Study On The Damage Instability And The Minimum Safety Thickness Of The Anti-outburst Layer Of Karst Foundation Pit Under Blasting Disturbance

With the continuous increase in the scale and quantity of foundation pit projects in China,the construction environment they face is also becoming increasingly complex.Especially in recent years,there have been many excavation projects in water rich karst areas.When using drilling and blasting method for construction,there is a joint effect of blasting disturbance,excavation unloading,and karst water pressure.The calculation method exceeds existing experience and specifications,and the risk of water inrush in the foundation pit significantly increases.The instability mode and safety thickness calculation of the anti outburst layer damage control in karst foundation pits under blasting disturbance have become key issues to be solved.In response to the above issues,this article takes the karst foundation pit of Suoyuwan South Station on Dalian Metro Line 5 as the engineering support,and conducts experimental research on the dynamic characteristics and energy consumption laws of limestone under impact load;Formulas for calculating the minimum safe thickness considering blasting effects have been proposed for relatively complete and fractured outburst prevention layers;Established an elastic-plastic stress seepage damage（MHD）coupling model and numerical algorithm for the outburst prevention layer of karst foundation pits based on cumulative damage caused by blasting;And propose an intelligent optimization method for blasting parameters that considers the damage index of the karst foundation pit’s outburst prevention layer.The main research content is as follows:1.Based on the dynamic damage essence of the anti outburst layer under multiple explosions in karst foundation pits,dynamic tests were conducted on limestone under uniaxial impact compression and uniaxial cyclic impact compression.The deformation characteristics,strength characteristics,and energy dissipation characteristics of limestone samples under impact load were obtained through experimental data,and the dynamic characteristics and energy dissipation laws of limestone under different strain rates were summarized.2.Based on the mechanical model of the relatively complete anti outburst layer damage and instability in karst foundation pits,considering the influence of pressure diffusion angle,a cone shaped mechanical model of the anti outburst layer damage and instability on the side wall of karst foundation pits is proposed.The total potential energy function equation of the anti outburst layer system is established,and the expression of the anti outburst layer tip sudden change model and the bifurcation point set equation are derived.On this basis,the elastic modulus is modified using the cumulative damage variable of blasting,and a calculation method for the minimum safe thickness of the anti outburst layer of karst foundation pits considering the cumulative damage effect of blasting is proposed,and the corresponding calculation formula is derived.This method is applied to the calculation of the safe thickness of the anti outburst layer in the karst foundation pit of Suoyuwan South Station,and parameter sensitivity is analyzed to explore the diffusion angle of the rock layerθ、Karst area radius R₂,rock layer Poisson’s ratioμ、The influence of rock elastic modulus E,steel support axial force F₁,karst water pressure F₃,and blasting frequency N on the minimum safe thickness of the anti outburst layer provides a scientific basis for reducing water inrush disasters and provides three emergency response measures for local karst water inrush.3.Based on the hydraulic fracturing mechanical model under the action of burst prevention layer blasting with cracks,a splitting factor reflecting the pore water pressure gradient is introduced.Based on the dynamic and static superposition effect,the failure criteria for the extension of tensile shear composite and compressive shear hydraulic cracks are proposed,and the corresponding critical water pressure calculation formula is derived.A criterion for the splitting and water inrush of fissured anti outburst layers under blasting disturbance has been established,and the anti outburst layer between the side wall of the foundation pit and the karst water body has been divided into four zones.The"four zones"of the anti outburst layer（blasting disturbance zone,blasting water pressure increasing zone,karst hydraulic fracturing zone,and existing crack zone）have been proposed,and a formula for calculating the minimum safe thickness of the fissure anti outburst layer under blasting disturbance and karst water pressure has been established.The rationality and accuracy of the proposed method were verified by comparing the calculated and measured values of the supporting engineering.4.The essence of water inrush in karst foundation pits is the process of MHD multi field coupling.In order to reasonably describe the MHD coupling mechanism of karst foundation pits,taking into account the cumulative damage caused by blasting and load damage,the damage evolution equation of karst foundation pits was derived.A modified effective stress formula and permeability coefficient evolution equation were introduced,and an elastic-plastic MHD coupling model and numerical algorithm considering the cumulative effect of blasting were established.In order to solve the"singularity"problem of Mohr Coulomb（M-C）yield criterion corner points,a fully implicit return mapping integral algorithm for solving the elastoplastic damage model in principal stress space was established in different regions.The model algorithm has been validated through experiments and classical analytical problems,and has high accuracy.And applied to the engineering calculation of the anti outburst layer in karst foundation pits,the damage evolution law of the anti outburst layer under multi field coupling was obtained.5.In order to prevent water inrush disasters in karst foundation pits under the action of blasting,it is quite necessary to control the damage of the anti outburst layer by selecting reasonable blasting parameters.Traditional methods for selecting blasting parameters often only consider a single indicator of blasting vibration speed,without considering the damage range of the anti burst layer.In response to the above issues,an intelligent optimization algorithm for blasting parameters was established,with blasting cost as the objective function and blasting vibration speed and the damage range of the anti burst layer as dual index constraints.By using differential evolution algorithm（DE）to optimize the hyperparameters of Gaussian process（GPR）,a nonlinear mapping GPR model was established between blasting parameters and blasting vibration velocity,as well as the damage range of the anti burst layer.This improved the accuracy of predicting blasting vibration velocity and the damage range of the anti burst layer.Based on the nonlinear mapping GPR model,an improved DE algorithm was used to quickly search for blasting parameters in karst foundation pits,which improved the convergence speed and global optimization ability of the optimization.On site monitoring shows that using optimized blasting parameters for construction,the blasting vibration speed and damage range are within a reasonable range.