Study On Mechanism And Law Of Shaft Deflection In Deep Soil Strata

In China,some shafts have experienced deflection in deep soil strata,which has seriously affected the safe service of those shafts.It is urgent to accomplish systematic and in-depth researches on the mechanism and law of shaft deflection.In this paper,some methods,such as theoretical analysis,numerical analysis and physical model experiment,were comprehensively adopted to study the law of shaft stress and its deformation affected by coal mining in deep soil strata.Combined with engineering cases,the mechanism of shaft deflection was revealed,and the guiding principles for designing shaft protection rock(including coal)pillars were proposed in the thick soil and thin rock strata.First,considering that the shaft is a special underground structure passing through the complex strata,its deformation must be affected by strata movement.Therefore,a dynamic prediction model(i.e.Knothe prediction model)was established to describe the horizontal and vertical movement of shaft,which is based on the probability integral method and the Knothe time function.Based on the analysis of the characteristics of surface subsidence in thick soil and thin rock strata during coal seam excavation,a superior shaft dynamic movement prediction model(i.e.composite prediction model)was established.The analytical solution of vertical additional stress caused by mining was derived according to the composite prediction model.Two ideal mining models,parallel mining model and perpendicular mining model,were established in combination with the general mining laws.By analyzing the variation law between shaft displacement and model parameters and mining parameters and rock pillars' size,the theoretical equation for calculating the shaft protection rock pillars' size was obtained.Aimed at the shortcomings of existing movement angles or boundary angles in designing the shaft protection rock pillars in deep soil strata,the concept of allowable movement angle was proposed.The results showed that:1)Not only can the composite prediction model well describe the whole subsidence process of deep soil surface,but also it can invert the shaft horizontal displacement at different depths with high precision.2)The horizontal shaft displacement is proportional to the subsidence coefficient and the horizontal movement coefficient and coal mining thickness,and has a positive nonlinear relationship with the mining depth and a negative nonlinear relationship with the tangent of main influence angle and the protection rock pillars' size.3)The design parameters of stipulated in the current specifications are not suitable for special conditions such as thick soil and thin rock strata.While the shaft protection rock pillars designed by allowable movement angle are more in line with coal mine production requirements.Second,a 3D numerical model for strata movement under the mining influence was established.By simulating the influence process of mining on the strata movement,the variation laws of shaft deformation with the main influence parameters,such as pillars' size,mining thickness,soil thickness and Young's modulus,were obtained.The numerical solution of shaft protection rock pillars' size was obtained based on numerical calculation results.The study showed that:1)The numerical calculation model can realize full mining state of strata,and the typical values of the model calculation parameters are reliable.For example,with the same size of protection rock pillar,the difference between measured and simulated values of maximum horizontal displacement of the main shaft in Guotun Coal Mine is only 4 mm,while the measured values of subsidence coefficient and movement angles are basically same as the simulated values.2)There is a linear positive correlation between the maximum horizontal shaft displacement solved by numerical model and the coal thickness,a nonlinear positive correlation with the soil thickness,and a nonlinear negative correlation with the protection rock pillar size.3)With the same critical deflection displacement,the numerical solution of shaft protection rock pillar is larger than the theoretical solution,and the difference between them decreases with the deflection displacement.Third,based on the principle of simulating the gravity field with the seepage force field,a testbed that can guarantee the similar gravity field was first established to simulate shaft mining deformation in deep soil strata.Through physical simulation tests,the shaft deformation laws caused by mining in deep soil strata were studied.Combining the above research,the shaft deflection mechanism in deep soil strata was revealed as follows: under the conditions of asymmetric mining,the mining caused the soil around the shaft to produce different displacement magnitudes at different depths,resulting in the vertical and horizontal displacement of the shaft within the range of mining influence.The mechanism of fracturing in shaft lining caused by mining subsidence was revealed as follows: mining caused the soil around the shaft to produce the vertical and horizontal movement,inducing the shaft lining to experience the vertical and horizontal additional stress,resulting in the compressive and bending failure of the lining under the combined action of both additional stress and self-weight stress.Based on the research,the purpose of reasonably designing of rock pillar size in thick soil and thin rock formation has been achieved.The results have an extremely important guiding significance for the safety of shaft lining and for the prevention and control of shaft deflection.Meanwhile,they provide a scientific theoretical basis for mining shaft protection coal pillars.In this paper,there are 191 figures,30 tables and 151 references.