The deep foundation pit under study is located in waterrich composite ground consisting soft strata at the upper part and hard strata at the lower part. Threeaxis mixing piles are designed as the waterstopping curtain of the foundation pit. The threeaxis mixing piles are required to extend into the weathered rock so as to isolate the impermeable strata. Therefore, extending the threeaxis mixing piles into the weathered rock is a technical challenge to be studied. In the paper, analysis is made on the type selection of machines for the threeaxis mixing piles, a new type of drill bit is designed for the drilling machine, and construction parameters suitable for the geological conditions are studied. Conclusions drawn are as follows: 1) Threeaxis mixing piles can be used as the waterstopping curtain for foundation pits located in waterrich composite ground consisting soft strata at the upper part and hard strata at the lower part; 2) Because threeaxis mixing piles can be extended into weathered rock, “bored piles + threeaxis mixing piles” may be used as retaining structures in geological conditions similar to those mentioned above to replace the diaphragm walls so as to reduce the investment. 