Field Test Research On Deformation Behavior Of Deep Foudation Pit In Soft Clay Area

With the rapid development of construction and amelioration of the urban rail transit network in Shanghai, projects of the metro construction have showed six trends. That is the foundation pits become deeper, the scale of constructions is getting larger, the distance between buildings and subway is getting closer, the time limit for constructions is more urgent, the geologic feature is more complex, the hidden troubles are more and more. According to statistics, foundation pits with the depth of 10m to 20m account for 65 percents of total amount. The exaction of deep foundation pits in soft soil area often induces some accidents. The accidents bring great loss in economy and bad influence on society. The main reason is the lack of study on the loads and deformation properties during excavation. Under this circumstance, the engineers cannot be aware of the danger and take actions accordingly in time. In order to positively understand the loads and deformation properties in the process of excavation of deep foundation pits. In this dissertation, through gathering and analyzing a great lot of field data and making use of forward and back analysis methods, deformation properties of deep foundation pit, effect of soft clay rheological characteristics on foundation pit deformation, water and soil pressure of supporting structure and calculation method of vertical displacement of upright column are discussed and researched on the basis of the construction of the station in Shanghai Metro Line. Primary content includes several parts as follows:1. Based on the existing theory frame and construction operation and field measured data, different calculation methods of water and earth pressure are compared, the variation of water and earth pressure of supporting structure is studied systematically and variation character of water and earth pressure and some abnormal phenomena are explained theoretically.2. The wall deformation, ground settlement, structure settlement and vertical displacement of uplift column are analyzed on the basis of field monitoring data. In the meantime relationship between vertical wall displacement and ground settlement, the respondence of structure settlement to ground settlement and space-effect of foundation pit deformation are also elaborately studied.3. Brief review and conclusion of the study status of soft clay rheological characteristics, including the methodology employed, the knowledge acquired, etc. Deformation of foundation pit is divided into two phases, 'excavation deformation' and 'bracing deformation'. The effect of 'bracing deformation' on total foundation pit deformation in different situation is studied. Some effective method of minimizing 'bracing deformation' are put forward as well.4. Based on principle of vertical displacement of upright column, several affecting factors are analyzed: vertical displacement of upright column not only has relation to the upheaval of the foundation pit, the displacement and deformation of upright column due to vertical load, but also has something to do with temperature and concrete contraction. Then, a calculation method of upright column is established which isΔ=δ+Δl-s₀. At last, several harmful factors to the supporting system brought by vertical deformation of upright column are discussed and a lot of effective methods minimizing vertical deformation of upright column are given.