Study On The Deformation Of Protecting Structure In Subway Station Of Deep Foundation Pit In Unsaturated Soil

Deep excavation design and stability analysis should consider the impact of changes in groundwater level, on the design of support measures for metro and other vertical deep pit dug prescription. A foundation pit dewatering will be carried out before excavation, with the groundwater level dropping due to excavation, soil of foundation pit within a certain range will become unsaturated soils, increasing the shear strength in the short term. In engineering, we should take advantage of this effect, which can provide an effective boost for foundation pit supporting.In this paper, for the existing unsaturated soil deep foundation pit problem in subway station deep excavation construction in present, studying what impact it will have on the pit deformation, supporting structure stress and others if the foundation pit soil considers unsaturated soil properties. With "Saturation(water content) ® the Shear Strength Parameters ® Pit Excavation Simulation" as the research idea, this paper has done the following work:The paper has three main parts, the first part is the judgment of unsaturated soil, namely before excavation, foundation pit needs precipitation which will make the groundwater level drop, soil of foundation pit within a certain range will become unsaturated soils, the first part is to determine how much of this range, that is how many meters above the water line that the foundation pit soil can be considered as unsaturated soil. "Precipitation--Saturation--Matric Suction--Unsaturated Discrimination(how many meters above the water line by unsaturated soils considering)" is the research route in this paper. Through matric suction we obtained that foundation pit soil above the water line 0.81 m considered as unsaturated soil is reasonable, which provides a reliable basis for the later supporting structure deformation analysis of unsaturated deep foundation pit and verifies the accuracy of the method of establishing numerical model.The second part is how to consider unsaturated soil after the judgment of unsaturated soil that already knowing which foundation pit soil is unsaturated soil. Studying the law of changes between different saturation(water content) and unsaturated soil strength parameters through ordinary triaxial tests, obtaining the fitting formula which has certain reference value by fitting test results.This part discovers that unsaturated soil shear strength and saturation(water content) are closely related, the fundamental difference between unsaturated soils and saturated soils is the effect of matric suction, the relation curve between saturation and matric suction is soil-water characteristic curve, then thinking of using SWCC combined with saturated soil strength parameters to predict unsaturated soil strength parameters. Due to the lack of experimental data, referring to test data of related papers, using the prediction formula to predictunsaturated soil strength, it was well, but the prediction formula still lacks sufficient test data to support it.The third part is studying what impact foundation pit considering the unsaturated soil will have on foundation pit deformation and supporting structure stress, deformation, etc. and comparing it to the results of pit excavation simulation by saturated soil parameters, studying and analyzing differences between the two and summing up the law.You can find that the saturation change will produce a series of effect on unsaturated soil and foundation pit deformation, taking unsaturated soil into account is of great importance to pit excavation, this study provides a reliable reference for unsaturated soil subway station deep foundation pit excavation, design and assessment management, it has an important guiding significance for future construction of similar engineering.