Study On Load Transmission Mechanism And Vertical Bearing Capacity Of Pile In Karst Region

With the implementation of the strategy of developing the western part of our country,high-grade highway construction will cross a large number of western mountainous areas and a large number of engineering pile foundations will cross the karst development zone.Due to the hiding nature of karst development,it is difficult to obtain complete engineering geological data even in detailed surveys.Therefore,how to reasonably determine the law of load transfer and bearing capacity under the condition of limited geological data is the key point to ensure the construction and operation safety of pile foundation in karst area.Combined with the National Natural Science Foundation of China,"Study on Vertical Bearing Mechanism and Design Calculation Method of Foundation Piles in Karst Area"(Project No.51278187),large-size indoor model test,theoretical analysis and field test are used to analyze the pile foundation Carrying capacity and vertical bearing capacity are discussed,mainly in the following aspects:(1)The existing results of stress analysis of foundation piles in karst area have been discussed.On this basis,based on the similarity theory,taking the on-site project as the prototype,the vertical loading failure test of karst cave under the rules was carried out.The deformation and strain development law of the foundation piles and karst roof were obtained during the loading process.The results show that the thickness and span of karst roof are the important factors affecting the vertical bearing capacity of foundation piles.Under the condition that the strength and span of roof of karst cave remain unchanged,the failure mode of roof varies with the thickness of roof,for the pile-end punching,damage of punching and bending combination and the overall bending failure of three typical failure mode.This model is closely related to the thickness of the top of the cave.When the thickness of roof is relatively small,the unfavorable position of karst destruction is the transfer contact surface of tensile stress and compressive stress,in which the roof tends to produce partial punching failure.When the thickness of roof is relatively large,the transfer contact surface of tensile stress and compressive stress extended to the edge of the roof,at this moment,the roof is easy to produce the overall form of bending failure.The comparison of the strain increment plot of shear versus the failure model of the model test in the interior shows that the failure surface of the top of the cave is very similar in location to the maximum value of the increment in shear strain,so it can be assumed that the location of the increment in shear strain is the destruction of karst roof critical surface.As the ratio of roof thickness to span of cave decreases,the ultimate bearing capacity of roof decreases gradually,and the failure mode of roof changes gradually from the overall bending failure to partial punching failure.The load transfer curve of pile tip indicates that the ultimate resistance of pile increases with the thickness of cave roof.(2)Based on the results of indoor model test,the theoretical analysis of vertical ultimate bearing capacity of foundation piles in karst area is carried.Introducing the catastrophe theory,the mutation characteristics of pile foundation destruction in karst area are mainly described,and the mechanics simplified formulas and potential energy function formulas of three kinds of failure modes are established.The ultimate bearing capacity of pile foundation in karst area is deduced Cusp mutation bifurcation formula.Combined with the boundary conditions,the formula of the bifurcation set was deduced to get the formula of the ultimate bearing capacity of the pile foundation in the karst area,and the determination equation of the ultimate load of pile foundation in the karst area was obtained.(3)The non-deterministic method is used to analyze the roof stability of karst roof.Based on the limited geological engineering exploration data,the influencing factors of stability of karst roof and related physical and mechanical parameters of rock and soil are researched firstly.Because of the uncertainties in the top of underlying karst roof in the process of stability analysis,the interval number is used to represent the uncertainty of the calculation parameters.The interval mathematical method and the RMR index are combined with the Hoek-Brown empirical strength formula to form a method for determining the parameter interval.Based on the existing research,a limit analysis system for the top of cave under the pile tip is established considering the effect of punching shear,shear resistance and bending.Based on the study of the determination of functional function,a non-probabilistic reliability theory of roof under caverns is proposed by using non-probabilistic reliability method.(4)Compared with the indoor model test in Chapter 3,the fourth chapter uses the numerical analysis software ABAQUS to establish the analysis model of the vertical bearing of karst pile foundation,and discusses the span,pile diameter and elastic modulus and other factors on the effect of vertical bearing capacity of karst Pile.(5)Field test of vertical bearing capacity of rock-socketed piles in many karst areas was carried out by means of self-balance technique and routine testing technique,and its load transmission mechanism was further analyzed.Finally,the method of rockburst analysis and the method of uncertainty analysis are applied to the pile foundation engineering of an expressway in Hunan Province.The theoretical analysis results are compared with the actual design parameters to verify the feasibility of the design calculation.At the same time,The results of comparison with the experimental data verify the applicability and rationality of the stability of surrounding rock and risk analysis method based on the uncertainty theory established in this paper.