How to Mitigate Liquefaction Risks On Landscape Development Projects

Liquefaction is a condition where some soils become liquified and lose their ability to support buildings and landscape structures.

There are two types of liquefaction failures that plague earthquake-rattled California landscapes after a significant seismic event.

First, large-scale liquefaction events create widespread damage and require significant amounts of remediation to avoid endangering lives and property.

Second, small-scale liquefaction failures affect smaller areas and do not pose the same level of danger as widespread damage. Modifying the soil may be all the mitigation required for small-scale liquefaction risks.

Here are a few general methods used to improve at-risk soils to reduce liquefaction risks to structures and landscapes:

  • Excavate and re-compact susceptible soils to increase soil density
  • In-situ soil densification, such as, mixing mortar into the soil
  • Construct deeper foundations to withstand liquefaction forces; and,
  • Reinforce shallow foundations to withstand the lateral and vertical forces produced during liquefaction events

How To Remediate Large-Scale Liquefaction Risks

Large-scale liquefaction consists of lateral spreading and displacement. “Large-scale ground displacements are defined as those that exceed 1-3 feet horizontally and 4-6 inches vertically.”

Lateral spreads generally occur on gently sloping or level ground where a high water table is mixed with agitated soil. The flow moves on a “on laterally continuous layers of loose, saturated gravel, sand, silt or sensitive clay.” Lateral spreads are most common in flood plains and artificial fills.

“The main type of destructive movement in large-scale displacements is lateral extension accompanied by shearing or tensile fracturing.” In extreme lateral spreading events, roads and utilities can be disrupted which poses a threat to first responders and rescue efforts.

“Only removal and densification of liquefiable soils, or permanently lowering the groundwater by dewatering, can fully eliminate liquefaction hazards.”

Here are some techniques used to mitigate large-scale displacements:

  • Provide lateral support for structures with edge containment foundations
  • Modify liquefiable soils to make them denser
  • Modify slope geometry to make the terrain less likely to fail due to liquefaction; and,
  • Reduce the water table below the liquefiable soil level

How To Remediate Localized Soil Failures

Soil improvement is the most preferred method to reduce the impact of localized liquefaction failures.

Types of Localized Failures

Three types of small-scale failures are common during liquefaction failures.

  • Loss of bearing strength
  • Differential settlement
  • Soil spreads and flows

Loss of Bearing Strength

Densifying the soil is the the most common method to protect structures from damaging effects of localized failures. Engineers recommend a few different methods including “using vibro-compaction, vibro-replacement, deep dynamic compaction, and compaction grouting”.

Differential Settlement

Differential settlement occurs when part of the soil under a building or paving area fail and subside relative to the surrounding soil. It is hard to predict where differential settlement will occur during a liquefaction event and how severe it will be.

Mitigation for differential settlement includes grading the site. “The typical grading solution to this type of failure is to estimate the amount of potential vertical settlement, then design and construct a mat of compacted fill that is thick enough to form a uniform bearing surface….The main technique used is to remove and re- compact a soil mat to give the foundation a more stable base. A variation of the technique is to actually construct the engineered mat above the existing ground level instead of excavating below grade. In general, the thicker the mat, the greater amount of settlement it can accommodate. A raised mat has the added impact of providing greater separation from a shallow water table.”

Spreads and Flows

“Localized lateral spreads and small-scale flows are formed by the displacement of a surface layer in response to liquefaction of an underlying layer. This type of failure is dependent upon a gentle slope  or  a  nearby “free face” or open area that will allow the displacement.”

“The key to mitigating localized lateral spreads is to require an adequate setback from an open face or sloping ground. If the distance and geometry is restricted, then bulkhead walls or another form of retaining structure must be installed. For smaller-scale, liquefaction-induced settlements (less that 2”  total),  a  wide  range  of  techniques  is  used  for  site-specific problems. They may range from removal and re-compaction of shallow soils to support concrete slabs and perimeter spread footings to deep, drilled pier foundations and structural strengthening.”

Final Thoughts

The only way to eliminate the risk of liquefaction on problem soils is to completely remove and replace the materials with stable fill. However, mitigation techniques and structural solutions can reduce the risk of liquefaction damage to structures and landscape structures.

  1. Thorough geologic and seismic evaluation of the proposed project and site conditions, with appropriate treatment of the conditions found;
  2. Careful testing of the conditions encountered during the preliminary rough grading phase of development;
  3. Continuous review and inspection during grading and fill placement;
  4. Monitoring soil properties, moisture content, and relative compaction during construction; and,
  5. Proper post-development maintenance of the drains, outlets and erosion control.

“Considerable geotechnical judgment is needed to make appropriate recommendations for the mitigation of earthquake-induced landslide and liquefaction hazards. For complicated settings, a Registered Civil Engineer and a Certified Engineering Geologist working together should perform the review, inspection and evaluation of proposed mitigations.”

For More Information

The quoted parts of this article are from the California Geological Survey Special Publication 117A.

Other Articles About Investigating Seismic Risks

John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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