Cone Penetration Rate Effects on Field Characterization of Hydraulically Placed Fly Ash

Hydraulically placed materials such as mine tailings and fly ash are often silt-sized, non- to low-plastic materials that exist in a very loose state. Cone penetration testing (CPT) is widely used in practice to characterize the in situ state of these materials because laboratory characterization is unreliable due to the difficulties in preserving in situ state conditions during sampling, transportation, and testing setup. Current soil behavior and characterization methods are based on a standard penetration rate of 2 cm/s. The CPT response of these materials can show clay-like behavior during penetration (high excess porewater pressures) but sand-like porewater pressure dissipation behavior during static dissipation testing. Partial drainage conditions are likely present in the material around the CPT during testing. Studies on how partial drainage conditions affect classification and strength interpretation are very limited. In this paper, we present CPT results from testing performed at a coal ash impoundment with penetration rates varying from 6 cm/s to 0.4 cm/s. Soil behavior classification is compared using data from the CPT soundings performed at different rates. Results are compared to index test results from samples taken adjacent to the CPT sounding. Penetration rate effects on drained and undrained shear strength interpretation and state parameter estimation are also discussed, and results are compared to laboratory and field test results. Practical recommendations conclude the paper.