Study of Boundary Effect on the Electrical Resistivity Measurements of Cement-Bentonite Mix

Seepage cut-off walls are common seepage mitigation measures in dikes to minimize seawater ingress. These cut-off walls are usually made of cement-bentonite (CB). The quality of this CB wall is critically important. Non-destructive testing would ensure quality control/assessment without damaging the wall’s integrity. One such non-destructive technique is the measurement of electrical resistivity. Correlations can be established between electrical resistivity, strength, and permeability of CB mixes. Continuous monitoring of electrical resistivity across the CB wall would enable early detection of its properties instead of waiting for coring of samples after 14, 28, or 91 days of curing, which would introduce discontinuities in the wall. For in situ electrical resistivity measurement, electrodes of finite size are buried in an infinite soil medium, where A_s, the area of soil mass is much larger than A_e, the area of electrode. This is very different from laboratory measurement, of which A_s = A_e. Thus, the effect of boundary on these two situations is very different, and the correlation between resistivity and unconfined compressive strength, as well as between resistivity and permeability, established in a laboratory setup may not be suitable for use in in situ measurement condition. Hence, this paper aims to systematically study the impact of boundary on the electrical resistivity of CB mix. Consistent CB mix placed in boxes of different heights and widths (thus different values of A_s) was designed to study the boundary effect, with area of electrodes (A_e) kept constant. It was found that the electrical resistivity value increases with reducing A_s/A_e value, and A_s/A_e has to be greater than 25 to negate the effects of boundary on electrical resistivity. In addition, there is an issue on the direction of resistivity measurement. The in situ resistivity measurements were more conveniently carried out horizontally in an infinite soil medium, while the laboratory cored samples were normally measured in vertical direction. Thus, there is a need to study the anisotropy effect on electrical resistivity value of CB mix.