Abstract: Cushion layer is an important subarea of concrete faced rockfill dam (CFRD). Prior to the construction of CFRD for Shitouxia Hydropower Station in Qinghai, China, sandy gravel materials collected from riverbed were checked, and the results revealed that the weight percentage of fine grains in the samples screened under mesh size of 100 mm( directly screened cushion material) was likely inferior to the low limit required for cushion layer. Seepage destruction and filter tests were performed for the directly screened materials and transition zone materials. Seepage field in the dam and probable maximum hydraulic gradient values both in cushion and transition zones were forecasted by numerical simulation method. The applicability of tested samples were assessed on the basis of seepage characteristics for cushion zone materials in response to the functional requirements of water leakage and seepage destruction control. The directly screened materials were judged unsuitable for cushion layer. Moreover, a scheme was proposed to improve the filling material by adding fine grains screened under mesh size of 5 mm to the directly screened material. The improved materials were checked with seepage destruction and filter tests, and were demonstrated applicable for dam cushion layer. The dam has been built up and has performed well during reservoir impoundment even under earthquake. This example tells the necessity of research on filling material throughout the construction of any embankment dam. Proof tests and applicability assessment should be taken for the natural or produced materials as early as possible before the dam fill construction.

Key words: concrete faced rockfill dam, cushion layer, transition zone, hydraulic gradient, improved cushion material