Experimental Study On Engineering Properties Of Foundation Soil Related To Excavation Process

With the development of the urbanization, more and more engineering constructions appear, such as super high-rise building, underground mall, tunnel, subway and so on. Meanwhile, there is an increase in building height, depth of foundation and building scale. Almost every engineering is related to the excavation. The stress state of soil, under the action of engineering behavior, is no longer a simple loading. Rather, it mostly contains unloading rebound and loading recompression. Stress path is more complex.Not only the initial and final stress state, but also the stress history and stress path are the influencing factor of the mechanical properties of soil. It is obviously different in the condition of loading and unloading. However, the engineering design rarely considers these. It ignores the influence of the consolidation condition and the stress path that put on the intensive property of soil.Considering the impact on the surrounding environment, Foundation pit engineering design gives priotity to deformation. The foundation soil is not completely in the limit equilibrium state. However, the analysis of earth pressure still assumes that the soil is in the limit state; Soil mechanics parameters is strength index which is obtained under the loading path, and it does not involve the effect of stress history and the path of excavation unloading on the mechanical properties of soil. Foundation pit engineering design is more based on experience, its often do not tally with the practical engineering.According to an instance of foundation pit, this paper studied stress history, stress path, and their influence on stress-strain relationship of soil. Combined with the numerical analysis and the full scale test of the foundation, this paper expects to get the foundation pit engineering optimization design method. The main research contents and conclusions were as follows:1. the research on stress-strain relationship for foundation pit excavation and the normalization analysis of stress-strain relationshipThe excavation is an unloading process, and the stress path is complex. By mean of simplified analysis, three kinds of stress path, which is true of foundation pit engineering, is proposed.(1) stress path① active zoneAxial stress is constant, and radial stress is reduced. It reflects the stress state in the active zone. There is hardly and unloading deformation if the value of unloading ratio is less than Rq.② passive zoneAxial stress is reduced, and radial stress is increased. It reflects that the soil is extruded by the excavation retaining struction, and it causes that lateral pressure is increased, besides, the lateral pressure is greater than the overburden pressure. The initial strength is large, and the shear strength index of soil, which is obtained by the unconsolidation-undrained triaxial test, does not conform to "φ=0" theory.Axial stress is reduced, and radial stress is constant. It reflectes that the excavation retaining struction has little squeezing effect on foundation soil where the buried depth is large. Due to the decrease of the overburden pressure, the lateral pressure is reduced, too. But the change is smaller amplitude; the deformation is small while the unloading ratio is large, but there is rapid increase of the strain of soil if the unloading ratio increase to a certain degree (generally, above 0.7).(2) the normalization analysis of stress-strain relationshipAccording to the characteristics of the stress-strain relationship, the normalization equation expression is determined. The stress-strain relationship, that is obtained by the above stress path controlled triaxial tests, is put into constitutive model. Compile DLL (dynamic link library), and analyze foundation pit excavation. The numerical results are very close to the measured data, furthermore, the rationality of the stress path is verified.① active zone (Axial stress is constant, and radial stress is reduced.) The normalization factor of the stress-strain relationship of soil:qc+Ro·σ3c-the normalized equation:② passive zone (Axial stress is reduced, and radial stress is increased.) The normalization factor of the stress-strain relationship of soil:σ3+c/tanφ. the normalized equation:③ passive zone (Axial stress is reduced, and radial stress is constant.) The normalization factor of the stress-strain relationship of soil:σm. the normalized equation:In addition, q= σ1-σ3, qc= σ1c -σ3c, σm= (σ1c+2·σ3c)/3; q and qc are respectively stress deviation and initial stress deviation; σ3c and σ3c are respectively initial maximum and minimum principal stress.2. the research on the method for the optimization design of foundation pit engineeringThrough the analysis of the data obtained by soil test, numerical calculation and field test, it finally come down to the stress-strain relationship of soil under the unloading path of excavation, the calculation method of the earth pressure is determined. It is different from Rankine’s theory and Coulomb’s theory. The soil is not entirely in the limit equilibrium state. Based on the stress-strain relationship, this method determined the influence depth of excavation and potential slip surface, estimated the regionalization of stress state of soil, and obtained the expressions to calculate active earth pressure, passive earth pressure and horizontal soil resistance coefficient.3. the research on the meansurement method for earth pressureField test should reflect earth pressure of the undisturbed soil. Earth pressure cells were successfully implanted in the specified location through ground operation. Monitoring data can more truly reflect the working state of the soil and retaining structure in the process of excavation.the process of excavation retaining structure and the working state of the soil, and it can provide the basis for the analysis and calculation of earth pressure of foundation pit engineering and the reliability of numerical simulation. There is almost not so complete monitoring data in the existing foundation pit engineering, besides, it is the first application of the embedment method for the new earth pressure cell. It can be used in practical engineering.