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!