Abstract: With the ever increasing of urban population, the speed of modernization and renewal, and the continuous improvement of residents’ living standards, upgrading of various wastewater treatment plants has become the development trend of the urban wastewater treatment projects. This paper presents a case history of a comprehensive deep excavation for a wastewater treatment plant in Shanghai. The gravity supporting structure formed by soil-cement mixing with HP pile insertions is numerically simulated by using the commercial finite element analysis software PLAXIS. The HS-Small soil constitutive model is implemented in the analysis of the deformation and stability of the supporting structure and the analytical results are compared with the construction monitoring data. The results show that :(1) for some special elements of the waste treatment structures, the footprint size is large and the required excavation depth is about 5m~7m. The conventional plate enclosure + internal support system not only increases the cost but also has the construction difficulty of the main structure, while the compound gravity dam can resolve the above problems; (2) The composite gravity dam supporting system has good feasibility and engineering application effect in the range of 6.95m local depth of shallow foundation pit, which can be used as a supplementary form of cantilever retaining in the future deep excavation support. (2) The HS model used to simulate deep excavation in soft soil area has good convergence and is close to the monitored data; (3) The field monitoring results show that the H-section steel and steel pipe in the composite gravity dam can provide large lateral stiffness, resist most of the earth pressure, and control the slope of the dam body to a certain extent. (4) Composite gravity dam can solve most of the problems in practical engineering, and the pit depth is suggested to be between 5 and 7 m.

Key words: Composite Gravity Dam, Deep Excavation with Cross Strut Support, Finite Element Analysis, Monitoring, Displacement