Overview
This article is intended to describe the current state of drilling applied to geotechnical site hazard assessment. The article examines hazards such as landslides and slope stability, potential for seismic events, liquefaction, contamination, settlement and subsidence and how drilling data can be used to assess them and design potential structures accordingly.
1 Introduction
Drilling has historically been one of the most effective site investigation methods used to assess geotechnical hazards. This article aims to examine some of the most common geotechnical hazards and identify how drilling can be used as an accurate hazard assessment tool, leading to cost-effective risk mitigation.
The hazards described in the following section were identified through surveyed consultation with the US Department of Transportation “geotechnical personnel and corroborated by industry geotechnical engineers” [1]. This survey was part of the National Cooperative Highway Research Program (NCHRP) effort to identify sources of geotechnical risk in design-build projects. While the list of hazards was initially developed with this goal in mind, these hazards could apply to any project requiring geotechnical design.
2 Hazards
This section aims to provide an overview and description of some common geotechnical hazards and how drilling data can provide value when assessing these hazards. It is important to note that “hazard” refers to the actual potential source of harm, whereas “risk” refers to the consequences of the hazard occurring combined with the probability of the hazard occurring.
2.1 Landslides and Slope Stability
Landslides were ranked as one of the primary hazards in the NCHRP survey described above and are defined by the US Geological Survey as “the movement of a mass of rock, debris, or earth down a slope” [2]. Landslides can be further broken down into different categories depending on the type of movement and the material composition of the slide. These different landslides are presented below and have various slope stability implications and geotechnical design considerations.
Table 1. Abbreviated version of Varnes classification of slope movements [2]
In rock, oriented core drilling can identify potential planes of weakness in the rock mass, including prominent features like faults missed when rock mapping in the field. Performing various lab tests on the sampled rock core is an easy way to get an idea of the intact rock strength, which, combined with other factors, allows a geotechnical engineer to classify the type and probability of potential landslides and design accordingly.
Slopes composed primarily of soil will act very differently depending on the predominant grain size of the slope. This variation in landslide type also becomes a significant source of risk due to the potential consequences from one type of landslide to the next. For example, implications for a high-velocity loose rock debris flow event are extreme compared to a low-velocity earth flow event. Any type of drilling that allows for the understanding of the grain size of the soil is instrumental in classifying the potential landslide hazard, mainly if the driving mechanism of the slope failure may be caused by a material that is not possible to identify through field mapping. Similar to rock slopes, samples can be taken in a soil slope to determine parameters such as cohesion and friction angle that significantly affect whether the slope is likely to fail.
In a slope that contains both rock and soil, drilling can be used to delineate the extent of the rock and create an approximate model of the slope that can be analyzed. Geotechnical engineers can input this model into software to determine an appropriate factor of safety for design. Advantages of slope modelling include the ability to test different materials and structural configurations to get an idea of how they would perform in a given slope. Software models are typically based on parameters that can only be obtained through in-situ testing facilitated by drilling and sampling.
2.2 Seismic Risk
Site seismicity is one of the most important factors to consider when assessing the hazards of a site, as it is fundamental in determining the type of foundation necessary for a given structure. This hazard is more frequently a concern in coastal areas or areas otherwise susceptible to high magnitude earthquakes.
In Canada, the seismicity of a particular site is subdivided into different site classifications, known as “site class”, that goes from A to F, with site class A being the most favourable for structural design. The table below presents each site class and the material properties associated with their classification.
Table 2. Site class definitions from ASCE 7-02 and ASCE 7-05
As shown in Table 2, the parameters that contribute to the site classification: soil shear wave velocity, standard penetration resistance, and undrained shear strength. These parameters are most commonly obtained through drilling, sampling and in-situ testing. This classification is only applicable if 30 metres of soil or rock data is available.
One in-situ method to obtain shear wave velocity data is through the use of downhole seismic testing, which involves placing a geophone at discrete depths in drill casing and using a hammer to create shear and compression waves that travel to the geophone. This simulates the effects of an earthquake on the soil and allows for an estimation of the shear wave velocity of the underlying soil or rock. This information can also be obtained through seismic cone penetration testing, which is a similar concept but is limited to relatively soft soils. Non-destructive geophysical testing can also be completed to get an idea of the shear wave velocity profile but is not always reliable since the interpretation, and associated results are based significantly on the assumed ground conditions.
Standard penetration testing is an in-situ testing method that uses a drop hammer to advance a sampling device known as a split spoon to a specified depth. The amount of hammer blows it takes to advance the sampler specific depth intervals is referred to as the standard penetration resistance known as the N value or blow count. This is a very versatile testing method and can be used in combination with most drilling methods while also providing a disturbed sample. Downsides of this testing method include its inability to provide an accurate blow count if the soil has a significant variation in grain size (like sand mixed with cobbles). The sampler also will not advance through rock unless it is very soft. These limitations are shown in Table 2.
The undrained shear strength of the soil is also not applicable to rock unless it is very soft and does not apply to coarse-grained soils. A relatively accurate representation of the undrained shear strength is typically obtained in-situ through vane shear testing. This testing method involves placing a metal vane through a drill casing into the subsurface and measuring the amount of force it takes to shear the soil. Undrained shear strength can also be approximated in a lab using a direct shear test. This is only possible if the sample used for testing is relatively undisturbed. Undisturbed samples can be obtained using a Shelby tube or a sonic drill sampler. However, they are not always representative of the in-situ conditions of a site. Cone penetration testing can also be used to obtain a reasonably accurate idea of the in-situ undrained shear strength but is also limited by the assumptions made when correlating the data.
2.3 Liquefaction
According to the US Geological Survey, liquefaction occurs when “loosely packed, water-logged sediments at or near the ground surface lose their strength in response to strong ground shaking” [3]. What this means, in reality, is that the soil essentially becomes a liquid which, without proper design, can cause a structure to sink into the ground in the event of a large enough earthquake.
Since the possibility of liquefaction depends entirely on the presence of saturated, loose soils like fine-grained sand, drilling is an essential tool to assess this hazard. Drilling can be used to classify the soil and determine the presence and variability in depth to the water table through instrumentation installation. This is essential information, as the water table may vary in depth depending on the season or factors like the amount of precipitation in a given year. A water table near the ground surface would cause loose soil to be saturated, which would then be at risk of liquefying in a seismic event.
2.4 Contaminated Material
The presence of contaminated material is another hazard that could create a significant environmental risk to a project, depending on the ability of the material to reach sensitive ecosystems. Sensitive ecosystems that could potentially be affected by a contaminated material typically include bodies of water like lakes or swamps. However, particular attention must be paid to flowing water like streams, rivers or groundwater, which have the potential to carry a contaminant to multiple ecosystems at a time. This can be highly destructive, depending on the type and quantity of the contaminant. Some examples of potential contaminants include mine waste tailings and petroleum products like oil, gasoline or diesel fuel.
Drilling can be used to assess the potential impacts of a contaminant spill in most scenarios by allowing an engineer to determine the ability of a substance to move through the subsurface. By understanding the soil composition and instrumentation like piezometers, an engineer can determine the direction and velocity of groundwater flow in a particular area. With this information, the engineer can then predict how long it would take for a contaminant to reach a sensitive ecosystem that allows for mitigation through techniques including barriers, silt fencing, or stream re-routing.
2.5 Settlement
A significant hazard to consider when designing a structure that needs to be supported on soil is the potential settlement caused by the presence of highly compressive or weak soils. Examples of these soils include soft clays, organic silts or peat. The depth of these materials has substantial cost implications for a project as it may not be feasible to complete a traditional foundation design. Settlement can occur in the structure but also in adjacent structures if the design is not sufficient.
Drilling allows a geotechnical engineer to assess the type, depth and thickness of these problem soils and make recommendations for foundation options that reduce or eliminates the likelihood of settlement. Some standard mitigation measures for this hazard include excavation and replacement, the use of deep foundations like piles or caissons, geofoam and raft slabs. It may also be necessary to support an excavation using shoring to reduce the settlement risk of any adjacent structures.
2.6 Subsidence due to Subsurface Voids
Subsurface voids can be present in rock or soil at a site for various reasons, including the type of material, the site’s geologic history, and the presence of flowing groundwater causing subsurface erosion. For example, karst formations occur when a soluble rock like limestone mixes with water, creating underground voids that grow over time. Sinkholes are extremely common in areas known for karst topography and create significant geotechnical challenges when designing structures in these areas.
Another example of a situation that could lead to subsurface voids will be if rock or a hard, fine-grained material (like clay) overlies an erodible material like sand and gravel with water flowing through it. The flowing water could lead to pockets of erosion in the sand and gravel, thereby creating voids that could eventually cause subsidence if the overlying soil or rock were to fail.
The presence of subsurface voids, particularly in karst environments, is a hazard that is often difficult to address and mitigate. In rare circumstances, it can lead to a site being deemed not feasible to develop. Drilling and sampling could also be used to determine the type and thickness of the material, which is particularly useful in karst environments where the thickness of soluble bedrock governs the design of a structure. A geotechnical engineer can also use drilling to determine the presence of fractures that could act as a mechanism for water to travel through and cause erosion or dissolution of a soluble rock mass.
2.7 Underground Debris
On some sites, there is a potential for the subsurface to consist of debris, whether artificial or natural. Examples would include old landfills or deposit sites for material waste like blast rock or debris from logging. To properly design a structure or development in an area that has a potential for debris, it is essential to understand the composition of the material and the extent of where the material could potentially be. Many drilling methods are acceptable for assessing the type, quantity and extent of this geotechnical hazard. However, sonic drilling is particularly useful in this scenario as it can get through most materials and provide a relatively undisturbed sample. While there is often still a risk of not identifying all of the material types present at one of these sites, drilling provides the data needed to design the foundation of a potential structure accordingly by delineating the thickness of the debris.
3 Conclusion
Drilling combined with in-situ testing through instrumentation is one of the most effective methods for assessing geotechnical site hazards. With drill rigs constantly improving in their ability to access remote sites in various environments, the potential for continued hazard identification, assessment and associated project risk mitigation is constantly expanding. New in-situ technology continues to be developed for use with drill rigs, providing geotechnical engineers with access to much more subsurface data than was ever possible in the past.
4 References
[1] D. D. Granberg, “Guidelines for Managing Geotechnical Risks in Design-Build Projects,” National Cooperative Highway Research Program, Iowa, 2017.
[2] US Geological Survey, “Landslide Types and Processes,” 2004. [Online]. Available: https://pubs.usgs.gov/fs/2004/3072/fs-2004-3072.html. [Accessed 2021].
[3] US Geological Survey, “What is liquefaction?,” [Online]. Available: https://www.usgs.gov/faqs/what-liquefaction?qt-news_science_products=3#qt-news_science_products. [Accessed 2021].
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