Triaxial seepage tests were conducted on the core-wall clay of a high rock-fill dam to investigate the change of hydraulic conductivity with axial strain. It was found that the compaction density and confining pressure were the two main factors influencing the change of hydraulic conductivity during triaxial compression. When the current confining pressure was greater than the pre-consolidation pressure of a compacted specimen

the specimen kept being compressed and became denser during the compression process

resulting in a decreasing trend in its hydraulic conductivity until it eventually reached a stable state. On the other hand

if the current confining pressure was far less than the pre-consolidation pressure of the specimen

the specimen deformed in a localised shear band which weakened the impermeability of the specimen

and as the shear band continued to dilate

an increasing trend in the overall hydraulic conductivity was observed. This study highlighted an important fact that heavily compacted clay under low confining pressures had a high susceptibility to localised shear bands of high permeability

which could be used to explain many historical leakage problems observed in clay-core dams.