Research On Lateral Unloading Stress Path And Wall Deformation For Foundation Pit

The development of underground space has become a new tendency with the advancing of the reforming and opening in our country. Most of the development has relations to open excavation. The design of excavation controlled by strength has switched to the one controlled by deformation step by step in crowded cities. Undermentioned work has been done in this paper for the pit deformation.1. The soil's stress path has important effects on the retaining wall's lateral deformation and the soil's deformation during the excavation of foundation pit. An experiment simulating the soil's stress path behind the wall has been carried out with the stress-controlled triaxial instrument. The principle stress ratio keeps constant during consolidation and the soil samples are permitted to drain during unloading process. Unloading means cell pressure to be reduced while the normal axial pressure keeping constant. The conclusions have been deduced from the detailed analysis of a set of experimental results as below: 1) The formula of tangent Young's modulus Et has been established during the unloading process; 2 )The shearing features of the samples are similar to undrained test even though there are draining paths so the Possion ratio of the sample is about 0.5; 3) Unloading paths have little effects on the shearing strength parameters. The strength and deformation parameters of the unloading test have been compared with those of the general triaxial experiments.2. The lateral deformation of the diaphragm wall in clays has been analyzed for braced-cut during excavation. In order to calculate the wall's lateral displacement, several diagrams have been cited from a F.E.M results .The main factors have been accounted in these diagrams as below: the wall stiffness, the strut stifmess, the width of pit, the elastic modulus of soil, the depth of the underlying hard layer, the strut preloads and the factors of safety against basal heave. Some formulas have been put forward from the analysis with the combinations to engineering practice if the excavation is not too fast: 1) Several formulas to calculate the relationships between the maximum lateral deformation and the corresponding parameters; 2) A formula to predict the maximum lateral deformation; 3) Two optimizing formulas of the stiffness aimed for the wall and the strut system for deformation control; 4) A dominant strength formula for strengthened soil under the bottom. Then some instanceshave been put out to certify that the maximum deformations calculated by the method are good enough to meet the need of the engineering .It has also been pointed out that this method has many advantages to direct F.E.M for it needs less parameters and the parameters are easy to be obtained. This method can be easily to master and useful to practice but it can not calculate the whole wall's deformation and the position of the maximum accurately.3. The formula of the maximum lateral deformation has been proposed for tie-back wall when the anchors are embedded in unmoving soils or in rocks. This work has enlarged the application field of the previous deformation formula. Examples from engineering practice prove that the values calculated with above formulas can meet the need of the pit engineering.