Geotechnical and seismic assessments can help preserve structural integrity and avoid property damage.
Risks are abundant and varied in the world of commercial real estate. Some are apparent at the onset of the transaction and require an immediate tolerance check from the buyer or lender. Conducting a Phase I Environmental Site Assessment (ESA) at the onset of the process can account for your calculated risks but what about the things you may not see? One of these geological conditions is soil liquefaction. While it is only prevalent in certain areas globally, soil liquefaction is an event that can drastically affect your property. If you’ve never heard of soil liquefaction or what you can do to possibly reduce the risks associated with in on your property, let us give you a quick overview below.
What is soil liquefaction?
Soil liquefaction is a geological phenomenon that occurs when soil becomes either partially or completely saturated and is simultaneously disturbed, thereby losing strength and stability and taking on the properties of a liquid. Soil is composed of individual particles which, in normal circumstances, remain packed closely together. The soil below the ground surface is held in place by the weight of the soil above it, and it is heavy, on average about 100 pounds per cubic foot! When soil is in a saturated condition, the contact between particles can instantly disappear due to a sudden increase in water pressure that is greater than the overlying weight of the soil. This is most commonly caused by seismic activity. Once the soil becomes weightless, it softens and weakens so much that is actually loses its properties as a solid and takes on those of a liquid.
When does this happen?
There are actually three factors necessary for soil liquefaction. The first is a relatively loose and light weight soil deposit. This can be loosely compacted, relatively young soils (less than 11,000 years old) which is most commonly located in low lying areas near bodies of water such as rivers, bays, lakes, and oceans. The second is the saturation of the soil with groundwater or as the result of a ‘wet season’, over-irrigation, leaking pipes, swimming pools, etc. The third is strong shaking or disturbance of the ground, which can happen naturally, as in the case of an earthquake, or by other means such as blasting, soil compaction, or similar tasks. All of these factors are needed to cause the ground to lose its ability to support itself or any structures resting upon or adjacent to it.
What kind of problems can this cause
Since earthquakes are the most common instigator of soil liquefaction, the first thing that comes to mind is the loss of structural integrity of and/or collapse of buildings. Liquefaction causes a sudden shift that the building is not prepared for, such as the foundation being pulled down into the soil causing the structure to lean and/or collapse. Other structures that can be compromised include bridges, dams, and retaining walls. Loss of bearing strength is not the only effect of liquefaction. Additional effects can include:
- Lateral Spreading: The ground can slide down gentle slopes or towards stream banks
- Sand Boils: Water, loaded with soil, can be ejected from a buried liquefied layer and erupt at the surface to form sand volcanoes
- Flow Failures: Earth moves down a steep slope with large areas of displacement and internal disruption of material (Landslides)
- Ground Oscillation: a surface layer of material, floating on a buried liquefied layer, is thrown back and forth, causing the surface to become deformed (Parking lots, sidewalks)
- Flotation: Lighter structures, like pipes, sewer lines, etc., can be moved to the surface when they are surrounded by liquefied soil
What can I do to reduce my risk or plan for the possibility of this geological condition?
While the prospect of soil liquefaction occurring on one of your properties may be ‘unsettling’, there are some things that you can do to reduce your risk if your property is located in an area prone to seismic activity. During the design of new construction, geotechnical engineers can conduct a geologic report and geologic hazard analysis, which should include soil mapping and geotechnical borings, to determine the amount of liquefaction induced settlement that could occur during an earthquake. There are design solutions to any case, though they will vary in cost.
Some these options may include:
- Soil Excavation and/or compaction;
- In-situ ground densification;
- Edge containment structures;
- Deep foundations and/or
- Reinforcement of shallow foundations.
If a hazard has been identified on a site with existing construction, you have a few options to protect yourself including:
- Avoiding any of the areas on the property identified as high risk;
- Purchasing insurance to cover any possible losses as a result of the findings;
- Improving the ground to be less susceptible to liquefaction; and/or
- Fortifying existing structures to withstand liquefaction of underlying soil.
An experienced geotechnical consultant can help to suggest measures to prevent serious damage to your property as well as creating solutions if you have already experienced a geological issue at your site.
By: Matthew Marcus, PE, RG (Globe St)
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