Study On Soil Arching And Performance-based Design Method For Piled Reinforced Embankments

A piled reinforced embankment consisting of embankment fill, single or muti-layer of geosynthetic reinforcements, piles, caps and foundation soil, is a new type of embankment. The system has been increasingly used to construct highways on soft soils in recent years worldwide due to its more rapid construction, higher stability, and smaller total and differential settlements over traditional soft soils treatment methods. Within such a piled embankment arching occurs, the most of the embankment load is borne by the piles and transfer to a lower firm stratum, so there is no need of staged construction of embankment filling. Compared with the conventional pile-raft foundations, the piles are not needed to be closely spaced, and not continuous slab but individual cap is placed on each pile head, which enable the piled reinforced embankment a cost-effective method.The interactions among embankment fill, geosynthetic reinforcements, piles and foundation soil are complex. Since the compression stiffness of the pile is greater than that of the foundation soil, the embankment fill mass directly above the foundation soil has a tendency to move downward. This movement is partially restrained by shear stress, Ï„, from the embankment fill mass directly above the pile-cap. The shear stress increases the pressure acting on the pile-cap but reduces the pressure on the foundation soil. This load transfer phenomena is termed as soil arching. The soil arching has a significant influence on the behavior of the piled reinforced embankments. If the degree of soil arching is not sufficient, too much embankment load will be born by the foundation soil and the pile-subsoil relative displacement will be reflected to the top of the embankment and unacceptable differential settlements may occur, which would harm the normal function of the embankment and its durability. However, too big value of the stress concentration ratio implies that nearly all the embankment load will be born by the piles and high costs. Consequently, athorough understanding of soil arching mechanism within the embankments is essential for engineering design.In this study, firstly, a total of 15 model tests were conducted to evaluate the effects of the pile-subsoil relative displacement, the embankment height, the cap-beam width and clear-spacing, and geosynthetic reinforcements with different tensile strengths on the stress concentration ratio and settlement in the piled reinforced embankments. The test results indicate that the stress concentration ratio varies with the pile-subsoil relative displacement and has upper and lower bound values. A higher ratio of embankment height to cap-beam clear spacing as well as a higher ratio of cap-beam width to clear spacing would result in a higher stress concentration ratio. The inclusion of geosynthetic reinforcements can increase the stress concentration ratio and reduce the total and differential settlements of the embankments. The test resultsalso indicate that the height of the equal settlement plane, h_e, is about (1.4-1.6) timesthe cap-beam clear spacing for 2D situation.Then, the experiments were simulated using a finite element code, PLAXIS. The height of the equal settlement plane was investigated in detail. The numerical results show that the height of the equal settlement plane is mainly influenced by the pile-cap clear spacing and has nearly no relationship with the pile-subsoil relative displacements, the reinforcements tensile strengths, and the mechanical properties of embankment fill. In-situ tests for piled reinforced embankment in Taijin Highway Project were carried out and soil pressures as well as settlements on pile-caps and subsoil were monitored during embankment filling and consolidation process of foundation soil. The test results indicate that the height of the equal settlement plane is about 3.5 times the pile-cap clear spacing for 3D situation. Based on the studies stated above, the soil arch within the piled reinforced embankments is divided into two types, one is completed and the other is uncompleted. When the completed soil arch is formed, no differential settlements will occur on the embankment surface, otherwise, apparent differential settlements will occur on the top of the embankment.Thirdly, An improved load-transfer hyperbolic model for the pile-soil interface is,based on the conventional hyperbolic model, developed, which considers the increasing of initial shear stiffness with the surrounding soil consolidating and loading/unloading behavior of the interface. Further studies on the negative skin friction of single piles under different pile-head loading were also conducted. The results show that the location of neutral point and distribution of skin friction of pile shaft and pile capacity vary with the surrounding soil consolidating and the pile-head loading conditions.Fourthly, an analytical model for the piled reinforced embankments was developed, which considers the influence of the embankment filling and the surrounding soil consolidating on the soil arching. The calculated results based on the model are in good agreement with the field measurements, which implies the validity of the model.Finally, a performance-based design method for the piled reinforced embankments was proposed.