Study On The Design Method And Behavior Of Piled Bridge Abutments On Soft Ground

Abutment is an important transition structure connecting embankment and bridge. As the border of embankment and bridge, abutment and its foundation are loaded from the embankment and the upper structure and are influenced by many factors. Pile foundations are usually used as the abutment bases in soft clay. Apart from the loads from the upper structure and the earth pressure on the retaining wall, pile foundations supporting abutments in soft clay are loaded laterally and vertically from the soft soil movement induced by embankment filling. At present, the behaviors of full-height piled abutment constructed on soft clay are lack of research. The existing technical codes are lack of methods to calculate the additional stress in the bridge abutment subsoil induced by two-rail embankment and also lack of the negative friction and the lateral thrust on the pile foundation from the soft soil movement induced by the embankment filling.Aiming at the widespread diseases existed in piled abutment zone of both rail and highway constructed on the deep soft clay and the lack of design methods and construction deficiencies in existing codes, this paper, based on two projects, uses field tests, theoretical analysis and finite element method to study behaviors of full-height piled abutment constructed on soft clay and propose a new design method which can consider the negative friction and lateral thrust on the pile foundations. The main research works are as follows:(1) Through analysis of field tests' results, the characteristics of the settlements of embankment, the lateral deflections and the pore pressure of the soft clay is interpreted, the negative friction on the pile foundation and the bending moments in the pile foundation are analyzed, the settlement of abutment and the earth pressure on the retaining wall during embankment filling are obtained.(2) A three dimensional finite element model of full-height piled abutment constructed on soft clay is set up to explore the settlements of abutment, the bending moments of pile foundations, the relationship between the deflections of pile tops (the maximum bending moments in piles) and the embankment load, the shear stress in the interface of the ground, the earth pressure on the retaining wall and the soil arching effects, which are compared well with the field tests. (3) The relative displacements between piles and soils are produced under different circumstances. The piles as active bodies produce shaft resistance between the piles and soils, while the soil mass as active bodies produce skin friction between the piles and soils. The shaft resistance and the skin friction are different. Six modes are summerized and analyzed according to the pile foundation practice.(4) The characteristics of the active pile and the passive pile are analyzed. The horizontal thrusts on the passive pile shafts are compared with the horizontal resistance on the active pile shafts emphatically. The results indicate that the horizontal thrusts acting on the passive pile shafts are larger than the horizontal resistance acting on the active pile shafts. The horizontal resistance, the lateral thrusts and the net pressure on the passive pile shafts show approximate triangle, approximate trapezoid and saddle distribution along the pile diameter with the load increase.(5) Based on Boussinesq's solution and the superposition principle, the formulas of the additional vertical stress in the bridge abutment subsoil are deduced. According to the plastic deformation theory, a formula to calculate the lateral thrust on the piles is deduced and proved correct by the tests.(6) A design method of pile foundations supporting abutments is proposed which can consider the negative friction and the lateral thrust from the soft soil movement induced by embankment filling. The computing program is written and is used to design the pile foundations.(7) Two field tests for piled abutments in Beijing-Shanghai high speed railway and three field tests for piled abutments(Wenzhou abutment in Duiwu mountain bridge, Fuzhou abutment in Ao river bridge and Fuzhou abutment in Feiyun river bridge) in Wenzhou-Fuzhou high speed railway are carried out and the results of Wenzhou piled abutment in Duiwu mountain are analyzed emphatically. A device has been invented to fix the earth pressure sensor on the surface of the bored pile shaft and successfully measure the lateral thrusts from the soft soil movement induced by embankment filling. The results indicate that the lateral thrusts are parabola distribution with depth. After the completion of embankment construction, a large down-dragload and lateral deflection occurred in piles installed in soft and loose soils. The abutment moved forward about 22mm. The battered piles in pile foundations bear large vertical load and horizontal load. The axial forces of the battered piles are larger than those of the vertical piles in pile foundation.