What is the purpose of a geotechnical site investigation?

By: C.K. Satyapriya, M.S., P.E., D.GE

Before a structure is built—regardless of whether it is a pipeline, skyscraper, bridge, parking lot, or solar farm—engineers must understand what is underneath it to ensure it is constructed safely and responsibly.

The purpose of a geotechnical site investigation is to understand a site’s subsurface conditions such as types of soils or rock, groundwater, and engineering properties. The cost of a subsurface investigation is less than 1% of the construction cost. However, the information it provides can save the project multiples of the investigation costs and help maintain the schedule.

Subsurface conditions affect the type of foundation the new structure/facility will require, how it will likely affect adjacent structures, and how expensive it will be to build, among many other factors. By understanding those conditions before a project starts, geotechnical investigations provide information that is not visible from the surface. This information will help to anticipate how soil and other subsurface conditions will respond to the proposed project and allow engineers to plan accordingly.

HOW DO GEOTECHNICAL ENGINEERS CONDUCT A SITE INVESTIGATION?

No two geotechnical investigations are the same. Because each project and piece of land are unique, studies are tailored to each situation.

Before beginning an investigation, the first step for geotechnical engineers or geologists is to communicate with the owner regarding their proposed plans. Understanding the planned construction or use of the land will guide each step of the investigation.

Knowledge of the site

Next, it is essential for geotechnical engineers is to understand the region’s geology.

For instance, in Cincinnati, Ohio, layers of rock and soil are often present. If excavation is performed without the knowledge of subsurface conditions (for a pipeline, for example), the cost of excavation will increase exponentially when one of these hidden rock layers is encountered. Due to this unforeseen condition, project delays will result, increasing the project costs as well.

On the other hand, there is a lot of peat in northwest Ohio, which is not suitable to support structures because it decomposes and settles. Geotechnical engineers must determine the limits of the peat and then figure out how to either treat it in place or replace it.

Although both of the examples above are in Ohio, geotechnical engineers may have a very different approach in each of those locations based on their knowledge of local geology.

Preliminary Site Assessment

Once the site is identified, engineers will work with the client to determine what type of structure they have planned for the site and understand what subsurface information is needed.

In the US, the next step is typically a visual assessment combined with borings, or drilling holes at specific depths to take soil samples. Sometimes, test pits along with visual observation are done in restricted locations.

Engineers will look for visual cues, including visible signs of rock or water and how hard the soil is to push or dig.

They will also examine the soil itself. An on-site strength penetration test can determine the strength parameters of the soil. Depending on the client’s specific needs, engineers also send soil samples to a laboratory for additional testing.

Detailed assessments for proposed construction

Typically, a more detailed assessment requires more borings. However, since it is often cost-prohibitive to conduct too many borings, geophysical investigations are often used as a supplement.

By adding a geophysical investigation, engineers can increase the distance between borings and decrease costs. These investigations are somewhat like conducting an x-ray from the surface and can augment the information between borings. Specifically, they measure the resistivity of the ground and help identify the different soil and rock layers to depths of 100 feet or more.

Geophysical studies are also beneficial for finding underground utilities and/or anomalies.

Recommendations and Solutions

Conducting the investigation, however, is by itself never the complete solution. Engineers must always pair knowledge of the site with knowledge of the proposed construction.

Any time soil is removed from an area, it changes how the site will support a structure. Long before construction begins, engineers must assess how the site is likely to respond to the specific changes proposed. Preparing a site for a ten-story structure, for instance, is different than a one-story structure with three floors below grade.

Throughout history, there have been many examples of construction projects gone wrong. In the 1970s, the basement of the John Hancock tower in Boston was improperly excavated and led to damage to the neighboring Trinity Church, resulting in a $4.1 million judgment many years later.

Even worse, in 1963, a landslide at the Vajont Dam in northern Italy caused a massive wave that overtopped the dam and caused enormous destruction to the Piave Valley below, killing thousands of people.

Without proper geotechnical engineering, there is always the possibility of a disaster. Understanding subsurface conditions is critical to ensure structures can be adequately supported and built safely and responsibly.

WHEN DOES A GEOTECHNICAL SITE INVESTIGATION START?

Although there are various reasons to conduct a geotechnical site investigation, it is most common for studies to occur before a potential property owner buys a piece of land or before construction.

Assessments range in complexity from preliminary reviews to determine if a piece of land is suitable for a project to detailed assessments before construction begins.

Of course, it is rare that geotechnical reports come back with only favorable information, so often, additional testing can provide further details while geotechnical engineers provide solutions to any unanticipated subsurface problems.

CTL ENGINEERING USES EXTENSIVE EXPERIENCE TO PROVIDE SOLUTIONS

With 95 years in business, CTL Engineering has provided geotechnical investigations in all sorts of unique situations. CTL engineers are trained not only to conduct thorough investigations but also to provide cost-effective, practical solutions. Often that can mean exploring new technology or pushing boundaries. Because it is not cost-effective to put every structure on top of rock, CTL works with clients to find feasible alternatives.

In particular, CTL specializes in several markets, including:

• Single or multi-story buildings (especially healthcare and education),
• Transportation projects
• Water and Water treatment, including pipelines and sewers
• Energy, including power plants, windmill foundations, and solar farms
• Telecommunication, including thousands of towers.

For more information about conducting a geotechnical investigation or to learn more, contact one of our experts below.

C.K. Satyapriya, M.S., P.E., D.GE is president and chairman of CTL Engineering Inc.

C.K. has been the project manager for several subsurface evaluation projects in excess of $1 million. He has conducted several investigations with respect to slope stability and developed a computer program to model transient groundwater levels along slopes. He is experienced in deep foundations, piling, nondestructive testing, structural evaluations, and mining engineering. Mr. Satyapriya is a registered professional engineer in a number of states.  He is Diplomate, Geotechnical Engineering as conferred to him by The Academy of geo-Professionals.  He is a Fellow in the American Society of Civil Engineers and a member and committee person in the American Society of Testing Materials.