Any engineering or building structure must always require some site investigation. A complete analysis of the soil and groundwater conditions to a significant depth below the surface using boreholes and in-situ and laboratory tests on the materials encountered may be required, as well as a simple inspection of the surface soils with or without a few shallow trial pits.

The objectives of a geotechnical site investigation are to ascertain the conditions and properties of the soil, rock, and groundwater in the site, and to obtain extra pertinent information about the site.

The type, size, and significance of the planned structure should be taken into account in the subsurface exploration program for a specific site. The number and depth of the necessary soil borings are determined by these criteria, which aid in the design of the site investigation program. Locating subterranean utilities should be part of the planning for a site investigation (i.e., phone, power, gas, etc.). As a result, many days before the planned site investigation, a local “call before you dig” service should be informed where available.

The significance and foundation configuration of the structure, the complexity of the soil conditions, and any knowledge that may be available regarding the behaviour of existing foundations on comparable soils all influence the scope of the investigation.

Categories of Site Investigation

Structures and earthworks are divided into three “geotechnical categories” according to Eurocode 7 (Geotechnical Design).

Geotechnical category 1 refers to light structures like single- or two-story buildings, low retaining walls, and buildings with column loads up to 250 kN or walls loaded to 100 kN/m. The qualitative investigations in this category can be restricted to verifying the design assumptions, at the latest, during the supervision of construction of the works, provided that the ground conditions and design requirements are known from prior experience and the ground is not significantly sloping. Visual inspection of the site, occasionally combined with inspection of small test pits, or sampling from auger borings are considered to be the main components of verification.

Conventional building types are included in category 2 structures on locations without anomalous dangers, unusually challenging ground conditions, or extraordinarily demanding loading conditions. This category includes conventional substructures such as retaining walls, bridge piers and abutments, excavations and excavation supports, rafts, piles, and shallow spread footings. Quantitative geotechnical data is needed, however standard testing techniques in the field and lab, as well as for analysis and design, are judged sufficient.

Structures in category 3 are those that are extremely massive, peculiar in nature, involve anomalous dangers, or have an unusually difficult ground or loading conditions. This category includes buildings that are located in seismically active regions.

The investigations essential for category 3 include any extra specialist research that may be required in addition to those thought to be sufficient for category 2. The processes and interpretations should be documented with references to the tests if specialized or unusual test procedures are necessary.

Site Investigations and Professional Practices

Buildings and engineering structures built upon deep excavations require extensive investigations. They offer vital information on the soil and groundwater conditions to contractors submitting bids for the work, in addition to information for foundation design. So, by collecting accurate and competitive bids based on an adequate understanding of the actual situation, money is saved.

If the cost of excavation work represents a sizeable portion of the overall project, a reputable contractor will not take a chance on it; instead, a comparably high sum will be added to the tender to account for the unforeseeable conditions. It follows the saying that “You pay for the borings whether you have them or not“.

An engineer doing a site investigation may hire local workers for hand auger boring or trial pit excavation, or they may hire a contractor for boring and soil samples. The boring contractor may send samples to his own lab or to a third-party testing facility if laboratory analysis is necessary.

After that, the geotechnical engineer analyzes the soil mechanics for foundation design. Alternatively, the entire investigation could be handled by a specialized company with comprehensive capabilities for drilling, sampling, field and laboratory testing, and soil mechanics analysis.

If preferred, in-situ testing can take the place of laboratory testing. In any case, the engineer in charge of overseeing the field and laboratory work on a daily basis should keep the site investigation’s objective in mind and continuously evaluate the data in a manner similar to that used when writing the report.

The relevance of characteristics like weak soil layers, deep rock weathering, and sub-artesian water pressure can be explored in as much detail as may be necessary while the fieldwork is still on by avoiding the omission of important information in this way.

Whatever method the engineer chooses to conduct his site investigation, it is important that the people or groups doing the task are diligent and absolutely trustworthy. The engineer has a significant obligation to his employers to select a qualified organization and to satisfy himself through the field, lab, and office work inspections that the work has been completed accurately and thoroughly.

Information Required from a Site Investigation

For geotechnical categories 2 and 3 the following information should be obtained in the course of a site investigation for foundation engineering purposes;

1. The general topography of the site, including surface configuration, adjacent property, the presence of watercourses, ponds, hedges, trees, rock outcrops, etc., and the accessibility for construction equipment and vehicles.
2. The position of underground utilities such as sewers, water mains, cable television, and telephone lines.
3. The overall geology of the region, with a focus on the primary geological formations that underlie the site and the potential for subsidence due to mining or other factors.
4. The prior usage and history of the site, including any defects or failures of current or former buildings related to foundation issues and the potential for toxic waste contamination of the site.
5. Any unique characteristics, such as the potential for earthquakes or environmental factors like flooding, seasonal swelling and shrinking, permafrost, or soil erosion.
6. The accessibility and quality of locally produced building materials, including water for construction, building and road stone, and concrete aggregates.
7. Information on normal spring and neap tide ranges, extreme high and low tidal ranges and river levels, seasonal river levels and discharges, tidal and river current velocities, wave action, and other hydrographic and meteorological data for marine or river structures.
8. A thorough record of the soil and rock strata, groundwater conditions, and any deeper strata that may have an impact on the site’s circumstances in any way within the zones affected by foundation-bearing pressures and construction activities.
9. The results of tests conducted in the field and laboratories on soil and rock samples relevant to the specific foundation design or construction issues.
10. The results of chemical investigations performed on soil, fill materials, and groundwater to identify potential negative effects on foundation structures.
11. The findings of chemical and bacterial investigations performed on contaminated soils, fill materials, and gas emissions to assess the potential health hazards.

Items (1) through (7) above can be obtained by a general site reconnaissance (the “walk-over” survey”) as well as through research of geological memoirs, maps, and other published records. Walking around the site area closely will often provide major clues about subsurface structures.

For instance, hidden swallow holes (sinkholes) in chalk or limestone formations are frequently revealed by sporadic depressions and noticeable irregularity in the ground surface; soil creep is shown by wrinkling of the surface on a hillside slope or leaning trees; abandoned mine workings are shown by old shafts or heaps of mineral waste; glacial deposits may be indicated by mounds or hummocks (drumlins) in a generally flat topography; and river or lake deposits by flat low-lying areas in valleys.

The existence of springs or wells and marshy terrain covered with reeds are surface indicators of groundwater (indicating the presence of a high water table with poor drainage and the possibility of peat). In the case of huge projects across wide areas, geological expertise should be sought.

Information on potential long-term changes in groundwater levels should be sought after. Pumping from deep mine shafts or ceasing groundwater extraction for industrial purposes can slowly raise groundwater levels over a large area.

Aerial photography is a useful tool for site inspections on large sites. Photographs may be taken from balloons, drones, or model aeroplanes. A great deal about a site’s topography and geomorphology can be learned through expert interpretation of aerial photos. A well-established science, geological mapping from aerial images is done by specialized companies.

Both outdated publications and old maps should be investigated because they may reveal the location’s prior uses and are especially helpful when looking into historically significant locations. Maps, memoirs, and images or photographs of a location in the past are frequently available at local libraries or museums. For information on buried services and coal mine workings in Britain, contact the Geological Survey and local authorities.

The list’s items (8), (9) (10), and (11) are obtained by boreholes or other subsurface research techniques, as well as through field and laboratory testing of soils or rocks. It is important to characterize soil types and consistency in accordance with accepted standards of practice. The British Standard Code of Practice Site Investigations, BS 5930, outlines the common descriptions and classifications of soils in Britain.

Subsurface Investigation Methods

Methods of determining the stratification and engineering characteristics of subsurface soils are as follows;

• Trial pits
• Hand auger borings
• Mechanical auger borings
• Light cable percussion borings
• Rotary open-hole drilling
• Wash borings
• Wash probings
• Dynamic cone penetration tests
• Static cone penetration tests
• Vane shear tests
• Pressuremeter tests
• Dilatometer tests
• Plate bearing tests

Detailed descriptions of the above methods as used in British practice are given in BS 5930 Site Investigations. Brief comments on the applicability of these methods to different soil and site conditions are given in the sections below.

In most cases, geotechnical category 1 investigations use trial pits. For shallow foundations, they are helpful for assessing the quality of weathered rocks. The most reliable method for determining the stage of deposition and characteristics of filled ground is to use trial pits expanded to trenches.

In soils that remain stable in an unlined hole, hand and mechanical auger borings are also suitable for category 1 examinations. Augering, when done properly, gives the least disturbance to the soil out of any other boring technique.

In British practice, light cable percussion borings are typically utilized. The straightforward and durable machinery is ideally suited to the vastly different soil types in Britain, including the extremely stiff or dense stone glacial soils and weathered boulders with a consistency similar to soil. For specialized testing, large-diameter undisturbed samples (up to 250 mm) can be recovered.

Typically, the United States, the Middle East, and countries in eastern Asia use rotary open-hole drilling. The rotary drills are typically skid- or tractor-mounted and can drill through rock as well as through dirt. Sample sizes are typically limited to 50 mm in diameter, and hole diameters are typically lower than percussion-drilled holes. Although drilling fluids such as bentonite slurry or water are employed, specialized foams have been created to aid in collecting nice, undisturbed samples.

Wash borings are holes with a small diameter (about 65 mm) that are bored using a water flush and chiselling. Sampling is done using 50 mm internal diameter open-drive tubes or 50–75 mm standard penetration test equipment.

Investigations into over-watered soils employ wash probings. They are used, for instance, in dredging investigations, to find rock heads or a strong layer overlain by loose or soft soils. They consist of a small-diameter pipe shot down. The soils cannot be positively identified, and sampling is frequently not feasible.

Soil Sampling

There are two main types of soil samples which can be recovered from boreholes or trial pits;

(a) Disturbed samples, as their name implies, are samples taken from the examples of the boring tools are auger parings, the contents of the split-spoon sampler in the standard penetration test, sludges from the shell or wash-water return, or hand samples dug from trial pits.

(b) Undisturbed samples, obtained by pushing or driving a thin-walled tube into the soil, represent as closely as is practicable the in-situ structure and water content of the soil.

It is important not to overdrive the sampler as this compresses the contents. It should be recognized that no sample taken by driving a tube into the soil can be truly undisturbed. Disturbance and the consequent changes in soil properties can be minimized by careful attention to maintaining a water balance in the borehole. That is, the head of water in the borehole must be maintained, while sampling, at a level corresponding to the piezometric pressure of the pore water in the soil at the level of sampling.

This may involve extending the borehole casing above ground level or using bentonite slurry instead of water to balance high piezometric pressures. The care in the sampling procedure and the elaborateness of the equipment depends on the class of work which is being undertaken, and the importance of accurate results on the design of the works.

The presence of well-developed solution channels, caves, springs, sinkholes, and extremely irregular weathered bedrock surfaces with cavities are the characteristics of a karstic topography. Sinkholes are “closed depressions” in the earth’s surface that are created when limestone and other rocks close to the surface dissolve and the material on top of them subside or collapses into subsurface solution caverns.

When infiltrating acidic water comes into prolonged contact with limestone, sinkholes often result. For obvious reasons, sinkholes are the major geologic hazard in karst terrain. They can cause structural damage and failure of buildings, cut off highways, drain ponds and lakes, and allow direct infiltration of contaminated groundwater.

Sinkholes are the result of a slow, continuous process. However, the effects of the sinkhole near the surface can occur suddenly and catastrophically. There are two types of sinkholes:

i) Collapse Sinkhole, and
ii) Subsidence Sinkhole

Collapse Sinkholes
Collapse sinkholes develop when the limestone’s solution forms a vertical cavern or throat below the surface of the ground. Initially, the surface soil may be strong enough to span across the cavern. The bridging soils will eventually crumble as the cavern widens over time. Most people have heard about sinkholes in the media in terms of this.

Subsidence sinkholes
When the soil is relatively granular above the limestone formation, sinkholes might arise. In this situation, as the limestone erodes, the earth material on top fills the spaces left behind. This is known as ravelling, and if the earth keeps ravaging into the spaces in the limestone, the ground will eventually sink and produce a sinkhole.

The size of sinkholes can be influenced by the thickness of the overlying layer. The overlaying stratum can span a bigger cavern if it is sufficiently thick. For a collapse to occur, the cavern must enlarge. Additionally, sinkholes can grow to sizes large enough to engulf entire buildings.

Site Investigation for Sinkholes

Rocks that are prone to solutioning activity are known to underlie around one-fourth of the earth’s crust (Syed, 2004). Numerous reports of issues with construction on karst sites have been made worldwide. Similarly, reports have been made on the sudden collapse of previously undetected solution cavities that have caused harm to existing structures and infrastructures.

It goes without saying that thorough subsurface investigations are required when significant structures are to be found in karstic regions. However, it is still true that it might be challenging to find possible sinkholes, caverns, or solutioning activity that have not yet affected the ground surface.

Because a borehole only examines a small region, the typical geotechnical study method of drilling holes may not be able to find them. Experience has shown that utilizing borings to locate sinkholes or cavities is only 10% to 20% accurate.

Studying the local geology and hydrogeology and mapping sinkholes that have already formed in the project area are the first steps in the investigation of sinkhole potential. Both surface geophysical approaches and boreholes will need to be used for the large-scale site investigation.

The typical approach should be to use borehole methods for detection and delineation and surface methods for early reconnaissance (anomaly detection). In other words, the zone in question should be drilled and sampled to give observations for the purpose of evaluation when aberrant responses are recorded during the surface surveys.

Various approaches to investigating Karstic features are:

• Aerial and Satellite Photography
• Backhoe Trenches
• Drilling boreholes
• Modern Geophysical Techniques

Aerial and Satellite Photography

Identifying probable sinkholes can be done through aerial surveys. Large-scale zones of subsidence may be visible on old aerial photographs, which may aid in locating smaller, localized sinkholes. Although they can be located on the ground, an aerial survey is more practical.

Backhoe Trenches

Backhoes can easily and quickly explore a relatively large area. Trenches done this way can expose near-surface solution voids or sinkhole throats. However, they can not completely replace information obtained from a borehole.

Drilling Boreholes

Test borings are an important part of sinkhole or cavity investigations. The holes are drilled to the bedrock even if this requires drilling to much greater depths than would be necessary, otherwise. Standard Penetration Tests (SPTs) are usually conducted as the bore advances. This helps in knowing the strengths of various sediment layers. The data is used to draw subsurface cross-sections that help in inferring the presence or absence of sinkholes or cavity-associated features.

Geophysical Surface Surveys

A number of such techniques are currently in vogue that can locate cavities and sinkholes. The idea behind such techniques is to probe the subsurface without disturbing the ground surface. This is done by generating a wave, which when propagated through the soil, reveals anomalies. This can be investigated to find if the same is the presence of cavities or not.

Sinkhole Management in Construction – Case Study of Saudi Arabia

Syed (2004) published a case study about the Prince Abdullah Military City Project Site, which is in the oasis of Al-Hassa, in the Eastern Province of the Kingdom of Saudi Arabia, some 12 km west of the city of Hofuf. The report was published and presented during the fifth International Conference on Case Histories in Geotechnical Engineering, April 2004.

According to Syed (2004), there are several bedrock cavities at the project site. Numerous Reports, including those written in 1992 by a geotechnical consultant for a small area of the subject Site and those completed by the US Army Corps of Engineers in 1978, have documented these conclusions for the subject site.

The following terms best describe the karstic features seen at the project site: collapse sinkholes, subsidence sinkholes, dropouts, and bedrock cavities. In order to find and map the bedrock cavities at the site, a rigorous soil investigation program was launched. In-situ testing and sampling of overburden soil by Standard Penetration Tests (SPTs) and coring in the bedrock strata were all parts of the detailed geotechnical investigation program that was carried out at the site for the purpose.

Additionally, Core Recovery and Rock Quality Designation (RQD) measurements were made. Testing typical samples of subsurface materials in the lab was also included. To further confirm the competency of the stratum below, a thorough “Cavity Search Probing” was also carried out under the footprints of each building. With the aid of a wagon drill and “Pneumatic Driving,” a rock probe was driven into the bedrock while being cleaned out of the hole as it went.

The “Karstic Terrain” at the project site was mapped with the aid of these investigation and probing. The information was also helpful in developing plans for the project’s remediation and later construction. Given the karstic issues at the project site, a semi-rigid raft foundation was chosen. This foundation system became the best option for the situation because it is known for being quite robust to bridge over the underlying cavities.

Sinkhole Investigation at the Site

The Project Site spans an area of around 6 square kilometers and is divided into 8 zones. An initial preliminary reconnaissance investigation was conducted at the site, followed by a detailed investigation for each of the 8 Zones. There were already two investigations conducted at the project site prior to the preliminary reconnaissance investigation (conducted in March 1998), each conducted by a different agency at a different time. The range of work for the various investigations carried out at the project site can be found in Syed (2004).

All of these tests at the project site proved that the limestone that makes up the bedrock is primarily light brown, fine-grained, strongly to moderately worn, and jointed. The underlying rock strata’s Total Core Recovery (TCR), which generally exceeds 50%, was determined to be between 27% and 100%. However, the Rock Quality Designation (RQD), which ranged from 0% to 100%, was typically lower than 20%.

The presence of several small cavities inside the underlying limestone bedrock is one of this limestone’s common characteristics that has been mentioned in all the studies. The findings advised performing a thorough Cavity Probing after excavation during the building stage at the locations of the foundations (under the footprints of the facilities). Drilling probing holes at least 1.5 times the width of the foundations deep is the best way to accomplish this.

The following methods were applied at the project site to investigate the local geology and hydrogeology as well as to find and map probable sinkholes and cavities.

• Trenches and Test Pits
• Drilling Boreholes, and
• Cavity Probing using pneumatic driving of Probe.

Trenches, test pits, and boreholes were used to collect a lot of information that indicated the presence of bedrock cavities beneath the surface. However, before starting to pour the foundations, a thorough cavityprobing was done beneath the footprints of each facility to determine whether or not there were any cavities. 5,610 probe holes total (in 166 facilities in 6 zones) were made. This comprised an extra 219 numbers in 18 numbers of facilities located surrounding the problematic locations.

The Cavity search program was quite thorough and detailed. The probing was done by using a wagon drill to pneumatically drive a rock probe into the rock. As time went by, compressed air was utilized to clear the hole. The probe’s time through each successive 20 cm depth was meticulously measured to determine the rock’s resistance to penetration.

The time for penetration records will be quite short if there are cavities or loose zones in the underlying strata. For a 20 cm penetration, a time of less than 10 seconds is regarded as the presence of loose zones, however in the case of a cavity, there will be no resistance to rock penetration and it happens suddenly.

In the course of this operation, the following details were meticulously recorded:

• Time taken for 20 cm penetration
• Air escape in the finished boreholes
• Sudden fall of drill rod
• Boreholes, where time was 10 seconds or less

Any bore where the previous three observations were made were probed repeatedly until a cavity was identified or some logical explanations for the observations were found. If there were any cavities, grout was used to fill them in. To reveal the sinkholes or cavities, excavations were also carried out in the vicinity of these problem probing holes.

Treatment and Remedial Measures

The methods used to remedy Karst-related features may be as diverse as the cavities and sinkholes themselves. In order to expose the opening in the rock surface, the overburden soils must be removed if the “throat” of a sinkhole can be found.

The excavation is then backfilled once the throat has been sealed off or covered with an inverted filter. A less effective treatment is typically employed if the throat cannot be found or if the depth of the rock makes exposing the rock surface impractical.

The excavation base could subsequently be sealed with concrete and/or the entire region could be covered with a geotextile if a throat is discernible but the depth to rock is excessive. If there is no neck visible and the rock is too deep, the only practicable remedy is to remove soft or organic materials, cover the region with a geotextile, and then re-fill the hole with clayey soils.

The project site’s sinkhole and cavity issues, however, required a variety of treatments and corrective actions due to the unique site characteristics.

This can be grouped as:

i) during leveling and grading
ii) during foundation construction, and
iii) during external works construction, like roads, water supply, sanitary sewer, storm water drainage, and other items related to landscaping and irrigation works.

With the exception of a few, the majority of the sinkholes and cavities discovered during foundation construction were outside the footprints of the facilities. Large open-mouthed sinkholes of about 5m × 4m x 3m deep were discovered inside the footprint of some foundation areas. These were overexcavated to reveal the size of the cavities, and then compaction grouting was used to cure them. This innovative method, which is known as “the Grout injected with a slump less than 25 mm,” was developed in the USA.

Typically, a soil-cement mixture with large amount of silt particles to give plasticity and create internal friction is utilized in this. In most cases, grout does not penetrate soil pores; instead, it stays in a homogenous mass that allows for controlled displacement. Additionally, regular grout was employed to address the issue in this facility. Some of them were also backfilled with rock using a bridging technique.

Due to the short duration observed for a 20 cm probe penetration during a cavity search probe, there was suspicion of the presence of sinkholes or cavities in some other facilities. Therefore, more investigations were conducted to confirm the suspicions. Except for two sites, the time recorded for the additional probes in most cases did not record fewer than 10 seconds.

Deep excavations have to be done for the service pits and hydraulic lift pits at some of the facilities. These extra probing were nearby and had time records of less than 10 seconds, which suggested the presence of cavities. Therefore, it was determined to over-excavate the limited area in order to explore the underground caverns.

The cavities were exposed during excavation, along with several lateral solution channels. These were completely cleaned before being treated using two different techniques:

(i) Compaction grouting of the vertical cavities, and
(ii) Bridging utilizing boulders and geotextiles to seal the mouth of the lateral solution channels.

Foundation Design Considerations

Even with the best techniques and designs, construction in karst terrain is unquestionably not risk free, according to experts on the issue. However, the risk they pose justifies the search for a remedy that lowers the risks at a reasonable cost.

The solution in vogue are outlined as follows;

• Optimize the location on the site
• Treat defects that are present
• Use modified shallow foundations
• Use deep foundations
• Minimize future activation

For medium-sized buildings, the use of modified shallow foundations is a practical solution to the problems of Karstic hazards.

Utilizing such foundations involves:

(i) constructing a footing that spans or bridges over the cavity; and 
(ii) constructing a mat foundation that is rigid enough to minimize deflections that may occur due to Sinkhole formation beneath it.

The proposal accepted for the project site is a semi-rigid raft foundation. It has thicker borders or raft bands and resembles a mat. This was implemented in the project to get around the site’s sinkhole and cavern issues. This can span the cavities because it is sufficiently stiff. The same was analyzed and designed using PCA-MATS. A bearing capacity of 150 kN/m² was used to design the foundations in accordance with the Geotechnical Report. Given the rock’s worn state and the karstic terrain, a relatively low figure of bearing capacity was advised.

Conclusion

Sinkholes are geologic hazards that can pose enormous risk to the safety of infrastructures. Extensive geotechnical site investigation is required when there is a possibility of the existence of sinkholes and cavities in an area. In a report presented by Syed (2004), a Karstic terrain was encountered in a project site in Al Hofuf, Saudi Arabia.

The detection, delineation, and mapping of bedrock cavities, sinkholes, dropouts, and solution channels were the subject of a thorough investigative program. Under the footprints of 166 facilities in 6 zones, 316 boreholes, 10 test pits, and 5610 cavity probe holes were drilled. By employing a Wagon Drill to pneumatically drive a rock probe, the cavity was probed. As time went by, compressed air was utilized to clear the hole.

The site’s karstic features were mapped, and remediation efforts were made. The majority of the sinkholes and voids were filled in using grout, with the use of geotextiles in other locations. A novel method of compaction grouting was used to fix some of them. Some of them were also “plugged” with rock utilizing the bridging method. By creating masonry and/or concrete structures, the voids and lateral solution channels in utility trenches were sealed. The roadways’ ditches were paved.

The Site’s treated karstic features are functioning effectively, and for the past four years or more, no problems have been reported. A hard and shallow style of foundation called a semi-rigid raft foundation was employed. Such footings are suggested by experts because of their capacity to span the cavities.

Reference Article:
Syed, Ahmad Faiz, “Managing Sinkholes at Project Site, A Saudi Arabian Case History” (2004). International Conference on Case Histories in Geotechnical Engineering. 20. https://scholarsmine.mst.edu/icchge/5icchge/session06/20

Many civil engineering projects usually involve excavating into water-bearing soil formations. Before beginning such excavations, the proper system(s) for managing and controlling groundwater and surface water run-off should be planned. This can only be done in practice if you are aware of the ground and groundwater conditions you are likely to face through site investigation data.

It is necessary to take precautions to manage groundwater flows and pore water pressures in water-bearing soils in order to prevent problematic circumstances during excavation and construction. Effective management of surface water runoff is also necessary. Understanding the potential effects of an excavation can help determine which groundwater control measures are required to assure stability.

There are three groups of methods available for temporary works control of groundwater:

(a) Lowering groundwater levels in construction by means of water abstraction, in other words – groundwater lowering or dewatering.
(b) Exclusion of groundwater inflow to the area of construction by some form of very low permeability cut-off wall or barrier (e.g. sheet-piling, diaphragm walls, artificial ground freezing).
(c) Application of fluid pressure in confined chambers such as tunnels, shafts and caissons to counterbalance groundwater pressures (e.g. compressed air, earth pressure balance tunnel boring machines).

Groundwater Control Techniques

Techniques for the control of groundwater can be divided into two principal types:

(a) Those that exclude water from the excavation (known as exclusion techniques)
(b) Those that deal with groundwater by pumping (known as dewatering techniques)

Exclusion Methods of groundwater control

The aim of groundwater control by exclusion is to prevent groundwater from entering the working area. Creating a physically impermeable cut-off wall or barrier around the perimeter of the excavation to keep groundwater out is one of the most widely used exclusion methods. The cut-off often produces a basal seal for the excavation by penetrating vertically down to a very low permeability stratum.

The depth and make-up of any underlying permeable stratum have a significant impact on the costs and viability of constructing a physical cut-off wall. Base instability is possible when upward seepage occurs beneath the base of the cut-off wall due to the lack of or presence of a sufficient very low permeability layer. In these situations, dewatering techniques and exclusion techniques may be combined.

As an alternative, a horizontal barrier or “floor” might be formed adjacent to the cut-off structure to stop vertical seepage. Although it is not common, horizontal barriers have been built utilizing techniques such jet grouting, mix-in-place grouting, and artificial ground freezing.

A portion of the groundwater will become trapped inside the working area if a complete physical cut-off is established. In order to move further with the project, this must be removed, either by sump pumping during excavation or by pumping from wells or wellpoints beforehand.

The capacity of the exclusion technique to enable work to be carried out below the groundwater level with little impact on groundwater levels outside the site is one of its attractive features. This ensures that any groundwater-lowering problems are prevented. Exclusion techniques are frequently employed instead of dewatering techniques, especially in metropolitan areas, to reduce the danger of settlement damage brought on by reducing groundwater levels.

However, it is crucial to remember that practically all walls will leak to some degree when considering the use of the exclusion technique to prevent groundwater level from dropping in areas beyond the site. Particularly vulnerable to leakage are any joints (between columns, panels, piles, etc.) left behind from the installation process.

Several issues can arise if groundwater leaks through cut-offs into the excavation or work area:

(i) The seepages may hinder site operations during construction, necessitating the deployment of sump pumps or surface water management techniques to keep the working area dry.
(ii) The risk of settlement or other negative impacts may result from the leaking into the excavation being severe enough to locally lower groundwater levels outside the site.
(iii) If the cut-offs are a permanent construction, like the walls of a deep basement, even tiny seepages over time will be ugly and could interfere with any architectural finishes that have been applied to the walls.

Grouting or other treatments can frequently be used to address the major seepages that cause issues (i) and (ii). On the other hand, it might be quite challenging to stop or stop the little seepages of (iii). Costs significantly increase if a completely dry or leak-proof construction is required. If cut-off walls are going to be included in the permanent works, this is something that needs to be taken into account.

The techniques used in groundwater exclusion are listed below;

Steel Sheet Piling

In this groundwater control technique, steel sheet piles are driven into the soil to form a barrier against the intrusion of groundwater into the construction area. This is one of the most prominent techniques used in the construction of cofferdams. This technique is applicable to most open soils, however, it can be challenging when obstructions such as rock boulders are encountered.

Steel sheet pile walls may be installed to form permanent cut-off, or used as temporary cut-off with piles removed at the end of construction. They offer the advantage of rapid installation and are relatively cheap. Additionally, they can support the sides of the excavation with suitable propping.

The disadvantages are that the seal may not be perfect, especially if obstructions are encountered. Vibration and noise of driving may be unacceptable on some sites, but ‘silent’ methods are available.

Vibrated beam wall

In this method, a grout injection nozzle is driven into a specially made wide flange beam section using a vibratory driver-extractor that is attached to the beam’s base. A self-hardening slurry is injected into the ground with the designed beam while it is vibrated into the ground to act as a lubricant.

When the beam is withdrawn, the extracted beam element leaves a minimum of a 4 to 6-inch panel that is filled with the self-hardening slurry. A continuous cutoff wall is created by the consecutive beam element insertions and the overlapping of the earlier beam insertions.

The Vibrating Beam construction technology enables operations in small spaces with little room for above-ground mixing or staging. This method also decreases soil disposal costs, which can be expensive when dealing with hazardous locations, as less excavation is needed. Permeabilities in the range of 10-8 cm/sec are capable of allowing for depths greater than 50 ft.

This method is applicable in open excavations in silts and sand but will not support the soil.

Slurry trench cut-off wall

A slurry cut-off wall or slurry trench wall is an excavation made deep into the ground while simultaneously pumping an engineered slurry mix into the trench. A permanent low permeability barrier to groundwater and leachates is created by the slurry cut-off wall after it is completed. Slurry trench cut-off walls also have the capacity to prevent the transportation/movement of a range of heavy metals and organic contaminants including volatile organic compounds (VOCs), hydrocarbons, diesel, solvents and tar.

The slurry trench walls create a low permeability curtain wall surrounding the excavation and it is a permanent water exclusion system. Slurry trench walls are can be quickly installed and relatively cheap, but the cost increases rapidly with depth. It is very applicable in silts, sands, and gravel up to a permeability of about 5 x 10^-3 m/sec.

Structural Concrete Diaphragm wall

Diaphragm walls can be used as cut-off walls for dams or excavation pits, as foundations, or as enclosures for structures. A diaphragm wall is a structural concrete wall constructed panel by panel in a deep trench excavation using either precast or in-situ concrete pours.

Diaphragm walls can serve as retaining walls, water-cut-off structures, deep foundations, basement walls, or as separating structures for underground facilities. They are constructed as ground-level concrete or concrete reinforced with steel walls. They are thought to be almost water-impermeable and deformation-resistant.

Diaphragm walls are permanent structures that support the sides of the excavation and often form the sidewalls of the finished construction. They can be keyed into rock and have the advantage of minimum noise and vibration. However, high cost may make the method uneconomical unless walls can be incorporated into a permanent structure.

It is applicable in side walls and shafts in most soils and weak rocks but the presence of boulders may cause problems.

Secant (interlocking)and contiguous bored piles

A retaining wall made of closely spaced bored piles can be used to construct a deep basement or a cut-and-cover tunnel. The piles could be constructed so that they almost touch each other (contiguous). A watertight retaining wall can be created by grouting the spaces between the piles.

This method of construction results in a more effective form of structure when the piles interlock. The piles will typically need propping during soil excavation before the permanent floor and/or roof structure is finished.

Jet Grouting

In jet grouting, a stabilizing fluid is injected into the subsoil (or the soil being treated) under high pressure and high velocity. The high-velocity fluid jets are used to create different geometries of cemented soil in the ground and typically form a series of overlapping columns of soil/grout mixture that can prevent the movement of groundwater.

Jet grouting can be messy and create large volumes of slurry. There is a risk of ground heave if not carried out with care. Jet grouting is Relatively expensive and applicable for open excavations in most soils and very weak rocks.

Deep soil mixing columns

In this permanent water exclusion solution, overlapping columns are formed by in-situ mixing of soil and injected grout/cement using auger-based equipment. Soil mixing can be categorised into deep soil mixing and shallow soil missing. The mixture of binding agent and natural soil produces columns with very low permeability. This approach produces little spoil, and it is less flexible than jet grouting. It is relatively expensive.

Injection grouting using cementitious grout

Injection grouting is a process by which grouts are injected under pressure into open fissures, voids, cracks, and pores in a soil/rock mass. The grout fills the pore spaces, preventing the flow of water through the soil.

Equipment is simple and can be used in confined spaces. A comparatively thick zone needs to be treated to ensure a continuous barrier is formed. Multiple stages of treatment may be needed. The procedure is applicable to tunnels and shafts in gravels and coarse sands, and fissured rocks.

Artificial ground freezing

Ground freezing is a temporary water exclusion solution during construction. In this solution, a ‘wall’ of frozen ground (a freeze wall) is formed using brine or liquid nitrogen, which can support the side of the excavation as well as exclude groundwater.

This approach may not work if groundwater flow velocities are excessive (> 2 m/day for brine or 20 m/day for liquid nitrogen). Liquid nitrogen is expensive but quick; brine is cheaper but slower. Liquid nitrogen is to be preferred if groundwater velocities are relatively high. Plant costs are relatively high.

Compressed air

In this temporary groundwater control solution, increased air pressure (up to 3.5 bar) raises pore water pressure in the soil around the chamber, reducing the hydraulic gradient and limiting groundwater inflow. Potential health hazards to workers. Air losses may be significant in high-permeability soils. High running and set-up costs.

It is suitable for confined chambers such as tunnels, sealed shafts and caissons.

The level of risk associated with foundation or slab movement at a site is determined by the expansion potential of soil or sedimentary bedrock formation. Any soil or rock element that has the ability to expand in volume with increasing water content is often referred to as expansive soil.

The volume of soil decreases as a result of consolidation, which is a process that forces water out of the pores and fills them with soil particles. However, during swelling, water is absorbed into the soil, which is the reverse of consolidation, in that it forces the soil’s particles apart and causes an increase in volume. This swelling may result in issues very similar to differential settlement by lowering the soil’s ultimate bearing capacity and shear strength.

Swell and swelling pressure can result from elastic restitution after a load is removed, from water adhering to the surface of soil particles, or from the particles expanding as a result of the adsorption of water into the soil particles. Of these, the adsorption of water is the most common. To comprehend the phenomenon of adsorption, a basic understanding of clay mineralogy is required.

The soil structure, dry density of the soil, the initial moisture content, and the availability and characteristics of water are all factors that affect swelling properties in addition to soil composition.

Effect of Soil Structure on Expansion Potential of Soil

The way soil particles interact will depend on their orientation within the soil mass and their distance from one another. The particles may obtain different degrees of orientation depending on the circumstances present during the deposition.

The particle orientation for flocculated and dispersed soil structures is shown in Figure 2. The figure exemplifies the extremes of totally flocculated and fully dispersed soil structures. Particle orientation in most soils would fall between these two extremes. It is convenient to think about the structures that are exhibited while considering the effects of density and soil structure on expansion potential.

It is obvious that the interactions between micelles in a flocculated structure, such as the one in Figure 2a, are principally driven by the contacts between the ends of the particles and the faces of adjacent ones. In contrast to the dispersed structure, the flocculated structure has a greater gap between particles.

It follows that in contrast to the dispersed structure shown in Figure 2b, crystalline and osmotic swelling would be less effective in the flocculated structure. The soil structure would lean more toward the dispersed structure for highly overconsolidated clays that have been subjected to substantial overburden stresses.

Effects of Water Content on Expansion Potential

Swelling won’t occur until there is free water available. Studies have revealed that depending on the type of ions in the water and whether or not there are ions already adsorbed on the soil particles, the electrolyte concentration of the water may change the swelling characteristics. The soil’s expansive properties could be significantly changed if the ions in the water have the capacity to replace the ions that have been adsorbed there.

The expansion potential will also be influenced by the distance between particles, the hydration states of the cations, and particle orientation. As a result, it would be expected that soil with a high dry density and low initial water content would have a higher expansion potential than soil with a lower dry density and a higher initial water content.

Chen (1973 and 1988) conducted oedometer experiments on samples that had been compacted to the same initial density but with different initial water contents. The results of the findings are shown in Figure 3. It is obvious that the initial water content significantly influenced the percentage swell.

Chen (1973 and 1988) also performed oedometer tests on samples with different dry densities and the same initial water content. Figure 4 shows these findings for percentage swelling and swelling pressure. They demonstrate quite clearly that both the percentage swelling and the swelling pressure were significantly influenced by the initial density. As a result, the expansion potential of the soil increases with initial soil moisture content and soil density.

Effects of Dry Density on Expansion Potential

The expansion potential for a remoulded soil sample increases with the density to which the sample is compacted. Similar to this, the swell potential decreases as water content increases.

Dry density is one of these parameters that is crucial. This is explained by taking into account how many soil particles there are in a given volume. The surface area accessible for the adsorption of water will increase with the number of particles in a unit volume, increasing the potential for swelling.

It was observed during laboratory studies that when the degree of compaction increases, swelling in the presence of water at a certain confining pressure increases. This can be attributed to the fact that more clay particles would occupy the same volume due to the greater compaction.

Studies with compacted clays have shown that soils compacted on the dry side of the optimum moisture content are likely to have a flocculated structure, whereas soils compacted on the wet side of optimum moisture content tend to have a dispersed structure, suggesting that expansion potential may be attributable to soil structure. Although soil structure may contribute to the mechanisms causing swelling, it’s possible that the initial moisture content at which the soil is compacted is more significant.

The capacity of the soil to absorb water is closely correlated with swelling potentials. It is clear that swell potential depends on the initial moisture content because the mass can only absorb a given amount of water. Compared to samples of the same density compacted wet of optimum, samples compacted dry of optimum exhibit higher swelling characteristics and swell to larger water contents.

Geotechnical engineering is a branch of civil engineering concerned with the engineering behaviour of soils. Geotechnical engineers apply the principles of soil and rock mechanics to the design of the foundation of all structures and infrastructures resting on the ground, and the use of geomaterials (soils and rocks) for construction. They are also interested in ground improvement, and protection of the soil and groundwater from pollution. Geotechnical engineering is also known as soil engineering.

Geotechnical engineers, therefore, design the foundations of buildings, bridges, dams, embankments, highways, earthworks and deep excavations,  natural and man-made earth slopes, landfills, retaining walls, tunnels, cofferdams, etc. In practice, they work closely with structural engineers, highway/transportation engineers, geologists, water resources engineers, etc.

Soil is a product of nature with so many different properties. Some soils are firm and stiff enough for construction purposes, while some soils are weak and marginal, hence unsuitable for construction purposes.  

The loads from all structures are ultimately transferred to the ground, and it is expected that the ground should be able to withstand the load without undergoing excessive settlement or shear failure. Furthermore, when geomaterials are to be used in civil engineering constructions such as highways, embankments, earth dams, etc, the material selected should have specific engineering properties that will prevent premature failure of the construction. It is the job of geotechnical engineers to recommend suitable materials or design the stabilisation/modification of existing materials.

During the design of foundations, the size and type of foundation can be altered to ensure that the pressure on the soil is within allowable limits. This is essentially the job of geotechnical engineers.

In some cases, the load coming from the structure may be too high, or the soil near the surface may be too weak, such that the geotechnical engineer may recommend a deep foundation such as piles. Using the geotechnical soil investigation report, geotechnical engineers recommend the depth, size, type, and allowable load on the pile. 

In essence, geotechnical engineers perform geotechnical analysis to assess site condition, perform field and environmental investigations, plan and conduct geotechncial exploration, review construction design proposls and approve geotechnical aspects.

A lot of theories are applied during the design of foundations and other earthen structures, and a geotechnical engineer combines them with soil investigation findings to decide on the most suitable foundation. 

Some important topics of geotechnical engineering are;

(1) Shear strength of soils
(2) Compressibility and consolidation
(3) Stresses in soil
(4) Phase relationship and physical properties of soils 
(5) Compaction
(6) Characterisation/Classification of soils

These topics form the basis of many geotechnical engineering design concepts such as foundation design, retaining walls design, stability of slopes, dam engineering, highway construction etc. Other important topics in geotechnical engineering are the flow of water through soils (hydraulic conductivity), soil dynamics, ground improvement, etc.

Geotechnical engineers are also interested in the improvement of marginal soils such as expansive soils, collapsible soils, etc in order to improve their engineering properties. 

Depending on the findings of the geotechnical soil investigation report, overexcavation and replacement of the weak soil with a more suitable material may be recommended. Other solutions can include, stabilisation of the soil with cement, lime, pozzolans, agricultural and industrial wastes, etc. Fibres and geotextiles are also used in soil stabilisation and improvement.

Other methods such as the use of vertical sand drains, deep soil mixing, stone columns, dynamic compaction, micro piles, and grouting can be used to densify weak soils and reduce settlement potential. 

Therefore, the practice of geotechnical engineering relies heavily on soil investigations which usually include laboratory and field testing of soil and rock samples. The results obtained from laboratory investigations are then used to carry out design with an appropriate factor of safety.

Geotechnical engineers are found in private consultancy, in academia, as borehole experts, construction industry, the oil and gas industry, the mining industry, military and warfare, water resources companies, etc. Their services can include; foundation design, design of earthen structures, groundwater extraction consultancy, verification of designs, design of machine foundations, structural design of highway pavements and embankments, ground improvement, etc.

Geotechnical engineers usually have civil engineering as their first degree, and may go ahead to obtain post graduate degrees in geotechnical engineering. They must also be registered/licensed to practice engineering in their country or state. This involves passing all the professional exams and interviews involved in the qualification process.

Some of the aspects of geotechnical engineering that are distinguished as a result of research and practice are as follows:

• Transportation geotechnics
• Geo-environmental engineering 
• Energy geotechnics
• Rock mechanics
• Soil mechanics
• Foundation Engineering 
• Marine/offshore geotechnics

Professional Bodies in Geotechnical Engineering

Some notable geotechnical engineering professional bodies are;

• International Society of Soil Mechanics and Geotechnical Engineering (ISSMGE)
• Academy of Geo-professionals
• Canadian Geotechnical Society (CGS)
• Chinese Institution of Soil Mechanics and Geotechnical Engineering China Civil Engineering Society  (CISMGE)
• Indian Geotechnical Society (IGS)
• Japanese Geotechnical Society (JGS)
• Kazakhstan Geotechnical Society (KGS)
• Korean Geotechnical Society (KGS)
• Mexican Society of Geotechnical Engineering (SMIG)
• Russian Society of Soil Mechanics and Geotechnical, and Foundation Engineering (RSSMGFE)

Since ancient times, people all over the world have practised the art of strengthening earthen structures with brushwood, bamboo, straw, and other materials of a similar nature. The additional strength added by such reinforcements gave birth to mechanically stabilised earth (MSE) walls and structures.

In the year 1963, a French architect and engineer named Henri Vidal was successful in obtaining a patent for a general configuration of applying the aforementioned principle to the construction of embankments. It was adopted for retaining walls, bridge abutments, dams, foundations, and several other applications on a global scale.

The following three fundamental elements make up the patented general configuration, as shown in Figure 1.

1. The earthfill (often chosen granular materials passing US sieve no. 200 with less than 15%).
2. Soil reinforcement. Currently, this takes the shape of metal strips, geotextiles, or wire grids that are attached to the facing unit and extend enough into the earth backfill. To reduce the length of the anchorage length required, special end anchorages can also be offered.
3. The facing unit (this is usually made of metal or concrete blocks made to maintain an aesthetic appearance of the structure and prevent soil erosion).

The Mechanism of MSE Wall

The friction generated at the soil reinforcement interfaces to stop the relative motion of the soil and reinforcement is an important component of the MSE mechanism. The reinforcements provide an apparent cohesion proportionate to the density and tensile resistance of the reinforcement and limit the lateral deformation of the reinforced earth mass.

Depending on the type of structure and the loading conditions, the greatest tensile force in the reinforcement occurs at a distance from the mechanically stabilised earth wall. The reinforced earth is separated into the two zones shown below along this line of maximal tensile force;

1. An active zone behind the facing where the shear forces are directed outward giving rise to an increase of tensile forces in the reinforcements.
2. A resistant zone, where the shear stresses are mobilized to prevent the sliding of the reinforcement which is directed towards the free end of the reinforcement inside the embankment.

Full-scale experiments and observations show that the behaviour of reinforced earth walls is quite different from that of classical retaining walls. The locus of the maximum tensile force in the reinforcement has been found to be different from Coulomb’s failure plane.

The remarkable features of mechanically stabilized earth (MSE) are the following:

1. Strength – It can resist significant earth pressure and seismic force.
2. Flexibility – They are flexible gravity structures. It adapts to substandard foundation soils and large settlements. (This is one of its main advantages over rigid walls.)
3. Construction – It can be easily constructed by untrained labour.
4. Low costs – Costs are low.
5. Aesthetic factors – It has good appearance as the facing can be made of attractive designs.

Design of Mechanically Stabilised Earth Walls

Mechanically stabilised earth walls should be designed against the following types of failures:

1. Tension failure of the reinforcement in the earth (internal stability)
2. Bearing capacity failure of the base (external stability)
3. Sliding of the whole block ABCD along the base (external stability)
4. Overturning and tilting under the horizontal earth pressure acting on the mass.

Usually, the effect of surcharge in concentrated load is assumed to be distributed using the 2 vertical to 1 horizontal distribution as in Figure 3.

The following two types of design methods are used for determining the tension in reinforcements:

1. The working stress method
2. The failure plane method (limit state method).

In the working stress method, we assume the surface of maximum tension in the reinforcement based on experimental values and work out the necessary anchorage length required for the soil reinforcement. It is used as a general case to deal with all types of reinforced earth walls.

In the failure plane method, we consider the equilibrium of several wedges along a potential failure plane and estimate the tension to be developed in the steel reinforcement. We then design for maximum tension. As the working stress method is the more popular and general method, we will discuss only this method in this chapter. The conservative Coulomb’s failure surface is also assumed as the surface of failure plane.

Reinforcement Design of Mechanically Stabilised Earth Wall

The design procedure is as follows for a retaining wall without any surcharge:

Step 1: Adopt vertical and horizontal spacing of the reinforcement to be used. It may range from 0.2 to 1 m vertically and 0.7 to 1m horizontally.

Step 2: Assume the locus of maximum stress in the soil reinforcements. Different authorities have suggested different locus as shown in Fig. 26.2. The most commonly used is the conservative Coulomb’s failure plane.

Step 3: Determine the magnitude of horizontal pressure at various depths due to earth pressure as well as due to any superload on the embankment. Usually, the active earth pressure distribution is assumed and the pressure due to superload may be approximately evaluated by using 2 vertical to 1 horizontal distribution.

Step 4: Take each reinforcement strip and determine the maximum tension that will be developed in it. Let it be T_i. Thus, take the reinforcement strip at depth z from the top. The friction f developed will depend on the pressure on the strip and the coefficient of friction f = γztanδ.

If b is the width of the strip assuming that friction acts on both faces, the anchorage length La required to anchor T_i will be as follows:

L_a = T_i/2bf = T_i/(2bγztanδ)

where;
T_i = tension in soil reinforcement
b = breadth of the reinforcement
γz = pressure at depth z
f = friction developed between soil and reinforcement = tan δ
Ti = (γgK_A) × (area of influence of reinforcing strip)

For each strip, find L_a, the anchorage length required to develop the corresponding anchorage.

(Note: In the field, for easiness of fabrication and installation, all the strips are made of the same length equal to the required maximum anchorage length.)

Soil Reinforcement Selection

It is important to choose the MSE reinforcement carefully. Low-creep materials are necessary for rigid structures like retaining walls. Materials with some creep may be advantageous for embankments that may consolidate over time. Nonmetallic reinforcements are weaker than metallic reinforcements like galvanized steel and stainless steel (used to reduce corrosion).

Corrosion resistance is a benefit of plastics. Plastics come in both fabric and nonfabric forms. Fabrics are created by weaving or knitting textiles. Grids and strips make up non-fabrics; the latter is occasionally strengthened with glass fibre. In any case, these reinforcements should be chosen only after a thorough examination of their strength, creep, and durability properties. Based on the outcomes of laboratory tests provided by the manufacturer or by accredited laboratories, they should base their decision.

Reference:
Varghese P. C. (2012): Foundation Engineering. PHI Learning Private Limited, New Delhi