The Dares Salaam project in Tanzania locates in the coral gravel sandstone area. PHC piles with diameter of 800 mm and 1 000 mm are used as pile foundation. Carrying out the geotechnical test on coral gravel sandstone and studying on the law of pile sinking with difference diameter of PHC pile，we obtain the peak friction angle of PHC pile-soil interface of coral gravel sandstone debris and the residual friction angle. Summarizing the law of piling of 200 foundation piles with 1.5m long pile shoe and with diameter of 800 mm and 1 000 mm respectively，we get to know the depth of piles into the strongly weathered coral reef limestone and law for pile sinking. The penetration ability of the 1 000 mm-diameter pile is stronger，and its elevation control shall be taken as the main criterion，while penetration shall be the complementary for sinking stopping. But both are main criteria for 800 mm-diameter piles. The research results provide a guidance for PHC sinking and theoretical research under similar geologic conditions.