Model Test On Bearing Behavior Of Square Closed Diaphragm Wall As Bridge Foundation In Collapsible Loess Ground

With the implementation of the western development strategy, transportation infrastructure construction in the western region of China has blossomed and continues to thrive. Collapsible loess is widely distributed in the northwest of China, which has a special influence on the construction of bridge foundation in the collapsible loess site with its special loose and collapsible soil structure, such as influence of foundation collapsible deformation on the bearing capacity of the foundation structure. Compared with pile foundation, open caisson foundation and shallow foundation in the collapsible loess site, adaptability of a new type of closed underground continuous wall as bridge deep foundation in the collapsible loess area is explored in this paper, which has advantages of high rigidity, wide application, high construction efficiency and good bearing stability. In loess area, closed diaphragm wall is generally used as the deep foundation of bridge, which brings the problem that negative friction of the closed wall caused by loess collapsibility can't be ignored when closed wall is used as a bridge foundation in the collapsible loess area. Therefore, it is of great practical significance to study the loess characteristics, vertical bearing capacity and variation law of negative friction for closed diaphragm wall in the collapsible loess area.Model test similar materials of collapsible loess has been developed, which was applied in the indoor model test of closed diaphragm wall.The vertical bearing capacity and the development law of negative friction of underground closed wall in collapsible loess area were studied by the method of numerical simulation. The main research contents and results are introduced in the following:(1) Based on the collapsible mechanism of loess, sand, quartz powder, bentonite, gypsum, and industrial salt were selected as basic materials, and the ratios of similar materials of collapsible loess with different collapsible coefficients were obtained after a large amount of proportioning test. Based on the results of collapsible test, the similar materials and natural collapsible loess have similar collapsible characteristic that strong collapsibility has appeared after water immersion, which could be applied in indoor model tests preferably.(2) Vertical bearing capacity and negative friction performance of closed diaphragm wall as bridge foundation in collapsible area were researched through the indoor model test. Prepared similar materials were used as model soil, and organic glass with surface treatment was used as the wall of square model box with the size of 25×25×57cm. Axial force of wall body, lateral friction resistance of inner wall and exterior wall, additional stress of soil collapsibility, resistance of wall end, collapsible deformation of soil mass, and settlement of wall roof were tested, with which analysis on the law of vertical bearing capacity and characteristic of negative friction of closed diaphragm wall in collapsible loess area were carried on.(3) According to the closed diaphragm wall model test of vertical bearing capacity, the nonlinear distribution of lateral friction resistance of inner wall and exterior wall is significant, and the maximum frictional resistance is located on the lower part of the inner wall and exterior wall. In the stress process of model test, exterior walls bear the most part of force, which is followed by the inner wall, wall end resistance and soil resistance under the cap.(4)Based on the results of lateral friction resistance model test for closed diaphragm wall, bearing capacity of the wall significantly reduces, and the negative friction of exterior wall comes out like a parabola after water immersion of the soil around wall, which promotes inner wall frictional resistance, wall end resistance and pile cap soil reaction compared with the condition of before water soaking.(5) Based on the finite element software MIDAS/GTS, field test was simulated with modulus reduction method. The variation law of displacement nephogram and of the stress nephogram was analyzed in the process of loading and soil wet subsidence. By comparing field test, indoor model test, and numerical simulation, development trend of lateral friction resistance of inner wall and exterior wall, and the axial force of wall body are generally consistent with the results.The research results of this paper can improve the research theory of closed diaphragm wall in collapsible loess area, which could provide technical reference for engineering design and construction.