What is a Geotech Survey? And What Does It Test For?

Real estate development requires a lot of surveying to be done before construction begins. One of the things that you might hear about is a geotechnical or geotech survey. Understanding what this is and when and why you might need to get one done is important for the long-term stability of your building.

What is a Geotech Survey?

Let’s start with what a geotechnical survey actually is. It might be called a few other things too, including geotechnical investigation, foundation analysis and subsurface investigation. It might also be called soil analysis and soil testing, but these terms might also refer to checking for pollutants. However, soil analysis is a key element of a geotech survey as soil types can determine how foundations need to be constructed. Geotech survey is a more precise term and is the one we will use here.

The survey is generally performed by a geotechnical engineer and/or a geologist.

The point of a geotech survey is to gather the physical characteristics of the soil and rocks on a site or around a building to properly design the foundations and/or make proper repairs because of subsurface shifting and changing. Geotech surveys are particularly important in areas with high seismic activity.

How is a Geotechnical Survey Performed?

There is no one specific set of tools. A geotech survey can range from a visual assessment of the site to computers and lab tests. In most cases, it means a variety of investigations are done by the geotechnical engineer, which might involve ground penetrating radar, soil sampling, looking at maps to determine where there might be underground utilities or subsurface structures, etc. The specific nature of the investigation depends on the local geology, weather, the type of site (brownfield sites require different techniques from greenfield), seismic activity, etc. Bedrock location can be a key element for many large buildings where it is necessary to go deep to ensure stability.

Some things which might happen during a geotechnical survey include:

1. Ground-penetrating radar. This uses radar pulses to image what is going on below the surface without needing to dig. This allows you to find material changes, cracks, and voids. Primarily, it is used to locate bedrock and also to ensure the detection of unmapped voids, tunnels, and underground caves and water sources.
2. Penetration test and sampling. Push tubes are put into the fine-grained soils to take samples. These are then tested for things such as shear strength, water content, etc. Various types of penetration tests might be used depending on the qualities of the soil, but the standard test uses straight tubes. Cone penetration tests are less commonly used.
3. Soil testing to assess the risk of liquefaction. Liquefaction means that the strength and stiffness of soil are reduced during an earthquake or similar event. It generally happens in soils that are saturated with water. The increased pressure causes the soil particles to move around. This decreases strength and often causes building collapse. Water content testing can help ensure that structures are not supported by soil that is prone to liquefaction.
4. Blow counts. This is a test that is done on standard density and refers to the number of times you need to hit a sample spoon to drive it 6 inches into the soil. This is a measure of how fine or dense the soil is and helps determine its structural integrity.
5. Particle size analysis. This is used to determine soil gradation by separating coarse from fine particles. This helps determine compressibility and conductivity.

When Should a Geotechnical Survey be Done?

Typically, geotechnical engineers conduct these surveys for all new construction. Most jurisdictions require them for nearly all projects, including new construction and significant renovations. It’s particularly important if your site is sloped or if there are subsurface structures such as pipelines nearby. You will generally be required to submit the survey when applying for a permit. For example, in Santa Monica, California, geotechnical reports are required for all new construction, for large additions and major remodels, and for small additions if you are in a Seismic Hazard Zone. You may also need them for a swimming pool under certain circumstances, and repairs to damage caused by ground movement.

It is normal to do a new investigation after earthquakes or other incidents. For example, if a building is hit by a large vehicle, it might be wise to do a survey to check for subsurface damage and thus mitigate potential foundation issues before they get worse.

How do Geotech Surveys Help Developers

It’s worth considering doing a survey even if it is not required.

The primary point of a geotechnical survey is to ensure that the new construction fits in well with the subsurface environment. Having a geotechnical engineer conduct a good survey will save you money in the long term by allowing you to design foundations and earthworks that will be stable in the long term. Before repairs, it can help you make sure you are addressing all of the problems, not just the more obvious ones. With brownfield sites, it may allow you to find unmapped subsurface structures which could otherwise have created sinkholes or other issues.

You can spot problems before you start construction, so they can be fixed at the design change. Having to change plans halfway through delays construction and can significantly add to cost.

They also let you do your due diligence. The last thing you want is a lawsuit because a building you developed collapsed. Geotechnical surveys should be a standard part of site preparation, along with environmental testing (especially on brownfield sites).

It’s vital to ensure that the consultant you hire gets all the necessary information about the nature of your project and works with you on getting not just the legally required information but the information you need to design your building.

If you are building new construction, repairing damage caused by subsidence or earthquakes, or performing significant renovations, a geotech survey is a vital part of your site preparation. You need to work with a consultant who can customize the geotechnical survey to your needs and help you get the information you need to safely move forward.