Abstract: Several 300 m-high rock-fill dams with clay core are being constructed or will be constructed in China, raising an urgent need for evaluating the safety against seepage failure of the clay core after experiencing large shear deformation and subjected to high water heads. A series of vertical-consolidation, circumferential-shear and radial-seepage tests are conducted on a clay using a newly-invented ring shear permeameter. The variations of the hydraulic transmissivities of the shear bands are measured for the specimens with different compaction densities and under different surcharge pressures. When being largely sheared, the original arrangement and bonding among particles in a shear band are damaged. Under high surcharge pressure, the shear band is compressed more tightly and becomes denser, and thus little increase can be observed in its hydraulic transmissivity. Conversely, under low surcharge pressure, the shattered fragments in the shear band glide, rotate, roll and climb across each other, leading to a looser packing, and as a result, the hydraulic transmissivity of the shear band increases sharply. It seems that the preconsolidation pressure originating from compaction may serve as a threshold value to determine whether the permeability will increase for the test clay. The test results highlight an important fact that the heavily over-consolidated clay can generate shear bands of remarkably reduced seepage resistance during shearing, which can be used to explain more rationally the trigger conditions and positions of concentrated leakages happened in earth dams than the conventional hydraulic fracturing hypothesis.