Biocementation is an environmentally friendly, cost-effective alternative to imported granular fills, concrete, costly hauling of materials or export to a landfill. In-service performance of soils, foundations, and other earthen structures and their required maintenance is highly dependent on methods of stabilisation, ranging from expensive mechanical stabilisation to chemical processes.

As such, many alternative materials originating from bio-based sources are being explored as potential stabilising additives to improve weak subgrade soils (i.e., dispersive soils, erodible and collapsible soil, and soft or expansive clays). In order to prevent geotechnical failures induced by water infiltration and/or erosion, some relevant alternatives include the use of bio-derived enzymes, microorganisms, and polymeric additives.

Bio-based Innovations in Geotechnical Engineering

For sustainable road infrastructure development, various microorganisms and other bio-based materials, including secondary metabolites, enzymatic materials, and polymeric materials, have been studied as viable substitutes for traditional chemical stabilisers (Ikeagwuani and Nwonu, 2019). Thus, increasing the adoption of these bio-based materials and technologies in the building industry requires a deeper understanding.

The use of bacteria in bio-based admixtures produced through industrial fermentation processes is gaining popularity because of a number of positive aspects, including their quick biosynthesis rates, high yields of desired products, quick growth on cheap culture media that is readily available in large quantities, openness to genetic manipulation, and lack of pathogenic traits (Pei et al., 2015). By the outcomes of the microbial treatment of soil, at least eight different types of biotechnological processes associated with construction can be categorized as shown in Figure 1 (Ivanov et al., 2015).

Of these, two biological processes (as far as the fundamental characteristics of research techniques) stand out;

(a) bioclogging, the production of pore-filling materials through biological means, to significantly reduce the hydraulic conductivity of soil or porous matrix and,

(b) biocementation, the generation of particle-binding materials through microbial processes in situ so that the shear strength of soil can be increased.

This is further reinforced by DeJong et al. (2010) stating that these methods allow a holistic view and meaningful characteristics in the area of soil stabilisation. The micro-organisms best suited for these techniques (i.e., soil bio-clogging, biocementation, and bio-aggregation) are facultative anaerobic and microaerophilic bacteria, although anaerobic fermenting bacteria have been suggested (Ivanov et al., 2015).

Microbial Biocementation

One of the most researched techniques is microbial-induced calcium carbonate precipitation (MICCP), also known as microbial biocement, which occurs when biological action increases the pH of soils to create supersaturated conditions (Oyediran and Ayeni, 2020).

Recently, biocementation has gained attention as a practical method for soil improvement, notably because of its superior performance and environmental sustainability. For instance, an essential approach called microbially induced calcite precipitation (MICP) uses bacteria to create calcium carbonate precipitates and introduce “biocementation” between the sand grains (Cheng et al., 2013).

For instance, the alkalophilic soil bacteria Bacillus pasteuri uses the highly active urease enzyme to consume urea and break it down into carbon dioxide (CO₂) and ammonia (NH₃). In the presence of water, NH₃ is changed into NH₄⁺, while CO₂ equilibrates with carbonic acid, carbonate, and bicarbonate ions in a pH-dependent manner. The alkaline environment and carbonate needed for the reaction with Ca²⁺ and precipitation of calcite (CaCO₃) are provided by the rise in pH, as schematically depicted in Fig. 2.

Another significant but understudied example of biocementation and bioengineered soil structures is termite mounds (anthills). Ecologists, entomologists, architects, and soil chemists are very interested in these above-ground mounds, particularly those made by the termites known as Macrotermitinae that grow fungi.

According to Kandasami et al (2016), a critical component for termite societies with millions of individual termites is the stability of termite mounds, which are bioengineered granular ensembles. Termites are skilled engineers, as shown by an experimental investigation on the mechanobiology of mounds and mound soil of the fungus-growing termite Odontotermes obesus (Rambur). The physical and mechanical characteristics of mound soil were notably different from those of the nearby or “control” soil. Yet, the clay mineralogy of the mound and surrounding soils was the same. 

To build anthills, termites combine the finer soil fraction with their secretions to form boluses in the presence of water, thereby significantly altering the soil. In order to efficiently mould the soil, they control the amount of water close to the soil’s plastic limit when producing these boluses (Kandasami et al, 2016).

As a result of these processes, the strength of the soil can be increased tenfold as a result of the cementation caused by termites utilizing their excretions and/or secretions, which may not have been possible otherwise. The modification of the soil by termites reduces the likelihood of erosion and collapse of the mounds.

Studies have also shown that termites effectively bonded foreign objects, indicating that they have a variety of cementation skills. When compared to reconstituted soil, a slope stability analysis with intact mound soil showed a much higher safety factor for the mound.

Applications of Biocementation in Soil Improvement

According to Cheng et al (2013), the MICCP method is most efficient at a particle contact right as cementation begins, and it becomes less effective as cementation spreads outward around a particle contact. Due to the increased inter-particle interactions, reallocating the CaCO₃ crystals to two contact places (connection points) as opposed to one would be more beneficial. As a function of the particle radius squared, the contact stress also decreases simultaneously. As a result, smaller particles have two complementary advantages: improved MICCP and reduced particle contact stresses.

Ng et al. (2012) used the B. megaterium MICCP procedure to evaluate shear strength, and they observed that the ratio of treated to untreated shear strength rose from 1.40 to 2.64. Van Paassen et al. (2010) reported a minimum of 300 kPa compressive strength. Similar results were published by Nafisi (2019), who observed that the soil strength varies between 210 kPa and 710 kPa depending on the type of sand and carbonate mass.

Van Paassen et al. (2010) also observed a 60% reduction in the permeability of treated soils at less than 100 kg/m³ CaCO₃ precipitation; however, as solution concentration increased, calcite crystal clogged pore spaces and reduced permeability between 50 and 99% using 1 M cementation solution.

In triaxial testing, Cheng et al. (2013) reported that the mechanical behaviour of the bio-cemented sand increased the effective shear strength parameters (i.e., cohesion, angle of internal friction) with an increase in CaCO₃ concentration at all saturation levels. By increasing the frictional angle at a lower saturation level, the precipitated crystals improved the cohesion of coarse sand. Under the same saturation level and similar CaCO₃ content as fine sand, coarse sand showed a greater friction angle than fine sand.

Here is how the researchers addressed this: The primary consequences of tiny soil particles include:

MICCP method can be improved by;

(a) adding additional inter-particle contact points and
(b) lowering the stress per particle contact.

An exponential relationship between the UCS value and the CaCO₃ content of the treated soils, according to van Paassen et al. (2010), demonstrates that even though the same amount of CaCO₃ precipitated, the mechanical response can vary depending on the CaCO₃ precipitation/crystals’ mechanism of action. Additionally, the process requires significant urease activity in the microorganisms, and the mode of action (MICCP) may be strain-dependent (van Paassen et al., 2010).

Several documented research works show several Bacillus spp. have the ability to precipitate calcium carbonate with structural properties, but a limited selection of micro-organisms highlights the triaxial and California Bearing Ratio (CBR) tests for potential road/pavement applications (Table 1).

According to Mujah et al (2016), the factors affecting the bio-cementation of soils are;

• Temperature
• pH level
• Urease activity/bacterial concentration
• Degree of saturation
• The concentration of cementation solution

Limitations of MICP for soil biocementation

The fact that MICP can only be applied to particular soil sizes is one of its key disadvantages. The approach is currently only effective for treating sands with particle sizes comparable to 0.5-3 mm, according to a review by Fragaszy et al (2011). Hence, expanding the use of MICP for improving and stabilising fine-grained soils like silt and clay would be a huge challenge.

Furthermore, some researchers have conducted field trials and upscaled studies to test the viability of MICP for in-situ bio-cementation deployment in construction. For instance, van Paassen (2009) conducted an experiment to treat 1-100 m³ of sand in the lab and observed that, after MICP treatment, the strength of bio-cemented sand was greatly increased; however, distinct spatial variability was observed. The most important factor that still needs further focus is treatment homogeneity or the homogeneous distribution of CaCO₃ across the treated soil matrix from top to bottom.

Conclusion

The engineering, mechanical, and physical properties of biocemented soils can potentially be enhanced by MICP treatment. The intended uses of MICP include slope stabilization, settlement reduction, erosion management, soil self-healing, and prevention of liquefaction.

The benefits of MICP for soil improvement include cheaper costs compared to chemical grouting and other man-made materials, treatment dependability, and an overarching idea that encourages sustainability in tandem with future needs. Before the MICP method is directly applied in the field, further study should be concentrated on optimizing it at both the micro and macro levels.

Investigations must be done into specific issues such as medium flow and transport via heterogeneous media, treatment durability, mixing procedure permanence, and particle-level stratigraphic mapping. Further potential directions in MICP technology include the use of seawater as a salt alternative as well as self-healing in terms of pre- and post-shearing of biotreated soils after significant earthquakes and related aftershocks.

References

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Ikeagwuani CC, Nwonu DC (2019): Emerging trends in expansive soil stabilisation: A review. J Rock Mech Geotech Eng 2019;11:423–40. https://doi.org/10.1016/j. jrmge.2018.08.013.

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Mujah D., Mohamed A. Shahin M.A, and Cheng L. (2016): State-of-the-Art Review of Biocementation by Microbially Induced Calcite Precipitation (MICP) for Soil Stabilization. GEOMICROBIOLOGY JOURNAL http://dx.doi.org/10.1080/01490451.2016.1225866

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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