The Effect of the Hammer Energy Efficiency Ratio on SPT-Based Liquefaction Evaluation

The Standard Penetration Test (SPT) has long been used by the geotechnical community to evaluate the factor of safety (FOS) against liquefaction and to estimate liquefaction induced soil strength loss, vertical settlement and lateral spread. There are other investigative methods of evaluating liquefaction risk, the most common being the cone penetration test (CPT). However, the SPT method is often the first choice for geotechnical professionals because the testing is economical, the equipment readily available, and it returns physical samples that can be tested in the laboratory. The public and governing jurisdictions trust that the geotechnical community is using field-testing methods that provide relatively accurate soil strength data, which is critical to liquefaction evaluation. Because most jurisdictions in the United States do not require the use of calibrated SPT hammers, the majority of SPT liquefaction analyses are based on uncalibrated SPT hammer data. There appears to be a perception by governing jurisdictions that it is acceptable for geotechnical engineers to rely on their engineering judgment to determine SPT hammer energy. However, numerous past studies demonstrate that SPT hammer energy varies widely depending upon the hammer type, manufacturer and lifting mechanism. The variance in energy can be as much as 200 to 250 percent, which is too large to reasonably estimate using engineering judgment alone. This paper focuses on the influence of hammer energy on the SPT liquefaction analyses results. We examine the variability within the results as a function of measured hammer energy. Finally, the paper encourages public agencies to require the use of calibrated SPT hammers in the evaluation of liquefaction triggering, strength loss, dynamic settlement and lateral spread hazards. It is our hope that this practice will be adopted in local and state building codes.