At the Canning Dam and indeed all dams, care had to be taken to prevent water seepage between the foundation rock and the structure of the dam. At the Canning in addition to cutting back the foundation to solid unfractured rock, a cut-off trench was sited near the upstream face of the dam, down stream which a rock filled drain containing open jointed pipes was provided to intercept any seepage between the rock face and the concrete of the dam. The inclusion of an internal drainage system was considered innovative at the period. Near vertical tubular 8 in (200 mm) cut-off drains were provided at five-foot (1.52 m) intervals along the dam to relieve internal seepage through the concrete.