Evaluating the hydraulic barrier performance of soil-bentonite cutoff walls using the piezocone penetration test

Abstract

When using cutoff walls to prevent the migration of mobile contaminants in aquifers, we should ensure that these cutoff walls maintain both low hydraulic conductivity and high homogeneity before the start of their operation. However, the on-site assessment of the properties of their main component, soil-bentonite (SB), is challenging because of the difficulty in the obtention of high-quality solid core samples after installation. This paper discusses an experimental approach for on-site quality control/quality assurance (QC/QA) immediately after installation to enhance the reliability of SB cutoff walls as containment barriers. In particular, the piezocone penetration test (CPTU) was conducted using a large-scale soil tank in which SB – prepared with different contents of bentonite powder – was filled to simulate vertically homogeneous or heterogeneous SB cutoff walls. The response to the penetration test varied according to the bentonite content, suggesting that the piezocone penetration test can be used for QC/QA of constructed soil-bentonite cutoff walls. Corrected cone resistance values were larger in layers prepared with lean SB mixture than in those prepared with rich SB mixture, with a difference similar to that of undrained shear strength as evaluated by triaxial compression tests. It was also found that the hydraulic barrier performance of SB cutoff walls could be assessed via a pore pressure dissipation test during CPTU, within one order of magnitude accuracy in a short period. A basic process for QC/QA using the piezocone penetration test in the field is proposed for the practical interpretation of experimental results.