Understanding Geotechnical Investigations for the Layperson


You’ve decided to build an addition or put in a pool and were told that you need a geotechnical investigation. Now you’re wondering what exactly that is and why you need one. Look no further, the answers to your questions are here.

In the simplest terms, a geotechnical investigation is done to identify the characteristics of a piece of land so that recommendations can be developed regarding what needs to be done to prepare the site for the project and the project for the site. Specifically, the investigation provides information needed by the architect and engineers to design the project foundation, defines what needs to be done to prepare the site for the improvements, and provides geotechnical information that may be required for a building permit. Although all geotechnical investigations involve the same basic steps, each investigation is unique to the specific project and property involved. The steps are:

Review of Geologic Literature

Readily available geologic literature is reviewed to gain an understanding of the known site conditions. This may include reviewing geotechnical investigations previously done in the area or at the project site for a previous project, aerial photographs, pre- and post-grading topographic maps or seismic maps.

Site Reconnaissance

During this step of the investigation, the geologist or engineer walks the site to observe the apparent natural geological characteristics as well as to identify areas where previous grading or other man-made disturbances may have taken place. Based on what is observed and what is known about the general geology of the area, the extent and methods for the subsurface portion of the investigation are determined. This is where investigations vary in terms of methodology, which can affect both cost and time frame for completion. For example, a hillside property or a property near an active fault will require a more extensive and complex subsurface investigation than would a relatively flat parcel of land in an area with stable geology.

Subsurface Exploration

Drill Rig

A drill rig creating a boring

This involves either digging trenches with a backhoe, drilling one or more holes (borings), or digging one or more test pits. Trenching is typically done on relatively undeveloped land where the project and site conditions indicate a shallower investigation, down to about ten feet, is sufficient, or on sites that may be bisected by an earthquake fault line, such as those located in La Jolla, Bay Park, downtown San Diego, or Normal Heights. Borings are done where access is limited or where a deeper investigation is required. Test pits are used when site conditions and the project indicate only a shallow investigation is appropriate, but trenching is not appropriate due to the land’s current use. For example, homeowners may not appreciate a trench being dug in their yard for an investigation!

In addition to trenching, drilling, or digging; cone penetrometer tests (CPT) or seismic refraction surveys may also be used. CPTs are done in softer soils and they involve pushing (advancing) a probe through the soil while the probe takes nearly continuous readings, providing a great deal of detailed information about the subsurface soils. Seismic refraction surveys are typically employed in areas of dense rock and simply give a reading as to the rigidity of the ground. The subsurface exploration reveals the earth materials and geologic conditions of the site and allows for representative sampling of those materials for laboratory testing.

Laboratory Testing

Lab testing involves performing a variety of tests to determine the physical characteristics of the near surface earth materials sampled. Typical tests include natural and maximum density of the soils; expansive characteristics of the soils, which indicates how much it may increase in volume due to moisture changes; tests to determine bearing capacity, indicating how much weight the ground can hold without displacement; shear testing of soil, which is utilized in foundation and wall design as well as slope stability analyses, and tests to identify settlement potential.

Report Preparation

Finally, the information gathered is presented in a report that includes the findings, conclusions and geotechnical recommendations for the project. That report is used by the project architect, structural and/or civil engineers in the design process, by the grading contractor during any grading operations, and may be required by the permitting agency for issuance of a building permit.

 

This post is for general informational purposes only and does not constitute professional engineering advice or recommendations.