三峡二期围堰是在葛洲坝水库中施工的，水深大，流速低，决定采用预平抛垫底措施，以减少围堰进占水深和堤头滑坍。对河床高程低于40m的深槽部位，采用沙砾料、石渣及块石平抛垫底，设计总投抛量约98．8万m3。为了施工和通航互不干挠，分左、右区，一区施工，另一区通航。 The second phase of the Three Gorges cofferdam is constructed in the Gezhouba Reservoir. The water depth is large and the flow rate is low. It is decided to adopt the method of preloading and laying the bottom of the dam to reduce the cofferdam’s water intake and slump. The depth of the river bed below 40m deep groove site, the use of gravel material, gravel and block stone flat bottom, the total design throw throw about 988,000 m3. In order to construction and navigation each other indifferent, points left and right areas, a district construction, another area navigation.