Mechanical Analysis And Control Of Overfilling Problem Of Pile Concrete In Karst Area

Karst geology is widely distributed in China,and with the increase of engineering construction,cast-in-place pile engineering is inevitably established in Karst areas with hidden Karst caves.Due to the limitations of underground engineering investigation methods,it is difficult to fully explore the detailed geological conditions.Therefore,it is often found that concrete is overfilled during the construction of cast-in-situ piles in Karst areas.The existing theoretical research direction of concrete overfilling is simple,mainly focusing on column hole stability analysis without adverse geological conditions or cavern stability analysis under various influencing factors.The proposed control measures also remain at the construction technical level,and It is not possible to systematically analyze the problem of over pouring of cast-in-place pile concrete in unfavorable geological areas.This paper systematically reveals the influence of various factors on the concrete overfilling in terms of mechanics,combining two analysis methods:elastoplastic mechanics analysis of plane axisymmetric limit problems and finite element simulation analysis of cavern stability,clarifies the mechanical mechanism of pile hole instability under different pile solution(soil)tunnel models,and proposes effective control measures,this provides theoretical basis and technical support for solving the problem of over pouring of cast-in-place pile concrete.The main content and conclusions are as follows:(1)Establish a plane axisymmetric model for pile hole expansion,and combine it with the unified strength theory to derive the elastic-plastic solution of the pile hole under the plane strain model,including the stress,strain,displacement,and stress solutions in the elastic zone.Based on the elastic-plastic solution of the pile hole,derive the formula for the minimum mud weight of the pile hole in a single soil layer,the analytical solution for the initial yield depth,and the relationship between the radius of the plastic zone,the hydraulic pressure inside the hole,and the radius of the expansion hole.(2)The modeling method and simulation experiment method of cast-inplace pile with solution(soil)tunnel are proposed,including model hypothesis,pile tunnel model establishment,parameter selection,calculation of critical height of soil tunnel,simplification of concrete lateral pressure,step-by-step excavation according to working conditions,establishment of boundary conditions of solution(soil)tunnel and crustal stress balance under adverse geological conditions.(3)Establishe a discrimination and evaluation system for the instability of pile holes and soil holes,which comprehensively considers the distribution of PEEQ plastic penetration zone,sudden displacement changes,maximum principal stress limit,and radius ratio Cs of pile hole soil plastic zone.Combined with the evaluation system,we analyze the impact of factors such as mud weight,soil hole size,and concrete slump on the stability of pile holes and soil holes,and obtain the general laws of pile diameter,soil hole displacement,and instability.Combining with engineering examples,we found that:1)The root cause of the larger concrete filling coefficient is the instability of bored pile holes caused by initial crustal stress,drilling fluid pressure,unfavorable geology and construction,and the direct cause is slurry leakage,hole expansion,hole collapse,etc.The influencing factors are: karst(soil)holes,processes and drilling tools,wall protection methods,concrete slump,karst ditches and grooves,cracks,etc.Increasing the slurry weight or reducing the concrete slump within a reasonable range can significantly reduce the risk of pile hole instability.2)The strength of the soil is a key factor in maintaining the stability of the pile hole.The minimum mud weight required in the hole and the maximum depth of dry drilling are influenced by the coefficient b of the central principal stress and the internal friction angle of the soil φ,The influence of soil cohesion c requires wall protection measures for the formation of pores in sandy soil.In the case study φ> At 16.5°,the protective wall can be constructed according to the requirements,φ> As the boundary point at 10 °,mud wall protection can be used to maintain the stability of the pile hole.3)Selecting the unified strength expansion theory and the stability theory of ellipsoidal hollow foundation to verify the correctness of the concrete overfilling theory,the results of the two calculation methods in terms of the calculation size of the plastic zone are basically consistent when the excavation depth z<4m under the case condition,and both show nonlinear growth with the increase of excavation depth,When 4m≤z<16m,the radius of the plastic zone increases nonlinearly with the increase of excavation depth,and the theoretical calculation of the plastic zone radius is more conservative.