Piezocone Evaluation of a Shallow Soil-Bentonite Slurry Wall

It has been well established that the confining stresses at which the soil-bentonite is tested at in the laboratory significantly affects the measured hydraulic conductivity. However, the stress state within the soil-bentonite cutoff wall (SBCW) is usually unknown; although it is generally agreed upon that it is less than the geo-static effective stress, defined as (γ' ⋅ z). With a highly compressible material like soil-bentonite, an overestimation of the confining stresses during laboratory testing would lead to an underestimated determination of the field hydraulic conductivity, thereby establishing a non-conservative cutoff wall permeability. A total of 57 piezocone penetration and pore pressure dissipation tests were conducted in a SBCW as part of a post-construction Quality Assurance (QA) testing program. The piezocone tests were used to provide an estimate of the in-situ effective stress of the SBCW, while the pore pressure dissipation tests were used to provide estimates of the in situ hydraulic conductivity. Results of the testing program showed that utilizing a pre-consolidation pressure correlation from the piezocone penetration pore pressure can provide reasonable estimates of the in-situ effective stress when compared to Arching and Lateral Squeezing theories. These results can aid in the laboratory evaluation of the hydraulic conductivity. Pore pressure dissipation results provided in-situ hydraulic conductivity results that compared favorably to laboratory flexible wall permeability tests.