 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 <unk> rock , a cut @-@ off trench was sited near the upstream face of the dam , down stream which a rock filled drain containing open <unk> 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 . 
