Regulatory Perspective on Penetration Testing in Gravelly Soils

The California Division of Safety of Dams (DSOD) regulates an inventory of over 1,200 dams. Many of these dams are founded on or constructed from gravelly soils, which can be challenging to characterize and evaluate using conventional techniques. Although most practitioners acknowledge the potential for penetration test values to be artificially elevated in gravelly soils, there is a broad range of opinions regarding the best practices for identifying gravel interference and applying corrections to site-specific data. This paper provides guidance for anticipating and evaluating gravel interference on penetration testing at dams.

Special provisions should be incorporated into geotechnical investigation plans for sites where gravelly soils are suspected or known to be present. The scope of these provisions can vary depending on the investigation objective, but the overall purpose is to ensure that enough contextual information is recorded to evaluate data quality. Once this information is available, the penetration test data should be evaluated for gravel interference by considering the geotechnical and geologic circumstances at each test interval.

We have also found probe-to-particle-diameter ratio to be an effective screening parameter that is preferable to any rule-of-thumb criterion based on gravel percentage alone. After data validation is complete, the higher confidence data can be applied to engineering analyses. This paper demonstrates how a unified geotechnical and geologic approach to gravelly soil characterization is beneficial for dam safety evaluations.