CS18C - Concurrent Session 18C: 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 data (blow counts) to be artificially elevated in gravelly soils, there is a broad range of opinion regarding the best practices for identifying gravel influence and applying corrections to site-specific data. This paper provides guidance for anticipating and evaluating gravel influence on blow counts at dam sites based on DSOD's experience reviewing numerous projects across California. 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 purpose of the investigation, but the overall purpose is to ensure that enough contextual information is recorded to evaluate data quality. Once the appropriate data is collected, each set of blow counts should be evaluated for gravel influence by considering the geotechnical and geologic circumstances at each test interval. We have found the particle-to-probe-diameter ratio to be an effective screening parameter that is preferable to any rule-of-thumb criterion based on gravel percentage alone. After this data validation is complete, the higher confidence data can be applied to engineering analyses. This paper also presents our perspective on estimating SPT penetration resistance from large diameter penetrometers, such as the Modified California Sampler (ModCal) and the Becker Penetration Test (BPT). Data is presented from several dam sites to compare DSOD's typical procedures with the range of procedures observed in practice. This paper demonstrates how a unified geotechnical and geologic approach to gravelly soil characterization is beneficial for dam safety evaluations.