Dynamlc Response And Long-term Settlement Of Soft Foundation Under Traffic Load

In recent years, with the rapid development of high-speed railway and highway, many researchers paid more attention to the dynamic response of foundation under the action of moving load. With increase of the vehicle operating speed, the vibrations of ground will become larger. When the speed approaches the critical velocity of ground, extremely large vibrations will occur in the ground, which will be a serious thread to the safety of high-speed vehicle. So, in this dissertation, the detailed research and exploration to the dynamic response of saturated foundation under moving load was carried on. In addition, large cumulative settlement and uneven settlement will be produced by the traffic load in the soft subgrade. This problem will seriously affect the vehicle’s normal operation, and will increase maintenance and repair costs, or even result in security incidents. This dissertation presented a new approach to predict the long-term settlement of soft subgrade under traffic load.Firstly, the integral transform method was used to study the dynamic response characteristics of saturated half space under the moving load. In frequency-wave number domain, the dispersion curve of body wave and surface wave of saturated soil, the Rayleigh wave equation of saturated soil, the Doppler Effect and the composition of the wave in dynamic response of the medium and so on were analyzed. The dynamic response of saturated soil under the moving load is associated with the waves produced in saturated soil. In time-space domain, analysis of the vibration and fluctuation conditions of the foundation surface were carried on.For the actual soil distribution and the influence of groundwater, the present study proposed the overlying elastic layer of saturated half space model. The model is divided into two layers, the upper thin dry layer is modeled by elastic medium and the underlying saturated half-space is considered as a two-phase medium that follows Biot’s theory. In addition, the interface between the upper dry layer and the bottom saturated half-space is considered to be either drained or undrained. In this dissertation, the dynamic response of the new model under moving load were studied. The efforts were concentrated on investigating the effect of the overlying dry layer on the dynamic response. The influence of the permeability of the interface (i.e. a drained or undrained interface) was also studied.In engineering practice, the permeability coefficient of the ground surface layer is not infinitesimal or infinity and should be considered as impeded boundary condition. The dynamic response of saturated half-space with impeded boundary to a moving load was investigated in this dissertation. Also, the influence of impeded boundary to dynamic response was studied.A simplified model of Metro rail and foundation coupling system was presented, and the dynamic response of the coupled system to buried moving loads was studied in this dissertation. Considering the layer of soil, the Transmission and Reflection Matrices method was used. TRM method can be valid for the high frequency and large layer thickness cases, which are difficult to be solved by the conventional propagator matrix method. The solutions of elastic layered foundation, coupled elastic layered foundation-beam system, saturated layered foundation and coupled saturated layered foundation-beam system were presented. In some simple conditions, the present solution can have a very good match with the existing solution, which demonstrates the validity of the solution in this dissertation. Many factors such as the load speed, load frequency, embedment depth of load, beam stiffness, sublayer were discussed by a numerical way.The stiffness and strength of saturated soft clay will attenuate under long-term cyclic loading, which will lead to the plastic deformation. Based on the existing cyclic triaxial test results, a modified formula of stiffness degradation was proposed, which builds the relationship between the decay factor and the number of cycles. And then, the model of stiffness degradation was embedded into ABAQUS software. So, the stiffness of soil units will decay by the law of the proposed model after each cycle. Accordingly, the accumulated plastic deformation after each cyclic loading was computed. At last, a practical example for long-term settlement under the traffic load induced in Xiaoshan soft clay was presented to demonstrate the new method. The result shows that the permanent settlement due to the traffic load is mainly contributed by the superficial layer of the subgrade and reduces quickly along with the increase of the depth. As a result, it is effective to reinforce the superficial layer of the soft subgrade. Furthermore, it is found that the diffusion function of pavement structure layers to the wheel load is noticeable. Consequently, the effect of the pavement structure layers in the computation can’t be ignored.