Model Experiment On Negative Friction Of Pile In Mining Area Using Transparent Soil

With the increasing demand of coal resources,coal mining under buildings,railways and water bodies,especially coal mining under buildings becomes a thorny problem.Due to good adaptability,pile foundation has become one of the most important foundation forms.Under the influence of underground mining,the negative friction at the pile side caused by mining seriously affects the bearing capacity of pile foundation,and greatly harms the stability of upper buildings.The current researches of negative friction at pile side mainly involve traditional natural foundation soil conditions,and there are few researches on the negative friction of pile foundation caused by the surface soil.The study on characteristics of pile foundation negative friction caused by the surface soil subsidence is of important value for the sustainable development of mining areas and town construction.In the paper,the characteristics of pile foundation negative friction caused by the surface soil subsidence were mainly studied through a variety of research techniques and methods,including comprehensive theoretical analysis,indoor model test and numerical analysis:(1)Based on the mining area surface soil subsidence rule and surface soil subsidence prediction-probability integral method surface soil subsidence prediction principles,a set of model test system that used the pile foundation transparent soil in mining surface soil subsidence(Patent No.: 201610067954.3)was developed.Besides,the system was adopted to conduct the contrast test on friction piles and end bearing pile.(2)Particle image velocimetry(PIV)was adopted.The contrastive analysis results showed that the measured surface subsidence was quite consistent with the calculation results through probability integral method,proving the test system was reliable.Besides,the displacement vector field,displacement contour map and the relative displacement of pile-soil of the soil mass near piles in the process of surface subsidence were analyzed.In the process of surface subsidence,pile endpoint soil first settled;with the increase of the relative displacement of pile-soil,negative friction at pile side first emerged on the pile end and then extended upwards along the pile body,and the position of neutral point also extended upwards from the pile end.When surface settlement intensified,pile body sunk sharply,causing the instability and failure of pile foundation.Through the contrastive analysis of the two kinds of pile type in the process of subsidence,it was not advisable to use end bearing pile foundation in mining surface subsidence area.(3)To study the influence of amount of the settlement of surface soil mass,pile top load size,pile length and pile diameter on the mechanical behavior of pile foundation in the process of surface subsidence,numerical analysis model was established based on the model test research.Moreover,the impact of the above factors on the mining area pile foundation negative friction characteristics was studied.(4)Based on the analysis of surface subsidence rule and the surface subsidence of pile in the soil of soil displacement field change,based on the mining surface subsidence area pile foundation reinforcement measures are put forward,in order to reduce the mining area of pile foundation negative friction resistance on the bearing capacity of the pile foundation,increase the safety of construction.