River diversion and closure and technology of cofferdam safety control with thick overburden is studied in a research project supported by the National Key Technology R&D program. In special topic 3 of the project, field test, laboratory test, physical modeling and numerical calculation are employed to investigate the engineering properties of thick overburden and cofferdam materials in the purpose of tackling the scale effect in engineering property tests on coarse  grained materials and determining the underwater dumping  filling density for the cofferdam with thick overburden. Wudongde, Baihetan and Shuangjiangkou Hydroelectric Station cofferdam projects are taken as background and case study for the special topic. The main content, technical route and important achievements of the topic are briefly presented. Detailed research approaches and achievements will be reported later.