Research Of Flow Plastic Soft Soil Composite Foundation Failure Mechanism And Processing Method

In order to solve such problems as the insufficient bearing capacity of foundation soil and post-construction settlement of soft soil foundation, the rigid pile foundations has been widely used in the treatment of flowing plastic soft soil foundation in China, including the CFG pile, the Pipe pile, the reinforced concrete bored pile and so on,. But at present, in the foundations treatment of practical engineering appeard the failure of foundation treatment, the landslide and collapse of the constructing subgrade, the instability and subsidence of the railway subgrade shortly after operation. This is mainly due to the following reasons as insufficient understanding on engineering characteristics of the lower shear strength and the higher thixotropy of flowing plastic soft soil, the unclear research on pile-soil interaction mechanism with significant difference between the modulus, the study of anti-shear contribution and the failure mode of CFG piles inthe foundation treatment of flowing plastic soft soil, moreover, using of traditional limit equilibrium method in the stability analysis of subgrade and overestimation the stability of embankment, that led to the inappropriate foundation treatment methods have been used in the practical engineering.Firstly, to solve the existing engineering problem of flowing plastic soft soil foundation treatment, this paper will start with the numerical analysis of working mechanism of single pile in the flowing plastic soft soil by using finite element analysis method. The systematic study on the influences of the different pile-soil modulus ratio, the different angle of pile and the sliding arc tangent, the anti-shear characteristics, the failure pattern of pile and the pile-soil interaction mechanism under different working conditions were carried out. The anti-shear contribution of pile is defined as the “apparent shear strength”. The results show that the stiffness of pile body itself, the modulus difference between pile and soil, the shear strength of soil, and the angle between pile and the sliding arc tangent of soil are the mainly influence factors of the “apparent shear strength”. The apparent shear strength increased with the gradually decreasing of pile-soil modulus ratio and the angle between pile and the sliding arc tangent of soil. Meanwhile with the gradually increasing of the difference in pile-soil modulus, the CFG pile will appear different apparent shear strength corresponding to the following different failure modes such as the anti-shear failure, tensile or compression failure, bending failure, soil streaming around failure, the toppling failure and the lateral sliding failure in sequence. Furthermore, the different angle between pile and the sliding arc tangent reflect different position of pile under subgrade, with the increasing of the angle between the pile and the shear plane, the failure modes of piles will transfer from the compression bending failure, the bending failure to the tensile bending failure and compression failure, and as the pile-soil modulus ratio is greater than 2000:1, the CFG rigid pile is not reasonable foundation treatment method for flowing plastic soft soil.On the basis of research on pile-soil interaction mechanism and the failure mode of piles, this paper proposed a new calculation method of Bishop Arc sliding stability based on the CFG apparent shear strength, this method using the limit equilibrium method and assumes the circular sliding surface. Then, the shear failure will appear on the pile and the soil along the sliding surface, the apparent shear strength has been used as the effective shear strength of CFG pile rather than simply used the shear strength of composite soil layer formed with reinforcement and soil. This method can both reflect the instability failure mechanism of the embankment supported bycomposite foundation treated with the rigid pileand evaluate its stability reasonably. After compared with a various methods of reinforcing soft soil foundation with rigid piles the results show that the safety factor is more reasonable based on the apparent shear strength.Based on the research achievements of pile-soil interaction mechanism and the stability analysis method of flowing plastic soft soil composite foundation, this paper proposes a new foundation treatment method combined both “drainage consolidation” and “composite foundation” methods, where the pile-soil modulus ratio is the key factor of this new foundation treatment methodby the consideration of both safety and economic.When the pile-soil modulus ratio is greater than 2000:1, the drainage consolidation method will be used firstly to improve the shear strength of flowing plastic soft soil foundation, then take the CFG pile composite foundation treatment method.This paper mainly focused on the study of the physical and mechanical properties of flowing plastic soft soil, the CFG pile engineering properties and the pile-soil interaction mechanism, revealing the failure mechanism of flowing plastic soft soil composite foundation and proposed a new calculation method of flowing plastic soft soil foundation stability, and finally obtained a new foundation treatment method in high-speed railway.