Research On Settlement Of Saturated Loess Cement Mixing Pile Foundation Reinforcement Under Embankment Load

With Chinese infrastructure construction scale widely expanding, the saturated loess’ s reinforcement is getting more and more attention of scholars. Saturated loess has some characteristics such as high water content, high compressibility and rheology. Because of locating in the lower place underground or soaking under the surface water for long time,the saturated loess has no collapsibility. Therefore, when dealing with saturated loess foundation, we can regard it as soft foundation. The cement mixing pile foundation is one of the most effective ways to reinforce soft ground.In this paper, as the background of reinforcing loess foundation from Lanzhou to Zhongchuan airport railway, composite foundation of soil cement mixing piles is applied to reinforce the saturated loess. Through settlement meters of single point are embedded in the composite foundation, monitoring the settlement deformation of foundation in the progress of subgrade filling and researching the settlement characteristics of composite foundation of soil cement mixing piles under the under embankment load. It is concluded that the following main results:(1) According to the design that the soil cement mixing piles are applied to reinforce the saturated loess, the bearing capacity of foundation and settlement meet the engineering demand. To accumulate more experience, in test section using three kinds of different amounts of cement that are 12%, 16% and 12% respectively. Tests showed that the appropriate increasing of the amount of cement can obviously decrease the settlement of composite foundation. Cement mortar has a critical value, however, when the amount of cement is 16%, if cement continues to be increased, settlement control effect of composite foundation becomes weakened obviously.(2) In the process of subgrade filling, with the increasing of filling height and time, the settlement of soil-cement mixing pile composite foundation is increased gradually. During the filling of subgrade, the change of settlement of composite foundation is bigger, but the filling is finished, the settlement become small. Settlement of subgrade filling process is two-thirds of the total settlement. At the same cross section, along with the increasing of height of the subgrade, the laws of settling curve are similar, but settlement is different. The settlement near the center of subgrade is the largest. The closer to the edge of the foundation the settlement is smaller.(3) Combined with the measured settlement data, contrasting the imitative effects of hyperbola method, three-point method and Asaoka method. Tests showed that the relative error of hyperbola method is minimum, and related index is the largest, and fitting precision is the highest, and imitative effect is the best. Three-point method is poorer than hyperbola method. Asaoka method is the worst. For this project that soil cement mixing piles is applied to the saturated loess, the settlement of the best fitting model is the hyperbolic model.(4) Compared with the final settlements of the three kinds of methods, it is concluded that the final settlement of the subgrade center is 113 mm, and the final settlement of the center of left rail is 73 mm when the cement adding amount was 16%. The final settlement of the subgrade center is 148 mm, and the final settlement of the center of left rail is 71 mm when the cement adding amount was 20%. The final settlement can provide certain reference value to determine the time of laying tracks.(5) According to the numerical simulation analysis and calculation by FLAC 3D, in the progress of subgrade filling the settlement near the center of subgrade is the largest and the closer to the edge of the foundation the settlement is smaller. Compared the numerical simulation results with the measured data, we can infer that the settlement of the numerical simulation is larger than the measured settlement of foundation. In numerical simulation, when the cement adding amount increases, the settlement of soil-cement mixing pile composite foundation decreases gradually. However, when the cement adding amount is increased from 16% to 20% in the practical engineering, the foundation settlement does not increase rather than decrease. So sometimes the numerical analysis has certain deviation with the actual engineering situation.