Variable penetration rate cone testing for characterization of intermediate soils

The characterization of sedimentary soil, and specifically 'intermediate' soils such as silty sands, non-plastic silts, clayey silts, and fine grained tailings are often challenging due to difficulties in sampling and testing and in interpretation of in-situ data. Cone penetrometer testing where the penetration rate is varied to control drainage conditions has been explored recently. The penetration rates required for undrained and drained conditions are proportional to the coefficient of consolidation, and a two order of magnitude change in penetration rate is generally required to move from one limiting condition to another. This paper utilizes experimental, analytical, and numerical research to examine practical implementation issues as well as theoretical details for proper analysis of variable penetration rate test data. Specifically, normalized relationships are presented to relate cone tip and pore pressure measurements to penetration rate. It is shown that partial consolidation during cone penetration at the standard rate of 2 cm/s exists if the t(50) from pore pressure dissipation test is less than 100 seconds. The errors induced by partial consolidation during penetration on interpretation of both soil behavior type charts and pore pressure dissipation data are then examined; a new solution for estimation of the coefficient of consolidation from pore pressure dissipation is presented. A new two-stage automated hydraulic control system capable of penetration rates from 20 cm/s down to 0.002 cm/s is then presented. A practical framework and chart for selection of penetration rates required to obtain drained or undrained measurements for a given soil is discussed. Finally, numerical cavity expansion results using a modified MIT-S1 constitutive model within FLAC indicate that the initial state, in-situ stress, limiting compression curve reference stress, and stress-dilatancy characteristics are the primary factors affecting the ratio of drained to undrained cone tip resistance.