Study On The Independent Stability Depth Of Pile Hole Considering Circular Arch Effect And Stress Concentration Effect

The stability of borehole wall and drilling hole pile is a problem in the construction process of widespread concern and a complex and difficult problem in the world.The instability of the borehole wall brings a great deal of difficulties and a series of engineering problems to the drilling engineering construction.The correct,scientific and clear understanding of the factors affecting the stability of the borehole wall during the drilling of bored piles,fully excavate the hole wall collapse mechanism and the reason,which is beneficial to guarantee construction quality,guarantee construction period and control construction cost,has important theoretical research significance,and also has important guiding value to the actual project.In view of the problem that the present theoretical calculation formulas of the self stability depth of the borehole wall can not take into account the influence of the pore size,a study based on the spatial symmetry theory developed by ?.?.?????????? was conducted.And the stability of the borehole wall is analyzed by numerical simulation.The main research results are as follows:(1)Based on the spatial symmetry theory developed by ?.?.??????????,and according to the ?.?.??????????'s active earth pressure formula on the cylindrical pit wall,the implicit solution of maximum independent stability depth of pile holes considering spatial effect(circular arch effect)is derived.Under the premise of detailed analysis of the implicit solution,a simplified formula for approximate calculating the maximum independent stability depth of a pile hole considering the effect of circular arch(the ?.?.??????????'s implicit solution)is presented.(2)The circular arch effect and stress concentration effect of the borehole wall are analyzed in detail by means of numerical simulation.The results show that both the stress concentration effect and the circular arch effect exist in the borehole wall soil after the excavation of the pile hole,and the circular arch effect has the unloading effect on the borehole wall,which is beneficial to the stability of the borehole wall.(3)According to the simulation results,the stress development of the borehole wall is analyzed.The results show that the simplified theoretical model of double hole infinite compression plate hole theory is not accurate enough to describe the development law of soil stress in the hole wall,and is conservative.(4)According to the comparative study of a large number of numerical simulation experiments.The results show that: the effect of pore size on the stability of borehole wall can not be ignored.The simplified formula considering the effect of arch is overestimated the maximum independent stability depth of borehole wall.The retaining wall theory underestimates the maximum independent stability depth of borehole wall.It is further proved that both the stress concentration effect and the circular arch effect exist in the borehole wall soil.(5)A large number of numerical simulation experimental results are analyzed and studied.An approximate calculation formula of the maximum self stabilizing depth of pile hole considering circular arch effect and the stress concentration effect is put forward.The rationality of the approximate calculation formula is proved from both qualitative and quantitative point of view.