Consolidation Calculation And Experimental Study Of Sand Well Composite Foundation

As the research of various widely used soft foundation treatment techniques becoming deeper, bagged sand well foundation treatment technique has become an effective way of available methods. Generally, it is believed that the bagged sand well works as a vertical drain channel which can accelerate the consolidation of soft foundation. In fact, the bagged sand well can also bear a part of the load at the same time as a result of forming the sand well composite foundation. In view of this particularity of sand well composite foundation, aiming at the working mechanism and consolidation behavior, a thorough study on the sand well composite foundation reinforced soft roadbed technique through theoretical analysis and model experiment is made in this dissertation under the support of High Technique Research and Development Program of China (863 program)—"The Treatment Technique of Double Direction Reinforced Foundation Based on The Settlement Control for Large Area and Non-uniform Soft Soil Foundation in Highway Construction".First, based on similarity theory, three indoor model tests including "soft soil foundation model test", "sand well composite foundation model test" and "gravel pile composite foundation model test" were designed and carried out. The test results show that, sand well can significantly improve the bearing capacity and settlement characteristic of the soft or weak subsoil. However, the "pile group effect" was also found, which means decreasing of the bearing capacity and increasing of the settlement of composite foundation if more sand wells were included under the bearing plate during the loading test. Considering of the "pile group effect", appropriate well spacing should be selected. Moreover, proper discount is also advised when the bearing capacity of the sand well composite foundation is estimated by loading test result of single-pile composite foundation.At the same time, under the comparative analysis of the pile-soil stress ratio curves between sand well composite foundation and gravel pile composite foundation from the test data, a conclusion had been made that bagged sand well has pile effect as a composite foundation which is inconsistent with the traditional sand well theory. Because of the similarity of work mechanism between sand well composite foundation and gravel pile composite foundation, the pile-soil stress ratio changing along with the exerted load of both show the same trend, while the pile effect of the former is weak than latter.At last, based on superposition principle and three-dimensional consolidation analytical solution, considering time dependent loading condition, the calculation formula of average consolidation degree of partially penetrating sand drain ground had been derived under the condition of single grade linear loading. The analysis of examples conducted by Matlab calculation program shows that under the influence of loading duration, soil consolidation degree of both the section with vertical drain soil and the section without it decrease compared to the consolidation degree under instantaneous loading,while the scope of this decrease is increasing along with the extending of loading duration. The comparative analysis between the calculation method of this paper and model test shows that the calculated results of the consolidation degree are closed to the value calculated from test data which has proved the feasibility of the method.