The bentonite cake on the sidewall surface of the trench for constructing slurry trench walls results from the filtration of slurry into the adjacent soil formation. The hydraulic performance of the slurry trench walls is deemed to benefit from the low hydraulic conductivity of the bentonite cake. This paper provides measurements of hydraulic conductivities of bentonite cakes formed on the Korean standard sand under various pressure levels. Two kinds of bentonite (Tixoton and Bentonil GTC4) were used to create the slurries with three different concentrations (4, 6, and 8% by weight). An estimation of the hydraulic conductivity of the bentonite cake based on the filtration theory was carried out. The range of hydraulic conductivities of the bentonite cakes was from 2.1×10⁻¹¹ m/s to 5.7×10⁻¹⁰ m/s. The hydraulic conductivity of the bentonite cakes deposited on the Korean standard sand were compared with that of the cake deposited on the filter paper to show the effect of the filter medium. The slurry concentration of 6% seems to be a value that most stimulates the effect of the filter medium on the permeability of the Bentonil GTC4 bentonite cakes.