The Technical Research On The Large-type Of Foundation Trench Protection Along The Qiantang River In Hangzhou Area

The foundation trench project is an old and contemporary soil and rock engineering subject, which many experts and researchers have been conducting research on the reasonable trench protection schemes for the specific geological conditions. Only in recent years, the construction of high-rise building on the sandy soil along the Qiantang River has been started. Although there have some small-scale engineering practices in this area, many problems exist in the trench protection design and construction. In addition, no extensive exploration have been made on the relationship between the theory and the practice. Therefore, it is necessary to make technical research on the large and deep foundation trench protection means along the Qiantang River according to the specific geological conditions of the representative project-CHINA RESOURCES SUN HUNG KAI PROPERTIES.In this paper, in accordance with the research results of other experts and scholars, analysis and comparison was made on the possible trench protection schemes based on the trench protection dimension and depth of the representative project. Great attention was given to the structure of the drilled and poured pile plus compound soil nail and the construction method of "central island" during the research process. The feasible scheme of the water-level reduction scheme, the reduction depth inside and outside the trench as well as the layout of deep well or well-point system were put forward in conformity with the trench seepage theory. The optimum water-tight curtain design, the surface settlement caused by water-level reduction using the limit-element calculation method and the environmental impact evaluation were made based on the trench environmental requirements. Analysis was made on the force bearing features of the compound protection structure and the underground water treatment was evaluated according to the project site testing and monitoring.It was stated that the drilled and poured pile plus compound soil nail protection system is feasible when the project site is spaciously limited to have a sloping, the trench deformation is strictly required and the excavation depth is around 10 meters in the sandy soil ground. The aforesaid scheme could ensure the foundation trench stability, effectively control the trench deformation, excavate easily and shorten the construction period without the inner supporting system. For the extra deep and large foundation trench, the application of the compound protection and supporting system could construct the trench in step without affecting the construction period, improve the whole system safety, reduce the trench protection cost, and easy for the construction process. In addition, it is necessary to arrange water-tight curtain for the extra-deep trench in the sandy soil foundation. Compared with other type of curtains, the continuously overlapped casting tri-axial cement mixing pile is the most economic and efficient water-tight form. For the extra large and deep trench in the sandy soil foundation, the gravity deep well is one of the reasonable and economic way to reduce underground water level.