Critical Site Investigation Items For A Geotechnical Report

Loma Prieta Liquefaction Damage

Damage caused by liquefaction in San Francisco's Marina district in 1989. Image courtesy of California Watch.

Earthquakes are a fact of life in California.

Improved seismic hazard investigation techniques make identifying and inventorying a site’s seismic risk more reliable than in the past.

The Seismic Hazards Mapping Act requires most developers and property owners to identify potential seismic hazards and develop a plan to mitigate the deleterious effects of earthquake induced landslides, ground motion, and liquefaction.

If you identified seismic risk factors during the site’s initial Seismic Screening Investigation, then you need to conduct a Quantitative Evaluation of Hazard Potential.

What is a Quantitative Evaluation of Hazard Potential?

A quantitative evaluation is a formal evaluation conducted by certified geologist and a registered civil engineer.

Your consultants produce a written report and related maps and graphics with recommendations for mitigating seismic risks found on the site. Here are some seismic risk conditions to have your consultants investigate on your project’s site.

Earthquake or Liquefaction-induced Landslides

Seismic activity can cause slopes to fail. Depending on the geology of your site’s slopes, earth movement during a quake may cause the slopes to fail and slide which could endanger human life and structures.

Liquefaction causes certain soil types to become liquified and lose its bearing capacity. When the soil liquefies, slopes may fail and endanger life and property.

Here are some items that your consultants should consider when conducting a quantitative evaluation. These items have been recommended for investigation by the California Geological Survey in Guidelines for Evaluating and Mitigating Seismic Hazards in California (Special Publication 117 A, 2008).

  • Description  of  the  proposed  project’s  location,  topographic  relief,  drainage,  geologic  and  soil  materials,  and  any   proposed grading.
  • Site plan map of project site showing the locations of all explorations, including test pits, borings, penetration test locations, and soil or rock samples.
  • Description of seismic setting, historic seismicity, nearest pertinent strong-motion records, and methods used to estimate (or source of) earthquake ground-motion parameters used in liquefaction and landslide analyses.
  • 1:24,000 or larger-scale geologic map showing bedrock, alluvium, colluvium, soil material, faults, shears, joint systems, lithologic contacts, seeps or springs, soil or bedrock slumps, and other pertinent geologic and soil features existing on and adjacent to the project site.
  • Logs of borings, test pits, or other subsurface data obtained.
  • Geologic cross sections depicting the most critical (least stable) slopes, geologic structure, stratigraphy, and subsurface water conditions, supported by boring and/or trench logs at appropriate locations.
  • Laboratory test results; soil classification, shear strength, and other pertinent geotechnical data.
  • Specific recommendations for mitigation alternatives necessary to reduce known and/or anticipated geologic/seismic hazards to an acceptable level of risk.
  • Description of shear test procedures (ASTM or other) and test specimens.
  • Shear strength plots, including identification of samples tested, whether data points reflect peak or residual values, and moisture conditions at time of testing.
  • Summary table or text describing methods of analysis, shear strength values, assumed groundwater conditions, and other pertinent assumptions used in the stability calculations.
  • Explanation of choice of seismic coefficient and/or design strong-motion record used in slope stability analysis, including site and/or topographic amplification estimates.
  • Slope stability analyses of critical (least-stable) cross sections, which substantiate conclusions and recommendations concerning stability of natural and as-graded slopes.
  • Factors of safety against slope failure and/or calculated displacements for the various anticipated slope configurations (cut, fill, and/or natural slopes).
  • Conclusions regarding the stability of slopes with respect to earthquake-induced landslides and their likely impact on the proposed project.
  • Discussion of proposed mitigation measures, if any, necessary to reduce damage from potential earthquake- initiated landsliding to an acceptable level of risk.
  • Acceptance testing criteria (e.g., pseudo-static factor of safety), if any, that will be used to demonstrate satisfactory remediation.

Investigating Liquefaction Risks

  • If methods other than Standard Penetration Test (SPT; ASTM D 1586; ASTM D 6066) and Cone Penetration Test (CPT; ASTM 3441) are used, description of pertinent equipment and procedural details of field measurements of penetration resistance (borehole type, hammer type and drop mechanism, sampler type and dimensions, etc.).
  • Boring logs showing raw (unmodified) N-values  if  SPT’s  are  performed;;  CPT  probe  logs  showing  raw  qc-values and  plots  of  raw  sleeve  friction  if  CPT’s  are  performed.
  • Explanation of the basis and methods used to convert raw SPT, CPT, and/or other non-standard data to “corrected” and “standardized” values.
  • Tabulation and/or plots of corrected values used for analyses.
  • Explanation of methods used to develop estimates of field loading equivalent uniform cyclic stress ratios (CSR) used to represent the anticipated field earthquake excitation (cyclic loading).
  • Explanation of the basis for evaluation of the equivalent uniform cyclic stress ratio necessary to cause liquefaction (CRR) within the number of equivalent uniform loading cycles considered representative of the design earthquake.
  • Factors of safety against liquefaction at various depths and/or within various potentially liquefiable soil units.
  • Conclusions regarding the potential for liquefaction and its likely impact on the proposed project.
  • Discussion of proposed mitigation measures, if any, necessary to reduce potential damage caused by liquefaction to an acceptable level of risk.
  • Criteria for SPT-based, CPT-based, or other types of acceptance testing, if any, that will be used to demonstrate satisfactory remediation.

For More Information

For more information about Evaluating and Mitigating Seismic Hazards in California, check out Special Publication 117 A from the California Geological Society.

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About

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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