Study On The Static And Dynamic Properties Of Soft Clay And Environmental Effects Of The Shallow-buried Underpass In Wenzhou

In recent years, the number of underground excavations which cause a lot of engineering accidents for environmental impact is countless, and the researches on these issues have also become one of the most popular researches in the geotechnical engineering field.The underpass engineering constructed by shallow buried covered excavation method generally belongs to shallow or even super shallow buried structure, which is different from the common tunnel excavation engineering due to shallow overlying soil layer and none self-bearing capacity. Meanwhile, the structure suffers from cyclic interaction of traffic load. In addition, the deep silt clay strata is widely distributed in Wenzhou, and it adds a lot of difficulties to construction of the underpass engineering by the shallow buried covered excavation method due to poor soil, high risk of construction, location of downtown traffic thoroughfare, and strict control of settlement. To solve this problem, researches need to be focused on the following three aspects:(1) the static and dynamic properties of saturated soft clay; (2) the effect of shallow buried covered excavation on environment analysis; (3) the improved silt strata grouting method.Based on construction of the underpass engineering of the second hospital of Wenzhou Medical University, researches are studied through the laboratory tests, numerical simulation, and field grouting construction, etc. The details include:1.With the static and dynamic triaxial test system GDS, the isotropic consolidation shear tests under different confining pressure and undrained condition, the KO-consolidation compression and tensile triaxial tests, and the static tests in different stress paths were conducted. The change of vertical deviatoric stress and pore pressure along with the development of axial strain under different confining pressure and calculation formula of pore pressure and strain under constant confining pressure were obtained, and the unified expression of pore pressure in different stress paths was established. The relationship between initial unloading modulus and average consolidation pressure and that between initial unloading modulus and stress path were acquired through silt rebound deformation tests.the soft soil stratum action partition of the tunnel and shaft excavation were analyzed based on the effect of stress path, and the soil constitutive model HS was chosen for PLAXIS analysis, the relative parameters of which were determined with drained triaxial test.2.A combination of the GDS dynamic triaxial apparatus and the piezoelectric ceramic bender element equipment were conjunctively used to conduct a series of stress-controlled cyclic triaxial tests to analyze the influence of cyclic stress history with relatively large amplitude on the small strain shear modulus of saturated soft clay, which suggested that the coupling stress path under cyclic confining pressure and cyclic deviatoric stress affected the attenuation law of G/Gmax-γ:The existence of confining pressure influenced the height of the backbone curve.3.Dynamic triaxial apparatus under cyclic confining pressure, through stress-controlled system and step by step loading method, was used to analyze the influence of coupled cyclic deviatoric stress and cyclic confining pressure in bidirectional stress path on the attenuation law of dynamic modulus. by comparing the fitting formula of Hardin-Drnevich and Stokoe, it was found that the influence of stress path gradient could not be considered in Hardin-Drnevich formula, while the Stokoe formula was better for fitting the attenuation law of dynamic modulus in different stress paths. Based on the Stokoe formula, the equivalent linear constitutive model was established considering the coupling stress path under cycle confining pressure and cyclic deviatoric stress.4.Due to the process of the underpass construction by shallow buried covered excavation method, through Plaxis finite element software used to analyze the shaft and tunnel pipe jacking excavation, and through the comparison between a simple numerical example and field monitoring value to verify the model reliability is In order to analyze the different factors that influenced strata stress field and deformation, the axial stiffness of transverse support bolt, the bending stiffness of underground continuous wall, the soil disturbance degree, and the combination of adverse conditions were considered. The degree and rule of the influence of tube wall stiffness, construction load, and disturbance degree of soil on construction environmental were analyzed.5.Plaxis finite element software was used to simulate the influence of the excavation of foundation pit on surrounding environment and pile foundation under different excavation depths, the result of which was compared with centrifuge test results in the literature to verify the model reliability. By nonlinear fitting method, the model for the ground settlement calculation induced by shallow buried covered excavation was established in the soft soil strata.6.1n order to reduce the negative environmental impact, the laboratory tests were conducted to compare the compressive strength of reinforced soil with different slurry. The field tests were conducted using double tube drilling rig with different grouting materials, for example, Portland cement+Sodium silicate+admixtures and HSC containing water and fine sand, etc. The appropriate grouting material and the ratio of slurry were determined, and the corresponding parameters of grouting were decided, which were inspected by site reinforcement effect.