Determination of Consolidation Characteristics in Fine Soils Evaluated by Piezocone Tests.

Abstract

There has been an increasing trend to use the piezocone penetration test (PCPT) as an in situ tool of choice for determining the consolidation and flow characteristics of cohesive soils. The focus of this research is to study various factors that would affect the determination of consolidation characteristics in fine soils evaluated by piezocone tests. Immediate changes in excess pore pressure and tip resistance after penetration arrest for dissipation were experimentally identified. Undrained shear strength, influence of stress history, and lateral stress coefficient effects on the penetration pore pressure were investigated. By using a two-stage slurry consolidation technique, cohesive soil specimens of very high quality were prepared in a specially designed calibration chamber (LSU/CALCHAS). Four standard piezocone penetration tests (reference) and twenty one miniature piezocone penetration tests were performed at various boundary conditions, stress level, and stress history. In order to capture the true excess pore pressure drop after penetration arrest in dissipation tests, data acquired at very close time intervals (0.01 seconds) using a digital oscilloscope were utilized. The oscilloscope results indicate a sudden drop in the excess pore pressure, especially at the tip of the cone, due to the normal stress reduction as soon as the penetration ceases. Hence, the interpretation of the dissipation results for the determination of the radial coefficient of consolidation (cr ) should be based on the initial dissipation values of the excess pore pressure and not the penetration pore pressures. Determination of the initial excess pore pressure distribution for a dissipation analysis should take into account the dissipation which has already occurred during piezocone penetration. The method proposed by Gupta and Davidson (1986), simulating the piezocone penetration process as successive spherical cavity expansions and taking into account the dissipation effect gave very good agreement with the dissipation results at the cone base. The predicted spatial pore pressure distribution during the dissipation phase showed only a qualitative agreement with the experimental results due to the limitations and simplifying assumptions in the method. The PCPT results were comparatively evaluated using state-of-the-art methods to estimate the undrained shear strength, Su; lateral stress coefficient, K0; overconsolidation ratio, OCR.