Abstract: Although saprolith is weathered and shaped like soil, it still retains the texture and the occurrence of its mother rock. It has relatively low strength (usually less than 2MPa) under certain moisture. After excavation and compaction it is partially broken into soil and becomes soil and rockmixed material.The study of saprolith has been carried out based on construction of the Tianshen-sqiao and the Lubuge hydropower project, which consists of a 180m high dam and a 103m high dam respectively.This paper studies the compactibility of a soil and rock mixture and other factore which atfact the compaction result, such as type and properties of mother rocks and specimen preparing method, variation of coarse grain content before and after compaction, and their effect on the engineering characteristics of the material. The paper discusses determination of compaction standard and selection of compaction equipment for this kind of soil based on both laboratory compaction tests with different compacting energy and field compaction tests.This paper also proves that saprolith is a good impervious material, which will bring about significant technical and economical benefit, together with short haul distance, less occupation of farmland and easy construction for water control works.