Study On Reinforcement Characteristics And Mechanism Of Thick Cushion Layer Rigid Pile Composite Foundation

Soft and weak soil layers are widely distributed in China,and the construction of transport road network projects inevitably has to pass through areas with soft soil layers.In order to meet the needs of engineering construction,the soft ground foundation must be reinforced.Among them,the rigid pile composite foundation method is the most commonly used method for the reinforcement of soft and weak foundations.According to this method,the pile usually needs to pass through the weak soil layer and the pile end falls on a bearing layer with good bearing capacity.The thicker the weak soil layer,the longer the pile.And the longer the pile body is,the more difficult it is to guarantee the construction quality of the pile under the existing technical conditions.To address this problem,some scholars have adopted the thick mattress layer rigid pile composite foundation method for reinforcement treatment,but there are few research results on the reinforcement characteristics and mechanism of this method.For this reason,this paper firstly analyses the general mechanism of the bedding layer in the rigid pile composite foundation method from the theoretical point of view.Then,centrifuge model tests are carried out to compare and analyse the settlement of the foundation and the distribution of pile-soil stress before and after the reinforcement of the rigid pile composite foundation with thick mattress bedding layer,in order to verify the effectiveness of the rigid pile composite foundation method with thick mattress bedding layer.Based on the material parameters from the indoor centrifuge model tests,a finite element model of the composite foundation with thick mattress bedding layer and rigid piles was established using PLAXIS 3D software to simulate the foundation strengthening process.The effects of parameters such as thickness of mattress bedding layer,compression modulus and shear strength index on foundation settlement and pile-soil stress distribution were further investigated,and the reasonable range of these parameters was analysed under the consideration of strengthening effect.Finally,based on the Chongqi Bridge foundation treatment project,the results of the field tests are compared with the finite element numerical simulation results to verify the reasonableness of the range of values of the relevant parameters of the mattress bedding.The research results can provide theoretical basis and guidance for the reinforcement treatment of deep and weak foundations.The main findings of the study are as follows:(1)After the reinforcement treatment with thick mattress layer rigid pile composite foundation method,the foundation settlement is significantly reduced,and the upper load is transmitted from the mattress layer to the shallow inter-pile soil and the stress is spread to the deeper soil layer through the pile body,so that the pile soil can share the load.This shows that the thick mattress layer rigid pile composite foundation can effectively strengthen the deep and weak foundation.(2)The thickness of mattress layer and compression modulus have a greater influence on foundation settlement,pile-soil stress ratio and pile stress,while the shear strength index of mattress layer has less influence on these parameters.When the thickness of mattress layer,compression modulus and shear strength index are taken within a certain range,the foundation reinforcement effect is better.When the value exceeds the range,the reinforcement effect is not good.(3)When using rigid pile composite foundation method for reinforcement treatment of deep and weak ground foundation,a larger thickness of mattress layer can be considered,and its compression modulus and shear strength index should not take larger values.The geometric and mechanical parameters of the bedding layer can effectively reduce the stress concentration at the top of the pile,as well as better play the role of inter-pile soil.