Accumulated Damage Of Strength In Fault Zones And Slope Stability Under Open Pit To Underground Mining

According to information on the Ministry of Natural Resources website,for the period 1969 to 2022 alone,as many as 60 large landslides were induced by blasting vibrations,of which more than 80 percent were caused by open-pit to underground mining.With the continuous mining of the deep concave open pit mines,the open pit slopes have been increasing in height and steepness,and their stability has been decreasing.At the same time,mining costs are rising,and more and more mines are going underground under ultra-high steep slope conditions.However,rocky slopes that have undergone opencast to underground mining are more complex in their stress state due to multiple blasting dynamics and disturbances from mining activities.In particular,the open pit to crumbling method of underground mining blasting is repeated multiple times in the closed underground quarry at the bottom of the high steep slope,the spatial location of the blast source is concentrated,the scale of blasting is large.Therefore,the cumulative effect of mining and blasting has a greater potential for dynamic instability of high and steep slopes containing fault zones.On the other hand,once the high and steep slopes are destabilized,their large number of landslides or crumbling and strong impact will not only directly threaten the safety of underground quarries,but also bring about serious geological disasters.Therefore,it is therefore significant to study the mechanisms of blast dynamic instability of slopes in opencast to underground mining,and even more important to control the stability of slopes.Taking the open pit ultra-high steep slope of Daye iron ore mine as an example,this thesis adopts a comprehensive research method of field test,indoor simulation test,theoretical analysis,and numerical simulation to conduct research on three aspects,including blasting vibration characteristics of open pit to underground mining slope,damage characteristics of fault zone of cascading slope under the cumulative effect of blasting,and dynamic instability characteristics of slope.The main research contents and results of the thesis are as follows.(1)The geological characteristics of open pit to underground mining on high steep slopes and the blast vibration characteristics of the slopes were investigated.Based on the background of the underground mining project in the east open pit of Daye Iron Mine,the F9 fault,a typical cascading slope fault in the high and steep open pit slope,was selected as the object of study,taking into account the engineering geological characteristics of the slope in the mine area.On the basis of statistical analysis of many years of scientific research data,the physical and mechanical parameters of the slope slip zone,the slip body and the surrounding rock have been analyzed and derived.At the same time,the characteristics of blast vibration parameters such as mass vibration velocity and frequency of high and steep slopes on site were analyzed in conjunction with the results of repeated slope vibration monitoring.(2)Model tests on the strength deterioration characteristics of cumulative damage from blasting in fault zones.Based on the similarity theory analysis principle and the blasting vibration parameters obtained from the field test,the model test method of shaking table loading was used to apply multiple blasting vibration loads to similar materials in the fault zone of the cascading rocky slope,and the effects of blasting vibration intensity,number of blasting vibrations,thickness,and water content of the fault zone on the mechanical parameters of the fault zone strength were studied.(a)The results of the study show that when the blast vibration intensity is less than or equal to 1.5 cm/s,the shear strength parameters of the fault zone are dominated by the cumulative weakening effect.When the blast vibration intensity is greater than(and including)2.2cm/s,the mechanical parameters of the fault zone are dominated by the inertia effect of the additional load.(b)The shear strength parameter of the fault zone gradually weakens with the increase of blasting vibration intensity and vibration number,and the magnitude of its shear strength parameter weakness shows a trend of first increasing and then decreasing with the increase of vibration number.The change of fault zone thickness has limited effect on the weakening of cohesion in the fault zone under the cumulative effect of blasting,but has significant effect on the weakening of internal friction angle.(c)As the water content of the fault zone increases,the shear strength of the fault zone gradually weakens,and the damage contribution of the water content to the internal friction angle of the fault zone is gradually greater than the cohesion of the fault zone as the cumulative blasting load increases.(3)Theoretical analysis of the cumulative blasting damage and deformation characteristics of fault zones.Based on the conventional geotechnical stress-strain strength theory,the damage mechanism of fault zones under the cumulative blasting action is studied,and the dynamic damage model of fault zones considering the cumulative damage factor of blasting vibration load is derived and established.(a)The results of the study show that the validated damage intrinsic model,under the coupling effect of blasting vibration load and surrounding rock pressure,can better describe the whole process of blasting cumulative damage deformation in the fault zone.At the same time,the damage variable evolution curves of the fault zone all increase monotonically at the beginning of blast vibration loading,and gradually converge after levelling off at the end of blast vibration loading.(b)The initial damage value of fault zone strength gradually increases as the blast vibration intensity increases,while the damage change of fault zone strength increases first and then decreases as the number of blast vibrations increases.The increase of surrounding rock pressure can slow down the damage change rate of fault zone strength.(4)Numerical simulation analysis of the crack evolution characteristics of fault zone damage under the cumulative blasting action.Based on the discrete element numerical simulation software PFC3 D,the crack expansion characteristics of the fault zone under the cumulative blasting action are analyzed,and the effects of different blasting vibration numbers,different vibration intensities and different surrounding rock pressures on the fine deterioration characteristics of the fault zone strength are obtained.(a)The results of the study show that: the fine-scale deterioration characteristics of the fault zone in the numerical model are mainly dominated by an increase in shear cracks when subjected to the cumulative effect of blasting,and the growth rates of cracks all show a trend of decreasing and then leveling off as the number of blasting vibrations increases.(b)The type of crack extension that increases within the numerical model gradually changes from shear to tensile cracking during the later stages of blast vibration loading.The increase in surrounding pressure is a key factor in slowing the rate of crack extension in the fault zone.(5)Study on the evolutionary characteristics of slope instability under the cumulative effect of blasting.Combined with the supporting project,a typical slope geological section was selected,and a full-size slope numerical model was established to study the sliding body displacement field,stress field,energy field and slope apparent damage form and crack distribution law in the evolution of the sliding surface formation process of the cascading rocky slope of the F9 fault in Daye iron ore mine under the effect of multiple blasting to landslide,and put forward the sliding surface formation process and slope.The sliding surface formation process and slope instability pattern under the cumulative effect of blasting are proposed.(a)Landslide displacement field characteristics show that the slope is mainly damaged by traction under the cumulative effect of blasting,and the cumulative permanent displacement of the slope body gradually increases with the increase of blasting vibration,and the displacement of the central part of the slope body shows a certain hysteresis effect under the effect of blasting vibration.Using the fractal theory R/S analysis,the fractal kinetic parameter displacement Hurst index of landslide displacement was determined,and combined with the landslide displacement vector angle criterion,the three stages of landslide evolution under blasting dynamics determined by landslide displacement alone were revealed,include: initial creep stage,isokinetic deformation stage and accelerated deformation stage.(b)Slope cracking and stress field change characteristics were obtained.When the blasting vibration intensity is less than or equal to 1.5cm/s,the slip zone area is dominated by mixed tensile-shear damage,and in the landslide body area is dominated by tensile cracking damage,and when the slip surface is formed,the slip surface at the back end of the slope body shows the cracking damage mode with slip-tension,and along the front end of the slope shows the damage characteristics with creep-slippressure tensile cracking.When the blasting vibration intensity is greater than(including)2.2cm/s,the damage of the slope occurs sequentially from the leading edge of the landslide to the trailing edge of the landslide,and the damage takes the form of mixed tensile-shear damage,while during the whole blasting accumulation process,the slope conforms to a three-stage development pattern of damage processes,including pullout,slip and shear slide out.(c)Landslide energy field evolution was obtained,at the beginning of the landslide evolution,the slip body only appears near the slip zone part of the strain energy concentration area,with the increase in the number of blasting vibration,the strain energy concentration within the slip body and the value of the larger area gradually increased,with the increase in blasting vibration intensity,the slip body elastic strain energy density value in gradually increased,the whole process of evolution of elastic strain energy from the rear to the front gradually transport law.(6)Research on the identification of landslide evolution stages and safety criteria of blasting vibration based on energy probability entropy.Based on the research results on the multi-field evolution characteristics of cascading rocky landslides under the cumulative effect of blasting,a study on the energy dissipation of landslide systems was carried out and the rate of change of energy probability entropy of landslide systems was obtained.Combining the characteristics of the change of parameters such as landslide displacement,slope stability coefficient,energy probability entropy and energy probability entropy change rate under the cumulative effect of blasting,the evolution stage of the cascading rocky landslide was comprehensively analyzed,and the safety criterion and control threshold of the cumulative blasting vibration effect on the cascading rocky slope of Daye Iron Mine was studied and proposed.(a)The characteristics of slope stability were obtained.The influence of the number of blast vibrations on the slope stability coefficient is greater than that of the intensity of a single blast vibration.With the increase of blasting vibration intensity,the key influence parameter to determine the stability of the slope is gradually transformed from the cohesive force of the slip zone to the internal friction angle of the slip zone,which reflects that the degree of damage accumulation of the slope increases with the increase of blasting vibration intensity,and the internal friction angle of the slip zone plays a key role in the destabilization of the slope by cumulative blasting dynamics.(b)Based on the slope stability coefficient discriminant method,the stability control thresholds for the number of blasting actions corresponding to the slope instability of the F9 fault in Daye iron ore mine are proposed,when the blast vibration intensity is0.5cm/s,0.9cm/s,1.5cm/s and 2.2cm/s respectively,the number of blasts corresponding to the slope instability of the F9 fault in Daye Iron Ore Mine is 14,060,9,712,6,203 and2,882,respectively.Based on the comprehensive criterion of energy dissipation theory,the number of blasting vibrations corresponding to the critical slope instability is determined as 11,500,7,500,6,000 and 3,000 times,so when in the process of carrying out underground mining,the comprehensive safety criterion model should be fully considered to determine the slope instability of the slope.(c)Compared with the characteristics of the landslide evolution stages determined by displacement alone,the evolution of the landslide model is divided into four stages based on the comprehensive criterion of energy dissipation theory,including initial creep stage,isometric deformation stage,accelerated deformation stage,and overall slip stage.(d)Based on the mentioned above,when carrying out research on the identification of landslide evolution stages,landslide displacement data,Hurst index in fractal parameters,slope stability coefficient and the rate of change of energy probability entropy of the landslide should be used as joint warning criterion indicators.When considering landslide pro-slip warnings,the slope displacement and blasting vibration intensity during the open pit to underground mining process should be strictly monitored,and it is also recommended that the landslide enters the overall slip phase as the forecast warning moment.