Experimental Research And Application Of High Slope Hard Rock Mechanical Peoperties Under Unloading Condition

The universal existence of high and steep rock slope in water conservancy and hydropower engineering,surface mining engineering,traffic engineering,national defense engineering,etc.is commonly under the unloading condition.However,present study of rock mechanic in loading condition or high stress environment of underground cavern is difficult to explore in these slope engineering.Thus,stability and deformation are studied to further supply theoretical basis of mechanism;it is important to research the rock mechanic of high and steep rock slope under the unloading condition,which is also directly related to its safety and economy.The influence factors of rock mechanical properties under the unloading condition including of initial confining pressure,specimen size,unloading ratio,and stress path,etc.Dabenliugou quarry slope was taken as research object,firstly,to study the evolution law of quartz sandstone mechanical parameters under different unloading ratio,then analysis the effects of initial confining pressure condition and specimen size condition toward to mechanics parameters of hard rock.Compared to rock mechanics parameters during different loading conditions,the rock mechanical characteristics in slope stress environment were obtained.And also the microscopic characteristics of rock unloading failure mechanism revealed by SEM and CT scanning technique were revealed.At the end,taking an example of Dabenliugou ditch material field slope which is monitored successively couple of years,to study the evolution law of deformation and stress of slope rock.At the base of evolution law of mechanical parameters and geological structure characteristics,numerical simulation was conducted to evaluate unloading deformation mechanism and the stability of the slope.Some main conclusions are as follows:?1?The brittle feature of rock failure under the unloading condition is more obvious;And the shear failure was occurred in common loading test,while the tensional-shear mixed failure was occurred in unloading test.As the decrease of confining pressure,the elastic modulus is decreased,the faster unloading ratio,the more obvious nonlinear relation shown.Compared with unloading condition and loading condition,the friction coefficient of rock is increased,while the cohesion keeps invariant or little decrease.?2?The faster the unloading rate,the increase of Poisson ratio is slower;under the condition of unloading,the cohesion of the rock sample decreases,while the internal friction angle increases.The faster unloading rate,the cohesion decrease more and the internal friction angle grow less.Under the three axis unloading failure stress path in the medium and low stress environment,the size effect of the internal friction angle of hard rock is not obvious,while the size effect of cohesion is remarkable.For instance,marble of large size,the cohesion decreases by about 21%compared with that of middle one,and decreases by about 54%compared with that of small one.However,the valve of strength parameter among three sizes varies about 10%.The unloading strength of rock exhibits nonlinear characteristics with the change of confining pressure.The larger the confining pressure is,the smaller the coefficient of friction is;the greater the confining pressure is,the greater the cohesion is.?3?The true triaxial tests of five stress paths are carried out on Jinping marble.The experimental results reflect the nonlinear characteristics of strength parameters of hard rock under different stress paths.Generalized Mises criterion is the rule to research strength parameters and strength criterion under different yield criterion path.There is a typical nonlinear relation between?₁ and?₃,both for loading failure and unloading damage.The peak value of the peak strength of loading test is slightly lower than unloading test,while the cohesion's value of peak strength of loading test is higher.Under three different unloading paths,cohesion's value varies significantly while internal friction angle varies slightly,at the same average lateral stress within the range of?₂₃;they are the Lode angle theta sigma reduces with the increase of lateral stress and the average.?4?Under low stress environment in high slope,hard rock loading test of micro crack appears three gradations which is divided to compaction,stable growth,instability and breakout.Implicit stratification is main influencing factors for rock rupture,the main fracture of basic and recessive bedding keep parallel and relatively smooth,show shear characteristics.Fracture crack expand more under unloading condition which the three-dimensional reconstruction map records the a ups and downs broken surface,its burst fracture is rough,the main damage is the frictional shear slip fracture between the unloading rock macro,which is the tension-shear failure mode.?5?Long-term monitoring data results indicates that the flow field of the monitoring Benz physical volume changes,without large cracks,no abnormal slope sliding deformation has,stress of bolt and anchor force tend to stabilize gradually.The slope is generally stable,but special attention should be paid to local buckling collapse risk.?6?Numerical simulation method is used to calculate and analyze the stress and deformation characteristics of the slope under excavation and support conditions.The results show that little influence to the deformation of the upper part of the slope on that of lower excavation,lower part of the slope deforms larger than that of the upper part.The deformation of the mini-slip plane between the rock beddings and the steep fault is larger;there are a certain degree of compressive stress concentration in the slope foot part and a small tensile stress area.During the excavation process of the overlying strata,the slope stress value of the foot part is gradually reduced.The plastic yield zone is mainly tensile failure and few shear zones.Under the normal working condition,the short working condition and the accidental condition,the safety factor is 1.35,1.18 and1.17 respectively;the slope is all in stable state.The engineering geologic analysis reflects that there are two kinds of failure mechanism which are slipping shear failure and buckling failure,and discrete element strength reduction method indicates that the high slope of the high flow field is the buckling failure mode.