Mechanical Analysis Of Isolation Pillars In Open-pit Combined Mining And Research On Safe Thickness Model Based On Thick Plate Theor

Currently,China is in a deepening transformation phase from extensive exploitation of shallow resources to green development of deep resources.Many large open-pit mines are facing a transition from surface to underground mining,with increasing difficulty and safety risks.In the process of joint mining from surface to underground,the determination of the thickness of the isolation pillars plays a crucial role in controlling safety risks,improving overall recovery rate,and ensuring economic benefits.Isolation pillars are an inevitable product of the transition from surface to underground mining,and the internal stress distribution characteristics and overall stability status of the isolation pillars dynamically adjust with the reserved thickness of the pillar during mining.This can easily lead to disasters such as roof collapse,collapse,landslide,and large deformation.Due to the complex properties of the rock mass,the complex development status of structural planes,and the complex stress distribution environment in underground mining,it is difficult to achieve accurate calculation and fine analysis of the engineering stability of isolation pillars using traditional classical mechanics analytical methods.Existing isolation pillar thickness models have a series of problems such as simplified stress and boundary conditions or inappropriate basic assumptions,which limit the application of isolation pillar theoretical models.In engineering practice,the determination of isolation pillar thickness often relies on empirical analogy methods.This paper discusses the theoretical conditions for the application of plate and beam theory based on the mechanical equilibrium equations and constitutive equations of isolation pillars.Using the thick plate theory,the mechanical model and the stress state of the lower surface of the isolation pillar are analyzed,and a multi-stress field joint action isolation pillar mechanical model is developed.Numerical simulation calculations were carried out to effectively analyze the stress distribution law of isolation pillars with different thicknesses,and the applicability of the mechanical model was visually verified.Based on the discussion of the failure criteria of isolation pillars,an optimization model for the critical thickness and safety factor of isolation pillars is established,and the applicability of the theoretical model is tested through monitoring and analysis in engineering applications.The specific contents are as follows:(1)A mechanical model of isolated mine pillars under the combined action of multiple stress fields was constructed.Based on the 3D mechanical model of isolated mine pillars,the horizontal stress was introduced as a volume force by using the unit pulse function and Dirac function,and a mechanical model of isolated mine pillars under the combined action of gravity and horizontal stress fields was constructed.The results of the numerical calculation using FLAC3 D show that the model has consistent regularity,and the average difference of stress distribution on different profiles is 4.61%.(2)The distribution law of internal stress in the isolated mine pillars was analyzed.The mechanical model of isolated mine pillars under the combined action of multiple stress fields was used to calculate the internal stress of the isolated mine pillars under actual stress conditions and dimensions,and the distribution law of the stress was analyzed.Under the coupled action of gravity and horizontal stress,the stress distribution in the isolated mine pillars is asymmetric about the center surface,and on the profile with the Z-axis as the outward normal direction,each stress component is symmetric about the lines x=0.5a and y=0.5b.The upper surface of the isolated mine pillar mainly shows compressive stress,while the lower surface mainly shows tensile stress,and there is a process of transformation from compressive stress to tensile stress.The stress inside the isolated mine pillar decreases with the increase of thickness,but due to the effect of gravity,the stress does not decrease to zero.As the thickness of the isolated mine pillar increases,the stress inside the isolated mine pillar tends to a certain limit.(3)An expression of safety factor based on the double shear strength criterion was constructed,and the distribution law of safety factor was analyzed.The overall safety factor of the isolated mine pillar is smaller closer to the lower surface.From the upper surface to the lower surface,the span of safety factor distribution gradually decreases,and the discreteness of safety factor decreases.(4)A method for determining the critical thickness of the isolated pillar is proposed.A method for determining the critical thickness of the isolation pillar is constructed based on the distribution law of the safety factor.Through the search algorithm,the minimum thickness that makes the safety factor of any point in the isolation pillar be greater than 1 is obtained.The effect of the width-to-length ratio(coefficient of variation)on the safety factor was analyzed.With the decrease of the width-length ratio(variation coefficient),the area with a low safety factor of profile gradually changed from the "X" type to the "一" type.The critical thickness of the isolation pillar does not increase monotonically with the decrease of the width-length ratio(variation coefficient),and there is a width-length ratio(variation coefficient)that maximizes the critical thickness of the isolation pillar.(5)The mechanical model of isolated mine pillars under the combined action of multiple stress fields was verified through industrial application.The data from the measurement points in the industrial application have a maximum average difference of less than 12% from the results of the mechanical model calculation,indicating good applicability of the constructed model.The mechanical model of isolated mine pillars under the combined action of multiple stress fields and the method for determining the critical thickness provide a new theory for determining the minimum thickness of isolated mine pillars,a new method for determining the thickness of isolated mine pillars with different exposed areas,and a new approach for safe,economic,and efficient joint mining of open-pit and underground mines.