Soil liquefaction is what happens when loosely packed, water-saturated sediments near the surface lose their strength in strong ground shaking. In the Southeast, the Charleston area is where it matters most for foundation design. Engineers screen a site for those conditions during a geotechnical exploration.
How Soil Liquefaction Happens
Liquefaction of soil is not a change in the mineral makeup of the ground. It is a temporary loss of strength when three conditions are present at the same time:
- Loose or loosely packed sediments, most often sandy soils
- Saturation, usually from a high water table
- Strong ground shaking
Liquefaction needs saturated, loose sandy soil and strong shaking at the same time. Take away any one of those conditions and the soil does not behave this way.
When this occurs at shallow depth, sand and water can come out at the ground surface. The USGS calls that a sand boil. It is surface evidence of what happened below, not a separate problem.
Why the Charleston Area Is Different
The regional record is documented for the South Carolina coast. According to the South Carolina Department of Natural Resources and the South Carolina Geological Survey, the Charleston earthquake of August 31, 1886, had an "estimated magnitude of 7.6."
SC DNR also reports that paleoliquefaction features have been found in South Carolina's coastal area. In a strong earthquake, subsurface saturated sand can liquefy and be ejected to the surface, leaving sand blows and vents or fissures.
Those features are preserved in the soil as evidence that saturated sand liquefied in past earthquakes. The same SC DNR account states, "Given the necessary conditions, a magnitude 5.5 quake can generate liquefaction features."
About 70 percent of South Carolina earthquakes occur in the Coastal Plain, mostly clustered west and north of Charleston. That Coastal Plain concentration, together with the paleoliquefaction record, is why sites in the Charleston area get this attention in foundation design.
Coastal sites around Charleston combine soft ground, shallow groundwater and a known seismic source. That is why liquefaction is a foundation-design question there, and why geotechnical engineering in Charleston includes screening for it on coastal projects.
A regional history does not tell you how a particular parcel will behave. That takes exploration at the site.
What Liquefaction Can Do to a Foundation
If the soil under a foundation loses strength during shaking, several things can happen:
- The soil may no longer provide the bearing support the foundation was designed for.
- The ground can settle, including uneven movement known as differential settlement.
- The ground surface can shift, and sand and water may come up at the surface.
When support is lost unevenly, some parts of a structure can move more than others. Those effects are why liquefaction is screened during design, rather than discovered after a building is already in the ground. The details vary from site to site, so the assessment has to be based on the soils and groundwater actually present.
How Engineers Screen a Site for Liquefaction
A geotechnical exploration screens for these conditions rather than assuming them from a regional map. Typical steps include:
- Drilling soil borings and recording SPT N-values.
- Running cone penetration tests.
- Evaluating groundwater, because saturation is one of the required conditions.
- Laboratory testing of samples where the design needs it.
- Then setting seismic design parameters and completing the liquefaction assessment, which appear in the geotechnical report.
The aim of the screening is to determine whether loose, saturated sandy soils are present and whether strong shaking could cause them to lose strength. Soil borings and cone penetration tests describe the soils. Groundwater evaluation shows whether those soils are saturated. Laboratory testing supports the analysis where it is needed.
Evaluating liquefaction potential is part of that assessment.
Design Options When Liquefaction Is a Concern
If screening shows the soil can lose strength, the design can respond in more than one way. The foundation choice follows from the site data, not from a preferred system.
- Ground improvement, including soil stabilization, to change how the soil behaves
- Deep foundations such as drilled shafts or driven piles, which can carry load below the susceptible soils
The soils, the groundwater and the planned structure decide which option fits.
How NOVA Approaches Liquefaction Screening
When NOVA scopes an exploration, the local geology and the intended use of the property set the plan. Our engineers pair borings and cone penetration testing with a look at groundwater, then report seismic design parameters and a liquefaction assessment where the conditions call for it.
That work is part of NOVA's geotechnical engineering services for projects in Georgia, Florida and the Carolinas.
Plan a Geotechnical Exploration for Your Site
On the South Carolina coast, NOVA's site explorations include soil liquefaction screening. To talk through a site, request a consultation.
