Abstract: For concrete faced rockfill dams, while the concrete face functions normally, the seepage stability of the dam is satisfied; while the concrete face cracks locally or does not work properly, it is pivotal for the transition zone to control seepage field in the dam and to keep seepage stability. Taking the concrete faced rockfill dam of Shuibuya Hydropower Station for example, the coefficient between cushion layer and transition zone is analyzed; it is indicated that tests are necessary to study filtering and protection of transition zone for cushion layer. Tests are performed with cushion layer and transition zone material of whole size gradation; and the seepage deformation and filtering effect is studied. Synthesizing the test results and the results in author’s papers published already, the mechanism for the transition zone to control the seepage field in superhigh concrete faced rockfill dam is concluded. The transition zone, together with the main rockfill zone, could drain efficiently water from cushion layer. Ones the concrete face cracks or does not work properly, the interior structure of the cushion layer would change with the fine particles moving downward along water flow. Fortunately, with the protection of transition zone, the particles would stop around the interface between the two zones; and seepage stability of the cushion layer would be maintained. In the first abnormal experience, some fine particles would move out from the transition zone; but the framework of the material would remain stable. With the support of the main rockfill zone, the transition zone could continue to function at both aspects of filtering and drainage.

Key words: concrete faced rockfill dam, transition zone, filter, seepage control