Research On The Deformation Mechanism And Control Method Of The Soft Basement Waste Dump In Open-pit Mine

With the continuous exploitation of open-pit mineral resources in China,accidents such as collapse and landslide of waste dump often occur,which cause great losses to people's lives and state property.In this paper,taking the waste dump of Dagushan open pit mine as the background,by means of field investigation,laboratory test,theoretical analysis and numerical simulation,the deformation mechanism and control methods of the soft basement waste dump are studied systematically.Based on the field investigation,the particle size distribution and zoning characteristics of the waste dump are studied.The influence of particle size distribution on physical and mechanical properties of waste dump is studied by triaxial test and numerical simulation.Based on the base friction test and digital speckle technique,the deformation mechanism and failure mode of the soft basement waste dump are studied.Based on the theory of limit equilibrium,the stability of Dagushan waste dump under complex conditions such as dumping,vibration and basement softening is studied,and the influence law of key parameters is also studied.The control methods of deformation of waste dump,such as optimization of waste dump technology and basement grouting reinforcement,are put forward.Main conclusions are as follows:(1)The particle size of the waste dump is obviously small in the upper part and large in the lower part.The intermediate particle size D50 and the average particle size (?) decrease linearly with the increase of the height of the waste dump(h/H).The mass percentage content of fine-grained dispersion(P<5mm)increases with the height of waste dump(h/H),and presents a quadratic function relationship.There is no lack of particle size section in each step of waste dump,and the grading is good.(2)The peak stress and the peak stress ratio in the triaxial test increase linearly with the increase of the fine particle content(P<5mm).The peak stress increases with the increase of confining pressure,and the peak stress ratio decreases with the increase of confining pressure.The deformation characteristics of bulk materials show dilatancy in low confining pressure and shrink in high confining pressure.The shear strength of the waste dump is in accordance with the exponential model:?=A·(?)B,and A=3.7142·P<5mm+0.3489,B=0.6808·P<5mm+0.0894.The fitting coefficients R2 of A and B are both greater than 0.95.(3)The stability of a soft basement waste dump is obviously worse than that of a hard basement waste dump.The deformation and failure pattern of a hard basement waste dump shows a collapse-traction-push feature,and it starts from the low-grade slope and develops towards the high-grade slope.The deformation and failure pattern of a soft basement waste dump is mainly controlled by the soft basement.The uneven settlement under gravity causes the slope toe to crack first;and then the slope body slides along the basement to the slope toe;and then gradually develops to the slope back.Thus,the cyclic subsidence-breakage-slip occurs until all the slopes above the soft basement slide.(4)Cracks in a soft basement waste dump,caused by uneven settlement,are dislocation cracks.However,cracks in a hard basement waste dump,caused by uneven horizontal deformation,are tension cracks.(5)First,the safety factor of Dagushan waste dump is reduced from 1.343 to 1.238 with the waste dump rising from+150m to+201m.Then,the weakened basement of silty clay reduces the safety factor from 1.238 to 1.129.Then the toe excavation of the waste dump reduces the safety factor from 1.129 to 1.029.At last,the mechanical vibration at the slope toe makes the safety factor of the waste dump change periodically in 6s.Under the condition of mechanical vibration,the minimum safety factor of waste dump is 0.747,which eventually leads to landslide.(6)The sensitivity of the dump stability to the angle of Grade ? slope,basement cohesion,the waste cohesion and the mechanical vibration strength is moderate;the sensitivity of the dump stability to the height of Grade ? slope,basement friction angle,waste friction angle,basement moisture content and the excavation angle of Grade I slope is high.The dump stability decreases with the increase of the height and angle of Grade ? slope,basement moisture content,excavation angle of Grade I slope,and mechanical vibration strength.Dump stability increases with the increase of the basement friction angle,basement cohesion,waste friction angle and waste cohesion.(7)The failure mode of forward covering dump is the arc-shaped landslide of Grade II and III slope.The failure mode of backward covering dump is a collapse-traction-push landslide.The failure mode of the forward presser foot dump is a local circular landslide of Grade ? slope.The failure mode of the backward presser foot dump is a base-type landslide along the interface of different graded granular materials.The forward presser foot dump has the smallest landslide area,the smallest sliding volume and high stability;its comprehensive effect is better than that of the other three types of dump.(8)The prevention and control methods of deformation of the soft basement waste dump mainly include the basement reinforcement and selection of reasonable dumping methods.The measures of dynamic consolidation,drilling grouting and drainage can be used to strengthen the soft basement;meanwhile,the forward presser foot dumping method with small landslide area,small sliding volume and high stability of the waste dump can be adopted to further reduce the deformation of the waste dump.