Study Of Unloading Damage And Energy Catastrophe For Jointed Rock Mass Of High Slope In Open Pit Mine

The stability of open pit slope has been a significant issue in geotechnical engineering,which restricted the profitability,safe and efficient production of mining.The steeply inclined ore bodies are common in metal and nonmetallic open pit mines.Additionally,the burial conditions of the ore body determines that the morphology of open pit slope is high and steep.The rock mass is a complex medium with multi-fractures,containing faults and joints and other structural planes.Moreover,during the service period(for several decades)of the mine,the quality of the engineering rock mass will gradually deterioration with blasting excavation and unloading due to excavation.The joints may be further expand because the frequent changes of second stress field,which resulting in the damage increase gradually.The effect of excavation is not only spatially but also accompanied by the downward of the open pite,the unloading effect has time dependence and is irreversible.The deformation and strength of engineering rock mass,complicated and difficult,is the core of rock mass mechanics.In this study,the Dagushan open-pit Iron Mine belonging to Anshan steel is chosen as background.Furthermore,the damage mechanics,Monte-Calo random sampling technique,Hoek-Brown strength theory,Rosenblueth probability moment estimation principle,energy conservation and cusp catastrophe theory were combined to study the damage evolution and catastrophe process of jointed rock slope of open-pit mine due to unloading.The main results are as following:(1)The geological survey for migmatite rock slope of the Dagushan open-pit Iron Mine was conducted,and two set dominant joints were determined through the optimization analysis with the fuzzy C-means clustering algorithm for joint orientation.Furthermore,the distribution parameters of joint dip,dip angle,trace length and density were determined by statistics.The Monte-Calo simulation technique was used to reconstruct the 3D joint network of slope rock mass.Additionally,the probability distribution type and parameters of initial damage tensor were obtained.(2)The triaxial compression tests were designed to obtain unloading mechanical and energy dissipation characteristics of the migmatite under high and low confining pressure.Additionally,the AE location technology was used to study the damage evolution process during unloading.For unloading tests,more than 70%of dissipative energy was consumed in accelerated deformation and failure stage,and 20%dissipated in the stage of unloading deformation.By contrast,about 70%of the energy was consumed in the loading deformation stage for loading tests,while the energy dissipated in the accelerated failure stage was less than 30%.(3)An analytical method is established to determine the disturbance coefficient by the mining damage evolution simulation of jointed rock mass which applying the Kawamoto damage tensor.On this basis,the strength of jointed rock with spatial-temporal and anisotropy undering mining unloading damage was obtained,which means that more comprehensive and objective factors are considered during the strength analysis of jointed rock mass.Furthermore,spatial-temporal evolution and anisotropy distribution of strength and deformation parameters of jointed rock mass in the southwestern slope of the Dagushan Iron mine were obtained by mining damage simulation.In the view of time series for open pit mining,with the increase of mining depth,the unloading damage of slope rock mass increases gradually.In spatial,the location nearing the slope surface,has the larger unloading damage,and the local rock mass may develop to the complete damage.Although mining caused the range of area which affected by unloading is large,the area of expansion due joint cracking and damage is small.Moreover,the sereve damage zone is limited to the position,about 50 meters from the slope surface.However,the disturbance coefficient of the region,apart from slope surface 100 meters,only have slightly changes.The damage of rock mass near the slope surface is affected by the structure of local slope,which is related to the potential shear slip zone of local slope.(4)The Rosenblueth principle was used to discuss the analysis method that estimating the probability moment of the mean matrix of damage tensor and standard deviation of the slope rock mass.On this basis,the empirical relationship between the Hoek-Brown strength parameters(m and s)and the rock mass classification index(GSI)were combined to determine the random strength of jointed rock mass.Additionally,the probability damage due to mining was obtained by the application of Monte-Calo random sampling technique.The reliability analysis of jointed rock slope for open-pit mine,considering probability damage due to mining,was carried out.In general,the safety factor and reliability index of general and local slope showing a decrease trend with the decline of stope.Moreover,the reliability analysis of local slope indicate that if the height of calculated slope is unchanged,the dynamic decline of the slope reliability can be reflected by considering spatial-temporal variation of strength for rock mass.(5)Based on the energy principle,from the perspective of slope gravitational potential energy,elastic strain energy,kinetic energy and dissipative energy,a catastrophe criterion for jointed rock slope instability considering spatial-temporal variation of strength was established.Additionally,the stability of slope was evaluated by energy catastrophe.When considering the mining damage,the mechanism of energy evolution during the failure of jointed rock mass slope for open pit mine was analylized.The results indicate that the safety factor obtained by the dissipative energy cusp catastrophe is same as that from limited equilibrium Janbu method.(6)When rock strip in the second expansion and stripping of Dagushan Iron Mine downward from-114 m to-210 m,the slope platform,located in the belt transport system(-114 m),appeared the cracking and sinking under the influence of the fault and unloading damage.The theories,such as mining damage,energy evolution and cusp catastrophe,were used to simulate the process of this phenomenon,and the simulation results are consistent with the actual situation.