Study On Method For Settlement Calculation And Loading Properties Of Soft Soil And Hard Crust Roadbed In Dongting Lake Area

In recent years, along with the rapid development of our country's economic construction, plenty of freeways will be built, while the soft soil and the hard crust foundation have extensive distribution along the freeway route. Hard crust and the soft soil will form a whole system under load, which makes the settlement and bearing deformation characteristics of the foundation are distinctly different from the soft foundation. However, our present level with settlement deformation of the hard crust and the soft foundation is still very scarce, and because of the complexity of the problem, the diffusion effect of the hard crust and the affect of the hard crust on the settlement deformation law of foundation only remains in the engineering test analysis and there is no quantitative theoretical answer now. Therefore, studies of the engineering characteristics and settlement deformation of the hard crust and soft foundation in the Dongting Lake scale, as well as the stress diffusion effect of the hard crust, are developed under the supports of Hunan provincial highway S204 road project.Based on summarizing the previous studies and using as a source of reference, a further study of the differences of the physical properties between hard crust of the Dongting Lake area and the soft soil under it was developed, combining the laboratory soil test and in-situ test. And based on the beams on elastic foundation theory, the law of settlement and deformation of hard crust with soft foundation under it was developed. Main results and conclusions are as follows:Firstly, a great deal of soft soil was sampled for a typical hard crust layer and soft foundation of the Dongting Lake area. A further study of the differences of the physical properties between the hard crust soil and soft soil under it was developed, combining the laboratory soil test with the engineering geological investigation of the region, as well as the law of the physical properties along the deepness of the foundation, and a preliminary identification method of the thickness of hard crust was introduced.Secondly, based on the shell body effect and the stress diffusion effect of the hard crust, as well as the elastic foundation beam theory, which considers the soft foundation as a linear elastic isotropic body, and the hard crust layer is a board with the limited width and infinite long, an analytic solution of beams on modified Vlasov foundation with lateral displacement was developed. The lateral and vertical displacements of the foundation were represented with separate variable method. The control deformation differential equations of beam, foundation and vertical attenuation functions were established according to the principle of minimum potential energy. Then, introducing four non-dimensional parametersγ1～γ4, it could get the lateral and vertical displacement with an iterative procedure. The parametric analysis was conducted, then. The computation and analysis of an engineering example was compared with actually measured and calculated settlement results by other methods, it shows that the proposed methods in this text are reasonable and feasible.Finally, an in-situ test was developed with a different hard crust thickness and soft foundation. The settlement deformation characteristics law and the bearing characteristic of the hard crust and soft foundation were studied, as well as the variation of the earth pressure and pore water pressure under the loading in the soft foundation. The test results showed the three developing stages of the settlement deformation of hard crust, inclunding linear elastic, elastic-plastic and limit state, also the test results showed the stress diffusion effect and the failure modes of the hard crust. Comparing the settlement and additional stresses by the theoretical calculations with in-situ test measured, it shows the proposed theory is reasonable.