Settlemtnt Calulation And Experimental Study Of Rigid-pile Composite Foundation

As an effective way of ground improvement, rigid pile composite foundation has been widely used in the construction of building foundations and roadbeds, getting good benefit. However, at present it is short of an intensive investigation for the mechanism of rigid pile composite foundation improving soft foundation, the interaction between gravel cushion and rigid piles, and the development of bearing capacity and deformation mechanism. So, 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", a thorough study on rigid pile composite foundation reinforced soft roadbed technique is made in this dissertation.Based on the analysis of existing rigid pile composite foundation model test, the similarity theory is introduced in model test against the arbitrariness of parameters choosing. In this paper, the relevant principle of similarity theory is adopted, then, the similarity criterion of rigid pile composite foundation model test is obtained by using dimensional analysis method. Key parameters of the model test are determined according to the similarity criterion and the analysis of conditions. A half roadbed is used to simulate the full-size roadbed with the consideration of symmetry, and four groups of model tests are designed: simple soft soil model test;rigid pile composite foundation test;Geogrid + Rigid pile composite foundation test and Geocell + Rigid pile composite foundation test. Through the four groups of model tests, a large number of valuable experimental data is obtained: the ultimate bearing capacity of soft roadbed before and after reinforcement, the load-settlement curves of single-pile and multi-pile composite foundation, the pile-soil stress ratio at the top and the bottom of piles, the axial force distribution of rigid piles. According to experimental data analysis and theoretical studies, the working mechanism is revealed of how geogrid and geocell reinforced cushion improves the stress distribution in shallow soft soil, increases the bearing capacity, and reduces settlement and differential settlement; the interaction between cushion, rigid pile, and soil in rigid piles composite foundation system, and load transmission mechanism are examined. Through experimental and theoretical research, it is found that upward penetration occurs at the pile top, and it causes negative friction to a specified depth, so the maximum axial stress occurs no longer at the pile top, but at the point of about one third of the pile length below pile top.With thoroughly analyzing the load transmission mechanism of rigid pile composite foundation and the consideration of the characteristic of the interaction of the pile, soil and bedding course, an basic differential equation of settlement calculation of the rigid pile composite foundation is established based on the load transfer method, according to the assumption and the simplified distribution mode about the vertical displacement of pile-soil element, through introducing elastic-plastic load transfer model and with consideration of both upward and downward penetration displacement of piles. Then, a new method of calculating the settlement of the rigid pile composite foundation is developed. This new method is applied to the analysis of an experiment, and the results show that the calculated results are closed to the real value. This method can decrease the cost of calculation and can be conveniently applied in the practical engineering.