Narrow and Deep Foundition Pit Engineering Practice under Conditions of High Permeable Layers in Sensitive Environments

A deep foundation pit is situated in the flood plain of Nanjing. The geological condition is that the flowing plastic mucky silty clay is distributed in the upper part, and a deep gravel layer with high permeability and strong bearing ability is distributed in the lower part. The foundation pit is a narrow cuboid with 250 m in east-west length, 50 m in north-south width, and 11~14 m in depth. Some adjacent sensitive buildings are existed in the south side of the foundation pit while some important municipal pipelines including natural gas pipeline and water supply pipe are close to the north side of the foundation pit. Based on the scheme comparison and pile testing, the combined supporting system composed of retaining pile and concrete inner support was adopted. To effectively control the underground water level, the triaxial cement-soil mixing column was taken as the waterproof curtain and the drainage was carried out through the dewatering well inside the foundation pit. Considering the mechanical property of narrow foundation pit and deformation control of the surrounding environment, the supporting structure and waterproof curtain in the south of the foundation pit were enhanced. A specific scheme of subarea and time-sharing construction was conducted to avoid the shape effect induced by pit excavation. The rational construction parameters for ultra-long waterproof curtain crossing different stratums were determined using pile testing. The detection and monitoring results show that the waterproof curtain is continuous and integral. Both the strength and deformation of structure and surrounding environment fit the requirement of design and specification. These situations can be a reference for the similar foundation pit engineering.