Study On The Long-term Settlement Of Overconsolidated Foundation In Soft Clay Area Induced By Traffic Loads

With the rapid development of national economy in recent years, more roads, railways, airports and other facilities are constructed,especially in developed southeast coastal areas. Due to the properties of low strength, low porosity ratio of soft clay, highways and airport runways built in soft soil area are easy to generate large settlement under traffic loads and cause the pavement cracking, which influences the normal facility usages and increases the cost of repair. Therefore, in order to avoid excessive post-construction settlement, the foundation is often preloaded by surcharge. Subsequently, the foundation soil is in overconsolidated state. The surface of foundation forms a layer of overconsolidated crust layer naturally, with the over-consolidation ratio decreasing gradually from top to bottom.Long-term dynamic settlement induced by traffic loads should be divided into two parts:the first is undrained cumulative plastic settlement induced by cyclic loading, the second is the consolidation settlement induced by the pore pressure dissipation. In this paper, the long-term settlement of the overconsolidated soft foundation under traffic loads is studied by combining with the methods of laboratory experiment and numerical analysis. The main work and results are as follows:1. In order to establish the undrained cumulative plastic strain model of the soft soil,the undrained cyclic triaxial tests are taken on the Wenzhou silty clay to obtain curve relationships of undrained cumulative soil deformation with the load cycle numbers; Cumulative pore pressure model, which is easy for engineering application, is established based on the existing pore pressure softening model.2. The distributions of dynamic deviator stress of Wenzhou airport runway foundation are computed by multilayer elastic theory. The long-term settlement of the surcharged foundation of Wenzhou airport is obtained by combing with the undrained cumulative plastic strain model and cumulative pore pressure model established above. And the undrained cumulative plastic strain and cumulative pore pressure models are verified by the measured settlement data of Wenzhou airport.3. The dynamic deviator stress distributions of soft soil subgrade of highway with crust under vehicle loads are computed by the finite element code ABAQUS. The unconditionally stable implicit algorithm is employed to time marching; Cumulative plastic strain model considering soil overconsolidation is established by the combination of the improved cumulative plastic model and the Cambridge model; Cumulative plastic settlement of foundation with crust under vehicle loads is acquired by layer summation method and then compared with the calculated results of foundation without crust.The research shows that:the long-term settlement of soft soil foundation of highway under traffic loads could be greatly reduced with rational utilization of hard crust layer.