Borehole Drilling: The Benefits of Multiple Bores

Borehole drilling is a critical factor in geotechnical engineering, which is why we sat down with one of our engineers, Geoff Webster, to learn more about this process and the benefits of multiple bores.

What are boreholes and why is borehole drilling important for geotechnical engineering?

To properly characterize a building site and to design a foundation that will support a structure, a geotechnical engineer must understand the types of soil deposits that will support the foundation. Rarely is the soil homogenous, and the soil profile will vary with depth and may vary across the site as well.

The process of identifying the layers of soil deposits that underlie a proposed structure and determining their physical characteristics is referred to as a Subsurface Investigation. A primary method of gathering subsurface information is the Soil Test Boring. A soil test boring is made by advancing a continuous-flight auger a predetermined distance into the ground. Power to advance the auger is provided by a truck-mounted drill rig. A cutter head is attached to the tip of the auger and auger flights carry loose soil from the bottom of the borehole to the ground surface where it may be observed and collected by the driller.

Why are multiple boreholes advantageous for some projects?

Soil is produced by the weathering of various rocks. Soil produced through weathering processes may be moved by physical erosional processes to locations distant from their origin. These are transported soils and they may be deposited over vast areas in random ways.

Other weathered soil may stay in place atop bedrock as residual soil. Both types of soil may exist in one location with transported soil overlying residual soil. The random nature of deposition means the soil profile of a given site may vary spatially across the site. The manner in which soil is deposited and the grain-size variation in deposited soil determine in part the strength parameters of soil. Structure foundations need to bear upon soils of similar strength to avoid differential movement. Borehole drilling multiple boreholes spaced at the corners and in the center of a proposed structure can ascertain a more complete characterization of soil under the proposed structure than could one borehole placed, say, at the center of the structure.

What are some of the technical aspects of bore holes?

Geotechnical engineers have developed laboratory test methods and mathematical formulations relating the physical characteristics of soil to the strength and performance of soil under loading. A key function of performing boreholes is gathering representative soil samples for use in laboratory testing. Soil samples are typically obtained every five feet of drilling, most commonly using a split-spoon sampler.

This device is pounded into the soil at the bottom of the borehole using a standard weight hammer to collect a 2-foot soil sample. In addition to collecting a soil sample, the number of blows required to advance the sampler a given distance produces a number called the N-value, a measure of the soil’s resistance to penetration. The N-value is the most significant soil value that may be obtained from a borehole, and is a seminal value that has been correlated to many strength and other soil parameters through laboratory testing and mathematical relationships.

What does RMG do to assess how many boreholes are needed?

A subsurface investigation for a given structure or development may be generally divided into three broad phases. Information gathered in each phase before borehole drilling allows RMG to determine a sufficient number of holes.

  • In Phase I we compile existing information regarding the proposed structure such as the type of structure and its intended use, building code requirements, and column and foundation loads.
  • Phase II includes gathering existing site and subsurface information. Such information may be gleaned from geologic maps, soil surveys, or existing soil exploration reports. This information may provide insight into the type of soils to be reasonably encountered and drilling problems that might be anticipated.
  • In Phase III, RMG engineers reconnoiter the site to assess accessibility, drainage patterns, and physical features that may provide clues to subsurface conditions.

While no hard-and-fast rule exists for determining the number of boreholes or the depth to which test borings should be advanced, information gathered during the three phases is used in conjunction with tried-and-true guidelines developed through time. For multi-story structures of light construction and normal loading, for instance, approximately 10-feet depth for each story up to three stories is typical, with at least one boring to a deeper depth to determine depth to bedrock, if present.

For taller structures of heavier loading deeper depths are required. With respect to borehole spacing, building codes and public agencies often specify a minimum number of borings per acre for site development, or a minimum number of borings per length for roadways. Otherwise, judgment and experience are used to ensure sufficient coverage.

Want to learn more about borehole drilling or RMG’s other geotechnical engineering offerings? Contact us today!







3 Drilling Methods To Consider For Soil And Rock Testing

In geotechnical engineering, different drilling methods are used to better understand the soil beneath the surface. There are three common methods to consider: solid stem auger, hollow stem auger and mud rotary. Here is what you should know about each method.

Drilling Methods 1 – Solid Stem Auger

Solid stem augers (SSA) use continuous flight augers which mechanically excavate and continuously transport cuttings to the surface. Augers are available in diameters of 3 to 14 inches. Solid stem augering has a number of advantages. It produces a moderate amount of easily contained cuttings and little or no fluid is required in the drilling process. Thin wall and split barrel sampling operations are supported. Because of lower torque requirements for solid stem augering, smaller drill-rigs can be used. Doing so simplifies site maneuvering and often incurs lower costs.

However, this drilling method also has a number of disadvantages. In order to obtain soil samples, the solid stem auger must be removed from the hole. Therefore, this technique is limited to stable soils which will not collapse when the augers is removed. Soil sampling during solid stem auger drilling is labor intensive, especially in deeper holes because the augers must be removed from the hole during each sampling procedure.

Drilling Methods - Solid Stem Auger

Drilling Methods 2 – Hollow Stem Auger

Hollow Stem Augers (HAS) are commonly used to set ground water monitoring wells for environmental and geotechnical applications, but can also be used to obtain soil samples. Hollow stem auger drilling uses large diameter (up to 14 inch outside diameter) continuous flight augers which mechanically excavate and continuously transport cuttings to the surface. A center bit, which is attached to the drill rod and bolted to the auger drive cap, is inserted through the cutter head to excavate the center of the boring. As the boring is advanced by adding sections of auger, sections of drill rod are added, maintaining the center bit at the face of the cutter bit.

Hollow stem augering has a number of advantages. The augers act as a temporary casing during and at the completion of drilling. This facilitates the soil and water sampling and the installation of monitoring wells. It is relatively rapid and little or no fluid is required in the drilling process. Additionally, this drilling method readily supports thin wall and split barrel sampling.

Hollow stem augering has a handful of disadvantages. For example, it is limited to drilling in poorly lithified to unlithified sediments. It also is limited to a maximum depth of 150 feet. Shallow bedrock or other hard to drill materials may reduce this depth significantly. High hydrostatic pressures when borehole drilling can cause problems with sand heaving up into the auger during soil sampling and well installation.

Drilling Methods 3 – Mud Rotary

The mud rotary method of drilling uses a drill bit that is mounted on the end of a drill rod and is advanced by the rapid rotation of the bit. The cuttings are removed by pumping drilling fluid (water or water mixed with bentonite or other fluid enhancers) down through the drill rods and bit and up the annulus, between the borehole and the drill rods. The drilling fluid also serves to cool the drill bit and stabilize the borehole walls.

Mud rotary drilling methods have a number of benefits. It is a very fast and efficient means of drilling—efficient drill-rigs can produce several hundred feet of hole per day. Mud rotary is adaptable to a wide range of geologic conditions. However, exceptionally large, poorly stabilized boulder conditions are unsuitable for mud rotary drilling. Sediment sampling is broadly supported and thin wall and split barrel sampling is available.

Mud rotary has a number of disadvantages. The use of drilling fluids may require support vehicles to properly manage and contain both the cuttings and the drilling fluids. The use of drilling fluids may also invade permeable zones compromising the validity of the monitoring well sampling.

RMG’s highly skilled team of geotechnical engineers have the knowledge and experience to assess which of the available drilling methods is best for your project. Contact us today to learn more about our services!