When maintaining the wearing surface in this way, the soil should be modified rather than stabilised.
Methods of Soil Stabilisation
Stabilization of soil may be achieved via mechanical, chemical, electrical, or thermal processes. Rarely are the last two choices chosen. The densification of soil through the use of mechanical energy is known as mechanical stabilisation, sometimes known as compaction. As air escapes from soil pores, densification happens without much change in water content. This technique works especially well in cohesionless soils where compaction energy can lead to particle interlocking and rearrangement. However, if these soils have considerable moisture variations, the approach might not work.
An increase in the fines content of the soil—that is, the proportion smaller than 75 μm—can also cause a reduction in the effectiveness of compaction. This is because particle rearrangement during compaction is hampered by cohesion and interparticle bonding. More successful than compaction for long-term stabilisation in these fine-grained soils is chemical stabilization/modification of their physio-chemical characteristics.
If a significant stabilisation response can be achieved in these soils, chemical stabilisation of non-cohesive, coarse-grained soils, soils with more than 50% by weight coarser than 75 μm, is also advantageous. When compared to the strength of the untreated material, the strength enhancement in this instance may be substantially larger—more than ten times higher. The most common methods of soil stabilization for roads include:
Mechanical stabilization.
Lime stabilization.
Cement stabilization.
Lime-Fly Ash (with or without cement) stabilization.
Bituminous stabilization.
Chemical stabilization.
Geotextiles, fibres, prefabricated materials, etc.
Selection of Stabilisers
The kind of soil to be stabilised, the intended use of the stabilised layer, the desired type of soil improvement, the required strength and durability of the stabilised layer, cost, and environmental circumstances are all important considerations when choosing a stabiliser. However, there are some broad criteria that make certain stabilisers more preferable based on soil granularity, plasticity, or texture. There may be more than one candidate stabiliser suitable for one soil type.
For instance, Portland cement can be used with a variety of soil types, but more plastic materials should be avoided since it’s crucial that the cement be thoroughly blended with the fines fraction (<0.075 mm). For Portland cement stabilisation, well-graded granular materials with enough fines to create a floating aggregate matrix (homogenous mixture) works very well.
Lime will cause soils with medium to high plasticity to become less plastic, become more workable, experience less swelling, and become stronger. Lime is used to stabilise a variety of materials by turning weak subgrade soils into a “working table” or subbase and combining them with weak granular base materials, such as clay-gravels, to create a strong, high-quality base course.
Due to the fact that fly ash is a pozzolanic substance that reacts with lime, it is virtually usually utilised in conjunction with lime in soils with little to no plastic particles. For increased strength, it has frequently been found useful to utilise a little proportion of Portland cement mixed with lime and fly ash. Lime, cement, and fly ash (LCF) have been used successfully to stabilise base courses.
Both asphalt and bituminous compounds are used to increase strength and weatherproof surfaces. Since it is desired to completely coat all of the soil particles, silty sandy and granular soils are typically good for stabilising asphalt.
Extreme weather conditions may also affect the best stabiliser choice, favouring the use of some stabilisers while discouraging the use of others, regardless of cost. Generally speaking, the hot, arid, and cold, rainy climates demand special attention.
Mechanical Stabilisation
The development of internal friction and cohesion, two naturally occurring forces inside the soil, is known as mechanical stabilisation. In some cases, compaction by itself is sufficient to stabilise the soil. The local soil can typically only be stabilised by adding a suitable quantity of soil or gravel components. When locally available soil or gravel materials with the right grading and plasticity are not available, mechanical stabilisation is used.
In order to change the particle size distribution and plasticity, mechanical stabilisation includes mixing or blending two or more chosen materials in the necessary amounts. Before final shape and compaction, mixing can be done on-site. A common application of mechanical stabilization is the blending of a granular material lacking in fines with a sand-clay. This blending of the materials has the potential to improve strength, abrasion resistance, imperviousness, and compatibility.
Lime Stabilisation
With lime stabilisation, the soil will become less plastic, more workable, less prone to swelling, and modified to give maximum strength. There is a recommended amount of lime for each type of soil, and adding more than that will have a negative impact on the mixture’s qualities. The quantity and kind of clay minerals in the soil determine how much lime is required (in percent by mass) to stabilise a material.
Lime stabilisation
Small amounts of lime (1 to 3 percent) may be used to stabilise some soils, such as clayey gravel with acceptable grading but moderately high plasticity, by lowering the plasticity index. The use of lime contents of 3 to 6 percent may result in considerable change in the material constitution.
The majority of plastic soil materials, including clayey sands (SC) and silty clays (ML), react effectively with lime in general. Materials with plasticity indices below 10% may not react quickly, nevertheless. The material’s reactivity to lime must be tested to find out. When treated with minor quantities of lime, the stabilised soil should preserve some cohesiveness of poorly graded clayey sand and gravels. They can become friable (easily crumbled or pulverised) and totally non-cohesive if too much is introduced, which will result in failures.
As a result, base material that has been treated with lime should adhere to the grading specifications that are typically given for untreated material. All lime-treated fine-grained soils generally show characteristics of reduced plasticity, enhanced workability, and reduced volume change. But not all soils have features that boost their strength. It should be underlined that there are numerous factors that affect the properties of soil-lime mixes. The most crucial factors are soil type, type of lime, quantity of lime added, and curing conditions (time, temperature, and moisture).
Cement Stabilisation
Portland cement can be used to either modify and improve the soil’s quality or to turn the soil into a mass that is cemented and has higher strength and durability. Whether the soil needs to be modified or stabilised will determine how much cement is needed. The stabilisation of soils has been accomplished with success using a variety of cement kinds.
Pavement construction has made extensive use of cement stabilisation. Cement, however, is typically not a suitable stabilising ingredient for a pavement’s wearing course. Without being covered by a wearing surface, the cementitious linkages formed cannot withstand the pressure of traffic. Additionally, unlike lime, cement cannot be reworked after initial mixing and subsequent setting. It is also not possible to rework cement using maintenance tools like graders. On the other hand, it can be utilised as a sub-base stabilising agent.
Soil stabilisation using cement
A variety of soils, from fine-grained clays and silts to sandy materials, can be stabilised using cement. When the plasticity index (PI) is low, cement is typically utilised with clays or silts for fine-grained materials. When the sulphate content of the soil is greater than 1%, cement stabilisation should be avoided.
The trial-and-error method is used to calculate the amount of cement needed to modify the soil and increase its quality. It is necessary to prepare multiple samples of soil-cement mixtures at various treatment levels in order to minimise the PI of the soil. The PI of each mixture must then be calculated. Since it was calculated using the material’s minus 40 percent, the value must be corrected in order to discover the design cement content using the entire sample weight represented in equation. The minimal cement content that provides the desired PI is selected.
A = 100BC (1) Where ; A = design cement content, percent total weight of soil B = percent passing 400 micron sieve size, expressed as a decimal C = percent cement required to obtain the desired PI of minus 400 micron material, expressed as a decimal.
Bituminous Stabilisation
When compared to cement and lime stabilisation, asphalt stabilisation of soils and aggregates is very different. A waterproofing phenomenon serves as the fundamental mechanism for the stability of fine-grained soils by asphalt. Asphalt is applied on soil particles or soil agglomerates to stop or limit water penetration, which would often result in a loss of soil strength. Asphalt stabilisation also makes the soil resistant to the negative impacts of water, such as volume, which can improve durability qualities.
There are two main mechanisms at work in non-cohesive materials like sands and gravel, crushed gravel, and crushed stone: waterproofing and adhesion. The asphalt layer on the cohesionless materials creates a membrane that stops or slows down water penetration, reducing the likelihood that the material would weaken in the presence of water.
Bituminous stabilisation
Adhesion has been named as the second mechanism. The asphalt serves as a binder or cement to hold the aggregate particles to the surface. Thus, the cementing effect boosts cohesiveness to increase shear strength. Criteria for the design of bituminous stabilized soils and aggregates are based almost entirely on stability and gradation requirements. For asphalt stabilised mixtures, the freeze-thaw and wet-dry durability tests are not relevant.
For hot, arid locations, bituminous stabilisation is more appropriate. The inclusion of bituminous binder aims to minimise water penetration through the soil and give non-plastic materials cohesiveness. Granular materials and materials that are easily granulated are the best candidates for bituminous stabilisation. There are restrictions when using bitumen-stabilised material as the wearing course for pavement. The binding action of bitumen alone won’t be adequate to stop ravelling from weathering and traffic unless significant amounts are applied. Typically, such a high bitumen content won’t be economical.
Bituminous stabilisation has been used to treat crushed rock, gravel, sandy loam, sand-clays, and other materials successfully. Bitumen can be used to stabilise fine-grained soils with increasing amounts of material passing through a 75-micron filter, however, doing so will result in increased prices and asphalt material requirements. The best candidates for this type of stabilisation are materials having a plasticity index of less than 10%.
Stabilisation with Lime-Fly Ash (LF) and Lime-Cement-Fly Ash(LCF)
Utilizing LF or LCF combinations can frequently be used to stabilise coarse-grained soils with little to no fines. During the burning of pulverised coal, a mineral byproduct known as fly ash—also known as coal ash—is produced. It contains compounds of silicon and aluminium that, when combined with lime and water, create a hardened cementitious mass with high compressive strengths.
Fly ash stabilisation in the field
Since fly ash provides an agent with which the lime can react, lime and fly ash are frequently employed together successfully to stabilise granular materials. Lime-fly ash or lime-cement-fly ash combinations can be used to stabilise any sand, gravel, or combination of sand/gravel soil. These soils shouldn’t include more than 12 percent fines, and their Plasticity Index shouldn’t be more than 25%. So stabilisation using LF or LCF is frequently acceptable for base and subbase course materials.
Stabilization using lime or cement is somewhat different from design with LF. The percentage of lime-fly ash, the moisture content, and the ratio of lime to fly ash can all be changed for a specific material combination (aggregate, fly ash, and lime) during the mix design process. It is well accepted that the quality of the matrix material directly affects engineering properties like strength and durability. The part made up of fly ash, lime, and fine aggregate particles is the matrix material. Basically, when the matrix material can “float” the coarse aggregate particles, increased strength and improved durability are possible.
The void spaces between the coarse aggregate particles are effectively filled by the fine size particles. To successfully fill the available void spaces and allow the coarse aggregate particles to “float,” a certain amount of matrix is needed for each coarse aggregate material. The optimum fines content is the amount of matrix necessary to achieve the maximum dry density of the entire combination.
It is advised that the amount of matrix in LF combinations be around 2% higher than the ideal fines content. The ratio of lime to fly ash also affects the strength development at the acceptable fines concentration. Different strength and durability values can be obtained by varying the lime-fly ash ratio.
Stabilisation with ground granulated blast furnace slag (GGBS)
Ground granulated blast furnace slag (ggbs) is a by-product from the blast-furnaces used to make iron. These run at a temperature of around 1500 °C and are fed with a precisely measured combination of limestone, coke, and iron ore. The leftover components create a slag that floats on top of the iron once the iron ore is converted to iron. This slag is regularly tapped off as a molten liquid and must be quickly cooled in a lot of water if it is to be employed in the production of ggbs.
GGBS
The quenching generates granules that resemble coarse sand and optimises the cementitious characteristics. In complex manufacturing facilities that can treat up to 500,000 tonnes of slag annually, this “granulated” slag is subsequently dried and ground to a fine powder to a precisely regulated fineness. Even though ggbs powder is a relatively slow-setting cement on its own, alkali is usually required to activate and accelerate it for practical use.
Portland cement often provides the alkalinity to activate and accelerate these capabilities because ggbs on its own only has slow cementitious properties. The alkali required for activation can alternatively be obtained from lime. Sulfides, as well as sulphates, are capable of causing disruptive expansion in stabilised soils, according to laboratory and field studies. Combinations of ggbs and lime have been demonstrated to be useful and efficient solutions for stabilising soil and to offer technical advantages. The integration of ggbs, in particular, is quite effective at preventing the expansion brought on by the presence of sulphate or sulphide in soil.
Stabilisation with geotextiles
Through their tensile strength capabilities, geotextiles can be utilised over very soft soils to help spread loads and so increase the site’s load bearing capacity. Every time any cover aggregate is to be added to a soil containing more than 10% fines, a geotextile is required as a separation layer. Geotextiles can also act as a separator to prevent excess fines from penetrating a granular material placed over it or as a water barrier to prevent moisture from entering the pavement. To manage and remove excess moisture, geotextiles can be utilised as filter media to build different drainage layers inside and next to the pavement.
The movement of traffic over low bearing capacity soils will be made easier by the use of geotextiles, particularly for expedient applications. The need for more traditional stabilisation materials may be diminished or eliminated by the use of geotextiles. The geotextile should meet the drainage or filtration requirements for the specific soil conditions when used for separation. To stop soil particle migration, the geotextile openings should be sized properly. The geotextile needs to be strong enough to adhere to survivability standards for subgrade situations and covering arterials.
There are design guidelines for situations when the geotextile is to be utilised as a reinforcement material or a water barrier. The geotextile often needs to be coated with a bitumen substance in order to function as a water barrier. While seams between geotextile sheets can be field seamed together using a variety of techniques, in the field they are typically only overlapped by a certain amount to avoid fastening issues.
Stabilisation using Fibres and prefabricated materials
Utilizing a pulverizer mixer, hair-like fibres are mixed into the moist soil to stabilise it. Sands and silty sands that are categorised as SW, SP, SM, and some SM-SC types of soils are the best materials for fibre stabilisation. Since the use of fibres in high-plasticity soils has produced erratic results, their application should typically be restricted to the aforementioned coarse-grained soil types. Uni-Mat, Hex-Mats, and any other fabricated material that can be utilised as a trafficked surface to sustain loads on a soft soil are examples of the fabricated materials mentioned for soil stabilisation.
Stabilisation with rice husk ash and lime sludge
Numerous industries across the world produce significant volumes of waste as a byproduct, including rice husk ash and lime sludge. These contaminants provide a serious disposal challenge and have dangerous consequences on the environment and nearby regions. The issue of their disposal can be greatly reduced by using this waste material in road construction. Studies on the use rice husk ash in stabilising soil masses have been carried out by a lot of researcher, and the findings showed that its application had a significant impact on the enhancement of soil qualities. According to some studies, it is particularly helpful for stabilising clayey soils.
The results of some studies are given below:
lt increases the liquid limit and plastic limit thereby decreasing the PI value of soil
It increases the unconfined compressive strength of soil.
It increases the soaked CBR of the soil.
The optimum proportioning of lime sludge and rice husk ash for maximum unconfined compressive strength and lowest plasticity index is 16% and 10% respectively.
The soaked CBR however kept on increasing at 15% and 20% rice husk ash.
Conclusion
Alteration of one or more soil properties mechanically or chemically to produce an improved soil material with the appropriate engineering properties is known as soil stabilisation. Stabilizing soils can make them stronger and more resilient, or it might stop erosion and dust production. No of the reason for stabilisation, the goal is to create a soil material or soil system that will endure over the design lifespan of the project under the design use conditions.
Engineers are in charge of deciding on or defining the appropriate stabilising strategy, methodology, and material requirements. The engineering qualities of soils vary from region to region throughout the world, as do the soils themselves. Soil testing is essential for the success of soil stabilisation. Prior to construction, ideally before selecting or buying materials, the chosen method of soil stabilisation should be tested in a lab.
Have you ever pondered the detrimental effect of water seepage on road pavements? It is common knowledge among engineers that frequent water seepage can result in road defects such as swelling of the subgrade layer and the formation of potholes and stains on roads. Perhaps you find yourself on a road construction project with water seeping out of the ground. What solution would you proffer?
This article will discuss the use of filter media in road construction as an alternative solution to water susceptibility of road pavements from groundwater absorbed into the pavement structure by capillary action. Thus, by controlling water seepage into pavement structures, we can reduce the formation of road defects and increase the lifespan of road pavements.
Detrimental effects of water seepage
Water is vital to human existence. However, given enough time, water can destroy even the best infrastructures. Therefore, water seepage and other water-related problems pose significant threats to road pavements. For instance, water action from groundwater or high hydrostatic pressure from surrounding water features leading to frequent water seepage can erode and damage road pavements. For example, look at the adverse effect of water seepage on a road near Guam Reef Hotel in Tumon, Guam, USA.
Road defect: Pothole
Water seepage is a prevalent issue that occurs after heavy rainfall. For instance, additional water is in the underlying road pavement layers when groundwater rises. This extra water then creates hydrostatic pressure against the road pavement resulting in soaked road layers, which reduces the bonding strength and load-bearing capacity of the premixed road materials.
Furthermore, the prolonged seepage of water into a road pavement in conjunction with traffic loads results in several pavement defects and problems, such as the formation of road stains and potholes, settlement, rutting, cracking, stripping, ravelling, and swelling of the subgrade layer. Therefore, it becomes necessary to give maximum consideration to water-related problems and how to reduce their damaging effects during the design and construction of road pavements.
Filter media in road construction
The granular layer serves as a filter media to prevent water seepage (Photo credit: cementconcrete.org)
A filter media is generally used in earthworks and other civil engineering structures, such as roads, dams, retaining walls, and embankments. For example, a subsurface filter media is essential in road projects because they help to prevent the decrease in the strength of the underlying layers of road pavement caused by increased water or moisture content.
Similarly, a layer of filter media is necessary when the surface of a road has its highest water table sufficiently below the crust of the road, and there is a likelihood of water rising to the subgrade or road surface through capillary action.
When filtering is targeted at preventing water seepage into a road pavement, a filter media is a layer of free-draining granular materials or clasts (e.g. cobblestones) underneath a road’s sub-base layer. The layer of granular materials or cobblestones of suitable thickness is usually provided to cut off capillary action between a road’s subgrade and its highest water table. Thus, granular materials or cobblestones prevent the build-up of hydrostatic or water pressure on the road pavement layer.
Cobblestones
Filtering in road construction significantly reduces road defects and failures resulting from water seepage. When filter materials are provided, the water that is supposed to rise into the underlying road layers will drain away into roadside drains. Furthermore, filtering with cobblestones is a suitable solution to controlling the capillary rise in waterlogged terrains where the subgrade is usually subjected to extreme soaking conditions due to high ground water table levels.
Selection criteria for granular materials and cobblestones as filter media
The filter media for roadbed drainage to remove seepage water and prevent damage to road pavements from uplift pressure may consist of either a single layer or several layers, each with different grading. The essential criteria are the grading and permeability of the granular materials and cobblestones. However, any filter material used as a filter media must be clean, hard, durable, dimensionally stable, and corrosion, dissolution, and frost-resistant.
Furthermore, the filter material must be free from deleterious materials that can adversely affect the efficiency and longevity of the material. These deleterious materials include clayey, elongated, flaky particles, chemically unsound or readily soluble materials, and excessively porous or laminated materials. Thus, the choice of filter material requires accessing a wide range of chemical and physical properties and sometimes depends on the judgment and experience of a designer.
The minimum acceptance criteria for granular materials and cobblestones as filter media are listed below.
Oven-dried relative density not less than 2.5
Maximum flakiness and elongation indices not greater than 30
Water absorption not greater than 3% by weight
Aggregate impact value, not more than 30
Aggregate crushing value, not more than 30
10% fines value not less than 100 kN
Los Angeles abrasion value not greater than 40
Aggregate abrasion value not greater than 20
Magnesium sulfate soundness value not more than 12% loss
Road construction procedures with cobblestones as filter media
A layer of cobblestones at a road section with prevalent water seepage action
The construction procedures follow the standard methods for constructing asphalt concrete roads, except for introducing cobblestones as a filter layer. The procedures are briefly discussed below;
Planning
Marking out of road alignment and dimensions
Earthworks, including excavation, grading and compaction of road subgrade
Placement of precast or casting of in-situ roadside flood drains
Laying of a suitable filter layer (cobblestones) with appropriate thickness where required along the stretch of the road
Laying, grading and compaction of sub-base and base courses
Laying and compaction of binder and surface or wearing asphalt courses
Lastly, it is essential to note that quality control is compulsory at every construction stage.
Alternative materials for filter media
Bituminous material
Bituminous layer serving as a filter media to prevent water seepage (Photo credit: cementconcrete.org)
An impermeable or bituminous layer covering a road section to seal water underneath the road’s sub-base can also serve as a filter media. Bitumen, made up of organic liquids that are highly viscous, sticky, and waterproof, is insoluble in water and water-resistant, thus, making it a viable alternative as an effective sealant and filter media. In addition, a filter media of bituminous material has significant advantages of availability and affordability and can be used over long stretches of a roadway.
Geosynthetics
Geonets
These materials can be used as filters in addition to or in place of traditional granular materials in road construction. Furthermore, geosynthetics are easy to install, have low permeability, and are often cost-effective, especially when granular materials are scarce and expensive or when the available ones do not meet project specifications.
Common geosynthetics used as filter media are geomembranes, geotextiles, and geonets. Also, it can be a combination of geotextiles and geonets to form a drainage geocomposite, whereby the geotextile act as a filter while the geonet serves as a drain. For example, geonets are designed in such a way that they can convey maximum anticipated seepage within their channels during their design life. Similarly, geotextiles are designed to dissipate pore water pressure at the base of roadway structures.
Conclusion
Capillary water that seeps into a road pavement attacks the bond between the asphalt binder and aggregates in the pavement. It is important however to note that keeping road pavements from coming in contact with water can be almost impossible. However, if you ignore and refuse to treat water seepage right away as a design or construction engineer, you put the road or a stretch of the road at risk of defects andfailure.
Therefore, a road or highway engineer needs to be aware of the potential sources of water on a road project and make provisions for them. For instance, providing a filter layer becomes necessary to prevent long-term damage or collapse if the water source is groundwater seepage. Lastly, it is vital to design and construct roads such that their water susceptibility is minimized and their service life is improved.
References
[1] Gourc, J. P. (2006), “Training Course on Geosynthetics: Geosynthetics in Drainage and Filtration”, International Geosynthetics Society (8IGG), Yokohama, Japan, September, available at: https://www.geosyntheticssociety.org/wp-content/uploads/2014/10/TrainingCourse_GeosyntheticsinDrainageandFiltration.pdf
[2] Engineering Geology Special Publications (2007), Aggregates for use in filter media, Geological Society, London, Vol. 17, No. 1, pp. 291-298, available at: https://doi.org/10.1144/GSL.ENG.2001.017.01.13 or https://www.scribd.com/document/400882158/Aggregates-for-Use-in-Filter-Media
