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Our world is developing fast. From mining projects to housing complexes to highways, major projects are constantly happening across the developed world. While geotechnical engineering is often not highlighted in the public eye, all projects that involve construction on the earth will require geotechnical input. With safety and building standards constantly evolving and improving, it is important that geotechnical design risks are being effectively understood and communicated so that well-informed design decisions are being made. The process for understanding these project risks begins with site investigations.
A geotechnical site investigation for design is the process of gathering available existing information, along with new information and compiling it to facilitate an understanding of the ground conditions at a particular project site. Drilling is one of the most common methods used to provide new data for geotechnical site investigations. One of the first places to understand the subsurface is historical water well drilling logs. It is clear that drilling is an incredibly versatile and necessary tool for geotechnical engineers and allows them to understand the subsurface better, identify potential hazards that could exist, and decrease uncertainty and project risk. Having more data allows an engineer to make more informed design decisions, particularly when working on highly complex ground conditions projects. While most geotechnical design concepts and decisions are made primarily with judgement and years of experience, drilling data fills in the gaps. This article explores the many drilling methods used today in geotechnical site investigations and describes the benefits, primary applications and considerations to make when choosing a drilling method.
2 Drilling in Overburden
Two main categories of material can be encountered in a drill hole, assuming the ground on the project site is native material: soil and rock. This section will focus on drilling in overburden (soil), one of the most common materials to find at a project site, depending on the geographic location.
2.1 Solid Stem Auger
Solid stem auger (SSA) drilling is one of the most common and cost-effective methods used in geotechnical investigations. The fast and easy setup time paired with the versatility in mounting on multiple rig types makes auger drilling a great choice for many projects. The following section will describe the process of auger drilling and how it works in practice, along with its benefits and downsides.
Solid stem auger drilling works using drill rods shaped like corkscrews to advance into the soil using rotation. Auger drill rods are usually about 1.5 meters or 5 feet in length and consist of multiple flights. The auger advances using the downward force from the drill rig combined with a clockwise rotation. Depending on the objectives of the geotechnical investigation and the soil type, it may be possible to obtain an undisturbed soil sample from the auger flights. When sampling, rather than spinning the augers counterclockwise to remove them, the augers are pulled directly out of the hole. As long as the soil is not too loose, this removes the entire soil column from the hole and allows the drilling inspector or engineer to pull samples directly off the flights (known as grab samples). A highly cohesive soil like stiff clay may require significant pulling power from the drill rig to remove the soil column for a sample. In some scenarios, the soil cohesion may prove too difficult to sample, and it will need to be rotated out, leaving the soil down the hole. Alternatively, very loose soil may fall off the flights, particularly if the desired sample is a few auger lengths downhole.
Solid stem auger drilling has benefits when considering the setup time and ability to move quickly between holes. The setup process depends on the type of terrain at the project site. Still, it often involves lowering the jacks (for stabilisation of the drill rig), raising the tower of the drill rig, and attaching the auger. Once those small steps are completed, auger drilling can begin. However, this drilling method does have a few downsides, including difficulty advancing through dense soils like gravel, cobbles, or glacial till. Sampling from an auger involves some interpretation, as it is sometimes unclear where material boundaries begin and end. As mentioned previously, it is sometimes difficult to obtain full recovery, and the samples are disturbed. Disturbed samples are great for field visual identification and can sometimes be used for moisture content testing in a lab. However, it is difficult to determine many engineering strength parameters from disturbed auger samples effectively. A common way of dealing with the lack of strength information is by completing some direct push method like dynamic cone penetration testing (DCPT) combined with the auger drilling. Direct push methods are explained in detail in the following sections.
In general, solid stem auger drilling is a very good choice for projects that contain softer soils and require the completion of multiple holes in a short period. While the sample quality is not as good as other methods and strength information is not as easy to obtain in solid stem auger holes, its efficiency and cost-effectiveness have made it one of the most common and well-known drilling methods in geotechnical site investigations.
2.2 Hollow Stem Auger
Hollow stem auger drilling is very similar to solid stem auger drilling and typically uses the same drill rig. As the name suggests, the drill rods are hollow but are otherwise the same as solid stem and contain flights. Using a hollow stem auger is to install various types of instrumentation and facilitate different sampling methods. These sampling methods and types of instrumentation, along with the general benefits of using hollow stem augers, are described in the following section.
As described in the last section, obtaining soil strength information in solid stem auger holes isn’t easy. One of the most common methods for addressing this problem is to use a standard penetration test in combination with hollow stem augers. This method works using a hammer dropped at a constant height and applying relatively constant energy to a drill rod attached to a sampler known as a split spoon. The number of hammer blows it takes to advance the sampler 0.3 meters, or 1 foot is the N value. This value has been correlated to multiple engineering strength parameters , including, but not limited to:
- Unit weight
- Relative density
- Shear wave velocity
- Friction angle
- Undrained shear strength
- Poisson’s ratio
A disadvantage to using split spoon samples in hollow stem augers is that they are highly disturbed. In addition, although SPT N values are useful, they require a lot of assumptions to correlate to the strength parameters listed above and are not always accurate. Many other methods can be used within a hollow stem auger cased hole, including Shelby tube sampling, vane shear testing, cone penetration testing, and downhole seismic testing. These methods are described in detail below.
Shelby tube sampling is inserting a thin-walled metal tube into the soil and removing it to obtain a sample. This sampling method returns an undisturbed sample, allowing for multiple types of lab testing, including triaxial and direct shear. Shelby sampling is usually completed in soft to firm clays, as it is not often possible to advance the sampler into granular soils.
Vane shear testing is typically completed in clay and uses small metal vanes to estimate the clay’s undrained shear strength value. The process involves inserting the vane into the soil and rotating it constantly, usually through an electric or manual mechanism. The measurement of the amount of torque it takes to rotate the vane in the clay allows an engineer to estimate the shear strength, while remoulding can estimate the clay’s sensitivity. This is an extremely useful test and one of the most effective ways to estimate the undrained shear strength of clay in the field.
Cone penetration testing (CPT) is an in-situ testing method that can be used on its own and combined with hollow stem auger cased holes. The benefit of performing a CPT in a cased hole is that it is possible to push deeper holes in more dense soil due to the rod support from the hollow stem casing. CPTs are an effective method of determining multiple strength parameters. The main parameters it records are tip resistance, sleeve friction and pore pressure. These values can be easily correlated to multiple useful engineering parameters, making CPTs an extremely useful in-situ testing tool for geotechnical investigations. Many different modules can be attached to a piezocone that allow for different testing types, including, but not limited to, resistivity, gamma radioactivity, seismic and ultraviolet-induced fluorescence.
Resistivity modules are used to determine the capacity for electrical current to flow through water in the pores of the soil and have applications in corrosion protection and identify the presence of contaminants.
Gamma modules are used to measure radioactivity (essentially a Geiger counter) and have applications in contamination detection and mineral identification. Some applications for this technology include contaminated landfill sites, tailings ponds and radioactive waste storage facilities.
Seismic CPTs are very common and make use of geophones in combination with an energy source (typically a sledgehammer and, in some cases, a truck-mounted hammer) to measure the travel time of seismic waves (shear and compression) to estimate the shear wave or compression wave velocity profile of the soil. The typical setup for a seismic CPT is to place a beam on the ground directly below the drill rig and advance the cone. The cone is then stopped in regular increments to record a seismic measurement, which involves hitting the hammer (or other energy sources) on the ground and recording the time it takes for the wave to reach the geophones in the cone. Through trigonometry, travel times, and various assumptions about the soil type (which come from the regular CPT data), a shear wave velocity of the soil can be estimated.
Cone penetration tests do have downsides in that they are a direct push testing method and cannot be pushed through rock or dense cobbles. In some cases, a CPT can be pushed through gravel, but it is very easy to damage the cone and usually very costly to repair or replace. While many drilling companies are beginning to adopt cone penetration testing, it is a fairly new technology and typically is completed by only a few companies specialising in it.
Downhole seismic testing is a method of obtaining soil’s shear wave velocity profile to facilitate seismic design in geotechnical engineering projects. This in-situ test requires casing and can be done in grouted PVC casing or directly in the hollow stem auger casing (although there are debates about its effectiveness in drill casing). The process is similar to seismic CPTs, but instead of the geophones being located on the cone, they are placed directly against the casing of the borehole and moved incrementally down the borehole. Similarly, an energy source (like a hammer) is used to measure the arrival time of the seismic wave and used to estimate the shear wave velocity.
Another benefit of choosing hollow stem auger drilling for a geotechnical investigation is that the method facilitates easy installation of any instrumentation that would normally require casing. Some examples of these instruments include standpipe piezometers, pore pressure meters and clinometers. The typical method for installing instrumentation is to drill the hollow stem auger to the desired depth, install PVC casing (and a well screen in the case of a standpipe piezometer) and then lower the instrument into the casing. Once this has been completed, the auger can be withdrawn, and the void is backfilled within the specifications of the governing authority or client. Either a flush-mounted or stick up well cover is installed and usually grouted in place. Flush mounted well covers are typically used for installations in urban areas like sidewalks or paved roads. In contrast, stick up covers are used in more rural or forested areas so that the wells can be easily found each time they need to be monitored.
Hollow stem auger drilling is generally a good option for many geotechnical projects that keep costs low while providing a reasonable amount of versatility in terms of the amount of data collected. However, the method has similar drawbacks to solid stem augers, as it is generally not possible to drill through particularly dense materials or loose gravels and cobbles.
2.3 Mud Rotary
Mud rotary drilling is another common and incredibly versatile drilling method used for geotechnical investigations. While auger drilling, described previously, is very efficient at completing multiple shallow holes, a mud rotary drill can very deep holes (sometimes up to 300 meters) through almost any ground conditions, including rock. This section will focus on mud rotary applications in drilling through soil; however, mud rotary drills can often also be combined with a core barrel to drill through and obtain core samples of rock.
The main premise of mud rotary drilling is the combination of rotation and the use of drilling fluid that serves various purposes. The fluid is usually a combination of bentonite and water (other fluids are also used depending on the application) and is used for clearing drill cuttings out of the hole and keeping the drill bit cool when advancing. The setup for mud rotary drilling is fairly simple and involves a mud pump, mud tank, water source and drill rods. Drillers circulate drilling mud through the rods and back through the pump, which creates a closed system. This circulation keeps the drill bit cool and maintains a straight hole. It is also possible to cut through very dense material using a tri-cone drill bit. Once the drill rods reach the desired depth, sampling methods similar to hollow stem augering can be used, as long as the hole is cased. Standard penetration testing is often used with mud rotary to obtain better samples than what comes up from the cuttings.
While mud rotary drilling can go extremely deep compared to auger drilling, it is time-consuming to set up since it has so many different parts. It is, therefore, more practical to choose mud rotary when drill holes need to be deep or if the hole has a combination of overburden and rock.
While this article focused mainly on solid stem and hollow stem auger drilling, it is important to note that mud rotary drilling is also incredibly common and useful in geotechnical investigations. The choice between auger drilling and mud rotary drilling usually depends on the depth of the hole and the material type. Projects with a need for multiple shallow holes in softer soils should choose auger drilling, whereas projects with a need for deeper holes in a larger variety of soil types should choose mud rotary. This is a generalisation, and other factors may influence which method to choose in a real geotechnical investigation.
 Kumar, R., Bhargava, K. & Choudhury, D., “Estimation of Engineering Properties of Soils from Field SPT Using Random Number Generation.,” INAE Lett 1, vol. 1, pp. 77-84, 2016.