Researches On Stability And Control Strategy Of Embankments On Soft Subgrade Reinforced By Rigid Piles

The slope stability of an embankment is a classical problem in geotechnical engineering.The composite ground reinforced with rigid piles is widely used to increase the stability,decrease the settlement and accelerate constructing of the embankment.The current design method for the pile-supported embankment mainly assumes that a simultaneous shear failure of the piles along the failure slip.However,many embankments designed with this concept occurred landslide accident,indicating the insufficient of current design method.In this study,centrifuge tests,numerical modelling and fitting analysises were adopted to investigate the stability and control strategy of embankments reinforced by rigid piles.The main contents are as follows.Centrifuge test and numerical modelling were used to analysis the progressive failure of rigid piles and the instability of the embankment.Both test and constitutive model that can simulate the post-failure behavior of the rigid pile were proposed.The analysis reveals unreinforced rigid piles in the composite foundation occur brittle bending failure in a local place firstly.This failure leads to the significant increase of bending moment in the neighboring piles,causing the bending failure of the piles.The progressive bending failure of piles contributes to the instability of the pilesupported embankment.The stability of pile-supported embankments would be overestimated remarkably when assuming a simultaneous failure of the piles along the failure slip.The progressive failure caused by local failure of piles should be considered in engineering design.Then,the influence of the pile material,pile cap and horizontal reinforcement on the progressive failure of rigid pile as well as the stability of the embankment was investigated.The stress condition and failure modes are obviously different for various types of piles because of the change of stiffness.Low strength DMM columns are more easily to occur shearing failure in the center of the embankment,which was developed toward the toe of the embankment under the embankment load.However,rigid piles tend to occur bending failure progressively from toe to center of the composite foundation.Piles with horizontal reinforcement or pile caps can decrease the tensile stress and bending moment in the pile,improving the stability of the embankment.Whereas,the failure mode that progressive bending failure is not changed.Double-layer reinforcement,increasing the area of the pile cap and the combined utilization of pile cap and horizontal reinforcement can further prevent the progressive failure to increase the stability.In order to evaluate bending failure of rigid piles under embankment load so that the stability of the embankment can be obtained,the influence of soil parameters(e.g.the thickness,the cohesion and the elastic modulus of clay),pile parameters(e.g.the center-to-center spacing,the width and the elastic modulus of piles)and the embankment load on tensile stress of rigid pile and the stability of the embankment were analyzed.The parameters above have a coupled nonlinear effect.Based on a lot of parametric analysis,a MARS model that can predicte the bending failure of rigid piles under embankment loading considering the coupling effect was adopted.This method can be used to ecaluate the sability of the embankment,and the predictions were compared with the centrifuge tests to demonstrate the accuracy of this method.Subsequently,the performance-based design methods of composite foundation with rigid piles was investigated.Firstly,the post-failure behavior of the reinforced concrete pile was studied.The analysis reveals reinforcement can increase both the ductility of pile and the stability of the embankment.Base on the methodology progressive failure control,the concept of key piles and non-uniform strength design method was proposed.The stability of pile-supported embankment could be increased efficiently by increasing the strength and ductility of key piles.Then,the tilting failure of rigid piles when the performance of embankments with rigid piles embedded in an inclined underlying stratum was analyzed.The embedment depths have an obvious effect on the stability.Based on the failure mechanism of the pile,the concept of critical embedded depth was proposed.According to the change of stress and failure modes for pile in different locations,the non-uniform embedment depths design method was suggested.