Study On Settlement And Mechanics Property Of Railway U-shaped Structure Subgrade Composite Foundation

Railway construction is entering leap-forward development period, while the problems of short construction period, large uneven settlement and shortage land resources are increasingly outstanding. Based on the design ideas of reinforced concrete retaining wall and box beam, this paper proposes a new type of subgrade structure—U-shaped structure subgrade in cantilever-type, which has the merits of large rigidity, small deformation, good integrality and little uneven settlement. The new type structure subgrade solves problems, such as the large land area of occupation on general slope subgrade and foundation treatment area. In this paper, the influence of settlement and mechanics property of composite foundation in U-shaped structure subgrade with different parameters of composite foundation, and the dynamic response of railway track-U-shaped structure subgrade-composite foundation coupling system are deeply and systematically investigated. The main research contents and achievements are as follows:(1) U-shaped structure subgrade and foundation are considered as a whole. It has the common action between them. Foundation coefficient is the most important factor influencing the deformation and inner force of U-shaped structure subgrade. When the foundation coefficient is6000kN/m3, the settlement of U-shaped structure subgrade, the differential settlement of slab and secondary stress of structure are tended to be stable. The height of U-shaped structure subgrade should control about5meter, and more stirrup or reinforcement steel bar is necessary in the stress concentration regions. Enlarging the thickness of U-shaped structure and enhancing the concrete strength grade can enhance the rigidity of structure and improve the capacity of resist flexure deformation. The concrete strength grade has larger influence to structure internal force than the thickness of structure, C30is the best strength grade of concrete.(2) Cushion is important to the pile-soil load distribution in U-shaped structure subgrade composite foundation, and reasonable value is0.4m. Cushion over thickness decreases the load pile sharing and diseconomy. Pile length has a significant impact on settlement and mechanics property of U-shaped subgrade structure on composite foundation, and reasonable value of pile shaft is15m, which should penetrate soft soil compressed layer, and insert soil layer with high load-bearing capacity and little compression amount. Decreasing the pile spacing or enlarging the pile diameter can increase the replacement ratio and decrease the settlement of composite foundation. Pile spacing has more influence than pile diameter, and the scheme of thin pile is optimal. The value of pile spacing is3.5m, and the value of pile diameter is0.5m.(3) In spatial coupling dynamics model of vehicle model-U-shaped structure subgrade-composite foundation, train loads transmit to composite foundation through steel rail, track slab, U-shaped structure subgrade and cushion. During the process of transmission, energy is absorbed by kinds of medium, and the value of each dynamic response indicators is damping gradually. The vibration of steel rail is larger than that of track slab. The vibration is attenuating gradually from track slab to supporting layer and U-shaped structure subgrade, but the difference is little. When the vibration transmits from cushion to composite foundation, the damping of each dynamic response indicators is about90%. Under the condition of single-direction and two-direction railway, U-shaped structure subgrade composite foundation can meet the stationarity and stability requirements of train high-speed running.(4) The time-history curve of each dynamic response indicators of steel rail, track slab, U-shaped structure subgrade and composite foundation have obvious wheel set and additive effect. The wheel set effect is receding gradually from steel rail to the deep composite foundation. The train speed has large influence on the dynamics property of U-shaped structure subgrade composite foundation. Increasing the train speed can aggravate the vibration and dynamic impact effect of steel rail, so the reasonable running speed is needed to meet the requirement of running safety.(5) Based on the M-B combining analytical method, programming the additional stress of U-shaped structure subgrade composite foundation using C programming language, and then we get the settlement of composite foundation combining with layer-wise summation method. The results have similar changing tendency to the conclusion drawn by finite element simulation, so the calculating method is feasible.