The Rheological Characteristics Of Soft Soil And Its Effect On The Performance Of Excavation Engineering

Shantou is a soft soil district with poor geological conditions, silt soil and muddy soil are covered as thick as about 10m ~ 30m. Shantou silt has remarkable rheological properties besides the characteristics of high water content, low strength, high compressibility, small permeability etc. In recent years, more and more large-scale excavations are proceeded with long period, the rheological properties of soft soil may bring adverse effects to the projects. It is an important issue of geotechnical engineering to study the rheological properties of soft soil, determine the deformation and stress of the variation with time, reduce the adverse effects of the rheological soft soil to projects, and provide a useful reference for the design and construction of excavation engineering.In this paper, a series of rheological tests are carried out with the reconstruction soil samples of Shantou silt, including one-dimensional oedometer tests, triaxial tests, direct shear tests and stress relaxation tests, considering the effects of stress history. The creep tests mainly measure the soil deformation varying with time. In general, creep deformation is small, has no clear boundary with primary deformation, and effects by multiple and complex factors such as water content, temperature, drainage condition and stress history. Stress relaxation tests measure the stress varying with time, which have a relatively short test time, but currently are lack of theoretical support. Since rheological tests consider the effects of time factor, the results better fit the actual situation of long-term construction projects.To establish an empirical creep model about Shantou silt based on tests. The classical Singh-Mitchell creep model is based on triaxial tests, however, this paper analyzes the results of direct shear creep tests and establishes a modified Singh-Mitchell empirical model which can appropriately describe the time-dependent deformation of Shantou silt. Numerical analysis of the long time progress of an excavation is proceeded with Plaxis finite element program. The foundation pit of Chenghai People's Hospital Building located in Shantou soft soil area is lying on a silt layer about 20m thick, with 5.5m deep excavation and three months of construction process. The deformation and internal force of the excavation support is impacted by time. The process of excavation is simulated with Plaxis finite element program to compare the difference of wall support and pile support, analyze the impact of step by step excavation, examine the deformation and internal forces of the support structure also with monitoring data. The selection of soil parameters is an important issue, which requires to perform sufficient and precise creep tests, this paper determines the parameters of Soft Soil Creep Model in Plaxis by oedometer tests.