Study On Mechanical Behavior And Damage Evolution Process Of Pile-rock Interface Of Rock-socketed Piles In Soft Rock

As a special pile foundation,the rock-socketed pile has the characteristics of high bearing capacity,small pile group effect and rapid settlement convergence.These characteristics meet the requirements of the bearing capacity of high-rise buildings,long-span bridges,ports,offshore oil drilling platforms.In recent years,more and more rock-socket piles have been applied to the construction of various large-scale infrastructures covering almost the national economy and people’s livelihood.Due to the characteristics of large bearing capacity,high test cost,and difficulty to perform destructive tests,which leads to the systematic and complete measurement data available for studying the bearing mechanism of rock-socketed piles are very limited.This limitation restricts the comprehensive understanding of mechanical behavior and rock damage process of the pile-rock interface.With the continuous deepening of the analysis,design and application of rock-socketed piles,the necessity and urgency of the refinement research about mechanical behavior and the damage evolution process of the pile-rock interface that determine the bearing mechanism of the rock-socketed pile continue to highlight.Based on the three-dimensional high-resolution X-ray CT scanning platform,Digital Image Processing(DIP)and Particle Flow Code(PFC),this paper conducted an experimental,theoretical and numerical simulation study on the bearing process of model piles embedded in soft rock.It aims to improve further the research on the inherent mechanical behavior and damage evolution process of the pile-rock interface that is governing the macro-bearing characteristics of rock-socketed piles in soft rock.The study contents and results are listed as follow.(1)Based on the three-dimensional high-resolution X-ray CT scanning platform and an in-house designed experimental setup for X-ray CT imaging,a multi-stage scanning experiment is conducted on the bearing process of the model pile embedded in soft rock.By using digital image processing technology to reconstruct,process and analyze the scanned image,the interaction mechanism of the model pile-rock interface in three-dimensional is determined from three aspects: i)the sliding and shearing mechanisms of asperities at the pile-rock interface,ii)the development and shape of the compression zone below the pile end,and iii)the radial crack formation in the surrounding rock.(2)By using the Hoek-Brown strength criterion and the cylindrical cavity expansion theory,the elastoplastic analysis of the rock around the pile during the sliding and shearing process is carried out and obtained the elastic-plastic solution of the normal stress increment of the bore wall.A few cases represented to illustrate the rationality of the obtained elasto-plastic solution of the normal stress increment of the bore wall.Afterward,through the sensitivity analysis of the parameters that affect the elasto-plastic solution of the normal stress increment of the bore wall,and the main and secondary influence parameters are determined.(3)Based on the elasto-plastic solution of the stress field of rock of the bore wall during the cylindrical cavity expansion determined the stress path of the bore wall rock in the sliding and shear process.On this basis,combined with the model experiment results,the formation mechanism of radial cracks and critical normal stress of tensile failure of rock in the bore wall is obtained.(4)The calculation method of the shear strength of the pile-rock interface during the sliding and shearing between the rock asperities is proposed by using the bilinear shear model and the slip-line field theory of the non-gravity obtuse wedge under unilateral pressure.Furthermore,by involving the elastic-plastic solution of the normal stress increment and reduction of normal stiffness of the bore wall,the calculation method of the shear stress of the pile-rock interface considering the change of normal stiffness of the bore wall is derived.(5)Based on the experimental results of the development and morphology of the compression zone below the pile end,the Hoek-Brown strength criterion and the spherical cavity expansion theory are used to study the bearing mechanism of the pile end,and created the relationship between the resistance of pile end and the ultimate pressure of the spherical cavity expansion.Then,a method for calculating the ultimate bearing capacity at the end of rock-socketed piles based on the approximate solution of the ultimate pressure of spherical cavity expansion is proposed.(6)On the basis of reproducing the model experiment using the PFC,the meso-damage evolution of the rock around the pile during the increases of displacement of model piles is studied,and the motion law of particles of the rock and the main generation mode of micro-cracks located on the side and below the pile are obtained.Furthermore,based on the statistics of micro-cracks,the corresponding relationship between the meso-damage and displacement curve located on the side and below the pile,and the shaft resistance,end resistance and the meso-damage rate of the rock are obtained.(7)Several sets of numerical models of rock-socketed piles with different roughness scales are established,and their bearing and meso-damage characteristics are studied.The effects of the roughness scale and the inclination of asperity of on the bearing characteristics of model piles and the meso-damage of rock around the piles are obtained.Moreover,the corresponding relationship between the shaft resistance and end resistance of piles with different roughness scales and the meso-damage rate of the rock is determined by introducing the data of shaft resistance and end resistance with displacement.