Mechanical Properties Of Rock Mass And Stability Evaluation Methods Of Slope In The Southwestern Basin Of Lancang River

With the construction of hydropower stations in the southwest basin of Lancang River,the stability of slope rock mass in reservoir area has received more and more attention.Due to the effects of hydraulic field,temperature field,stress field and chemical field,the rock mass of reservoir bank slopes usually shows various levels of weathering and unloading,which further results in the deterioration of rock mass.Thus,research on the mechanical properties of rock mass under various deterioration conditions is more conducive to evaluating the long-term stability of reservoir bank slopes.During the construction of hydropower stations in the southwest basin of Lancang River,many anti-dip rock slopes have been exposed and the toppling deformation of these anti-dip rock slopes even affected the construction of several hydropower stations.Therefore,research on the deformation mechanism and stability evaluation of anti-dip rock slopes are necessary to maintain the normal function of related hydropower stations.Although some scholars have carried out research on the slope rock mass of reservoir bank in the southwestern basin of Lancang River,there are still some deficiencies to be improved,e.g.,the mechanical properties of typical rock mass in the reservoir area,the long-term shear properties of discontinuities in the reservoir area,the mechanical relationship between rock mass and rock block in the reservoir area,and the deformation mechanism and stability evaluation of anti-dip layered rock slopes under various levels of deterioration,etc.Aiming at the above deficiencies,the paper mainly studies the mechanical properties of rock mass and the stability evaluation methods of related slopes in the southwestern basin of Lancang River,including the mechanical properties of slate,the mechanical properties of interbedded slate and sandstone,the mechanical properties of discontinuities,the deformation mechanism of anti-dip layered rock slopes under various levels of deterioration,the mechanical parameters of rock mass and the stability evaluation of anti-dip layered rock slopes under various levels of deterioration.The main research results are summarized as follows:(1)Based on the slake durability test,cyclic wetting and drying test,uniaxial and triaxial compression test,rebound hardness test,computed tomography and digital image correlation methods,the strength and deformation characteristics,crack propagation characteristics and failure modes of slate were studied.Then,the energy evolution model and damage evolution model were proposed based on the strength and deformation characteristics.Finally,two damage constitutive models were proposed to analyze the stress-strain relationship of slate.1)The cyclic wetting and drying along with confining pressure affect the strength and deformation characteristics of slate,the rebound hardness values have a negative linear relationship with the number of cycles and the mechanical parameters can be estimated by a series of exponential functions.2)The computed tomography and digital image correlation methods can non-destructively study the internal and external deformation characteristics of slate,and correspondingly,five crack types and three failure modes were concluded.3)The five energy parameters well reflect the pre-peak deformation stage of slate.4)The constitutive model based on damage’s random distribution assumes that the internal damage of slate obeys probability distribution model,and two strength criteria are used as discriminating conditions for the occurrence of damage,the model parameters are obtained through boundary conditions and maximum likelihood estimation method.Besides,the constitutive model based on damage’s partition assumes that the undamaged area,cracked area and fissure area are contained in slate,the stress state of each area is analyzed separately and when the damaged area does not meet coordinated deformation conditions,then the constitutive model can be built from the aspect of energy.(2)Based on the triaxial compression test,the strength and deformation characteristics,energy evolution characteristics and constitutive model of interbedded rock with various dip angles were studied.1)The interbedded rock displays five types of stress-strain curves,and the curve types are easily affected by the change of dip angle.2)The cracks of interbedded rock can be subdivided into eight common types and four uncommon types,and correspondingly,eight failure modes of interbedded rock are summarized and the failure modes are reflected in the fracture characteristics of in situ rock mass.3)Seven basic energy parameters and three derived energy parameters can analyze the energy evolution process of interbedded rock under compression condition,based on the energy parameters,the damage evolution model of interbedded rock can be established by combining energy criteria,probability model and damage mechanics.The damage process of interbedded rock is divided into three stages and the proposed dissipation energy per strain can be used to distinguish the brittleness and ductility deformation of interbedded rock.(3)Based on the direct shear test,the effects of cyclic wetting and drying,cyclic shear and normal stress on the shear strength of artificial and natural discontinuities were studied,among which,the effects of physical weathering and long-term shear were respectively quantified as damage and roughness,and one shear strength model considering physical weathering and long-term shear were proposed.1)The variation process of shear stress can be divided into three stages,and the difference of shear stress curves of discontinuities is mainly reflected in the rapid growth stage.2)The increase of shear number effectively reduces the roughness of discontinuities,and the increase in the number of wetting and drying cycles significantly reduces the ability of discontinuities to resist shear deformation,while the increase of axial stress causes the convex part of discontinuities to be sheared and then reduce the roughness.3)The shear strength model mainly considers the influences of roughness of discontinuities and damage of rock wall,the proposed model has higher calculation accuracy compared with Barton’s model,which also shows good application effect in the stability evaluation of a slope case.(4)Based on the field investigation,physical model test and numerical simulation test,the deformation mechanism of an anti-dip layered rock slope on the left bank in front of one hydropower station were studied,the study considered the deterioration of rock mass.1)In the physical model test,it is feasible to use the original rock powder as model material,and from the displacement vector map and global displacement cloud map,it is found that the slope model tends to have combined bending-fracture-based toppling failure,the failure surface is located in the shallow position of slope model and passes through the slope toe.2)In the numerical simulation test,the influence of rock layers and joints on the slope model is considered.The deformation mechanism of slope model is mainly reflected by the rock fracture characteristics,global displacement cloud map,displacement characteristics of monitoring points and bending angle characteristics,the test result is consistent with the field investigation result.3)The deterioration degree of rock mass is quantified and combined with the physical and mechanical parameters,based on which,the deformation mechanism of anti-dip layered rock slope under deterioration condition is studied,and the deformation process can be divided into four stages.Once the deterioration degree of rock mass is less than 0.571,the position of slope surface corresponding to transition point generally generates displacement first,and the deformation area and deformation depth gradually expand.Once the deterioration degree of rock mass is greater than 0.571,the overall displacement of shallow layer of the slope model is more obvious and the displacement of deep layer of the slope model gradually stabilizes after a rapid increase.4)According to the bending degree of slope rock mass,the bending angle can be divided into four intervals,namely the bending angle of rock mass above failure surface and potential failure surface,the bending angle of rock mass with obvious toppling deformation below failure surface,the bending angle of rock mass with unobvious toppling deformation below failure surface,and the bending angle of rock mass that is undisturbed.Considering the deterioration degree,fracture characteristics,bending angle characteristics,displacement characteristics and stress form of rock mass,the deformation mode of the slope model can be divided into brittle fracture-based toppling deformation and flexible bending-based toppling deformation,as well as fracture-based toppling deformation,bending-based toppling and combined bending-fracture-based deformation.(5)Based on the rock mass quality evaluation model and generalized Hoek-Brown criterion,the calculation method of mechanical parameters of deteriorated rock mass was derived,then combined with the generalized Hoek-Brown criterion,catastrophe theory,maximum moment model and failure probability model,the stability evaluation methods for anti-dip layered rock slope were proposed.1)The improved generalized Hoek-Brown criterion considers the influence of quality grade and deterioration degree of rock mass,and is more suitable for evaluating the mechanical parameters of rock mass.2)The calculation methods of ultimate failure length and residual sliding force of rock mass are based on the cantilever beam model,among which,the ultimate failure length and safety factor are obtained through the tensile stress criterion and improved generalized Hoek-Brown criterion,while the residual sliding force is obtained from the moment balance equation.The calculation method of residual sliding force can additionally add the influence of roughness.The above calculation methods assume that the failure surface of rock mass is caused by fracture and are more suitable for evaluating the stability of anti-dip layered rock slope with fracture-based toppling deformation.3)The energy potential function and equilibrium surface of rock mass are established referring to the catastrophe theory.The control variable integrates the factors that affect the stability of slope and thus,reflects the control of state variable.When there are more factors embodied by the control variable,the obtained equilibrium surface can better reflect the evolution process of stability state of rock mass,thus,it is extremely important to study the multi-field parameters of rock mass under deformation process.Another form of ultimate failure length and safety factor can be obtained from the catastrophe condition of rock mass.This method takes into account the deflection characteristics of curved rock mass and is more suitable for evaluating the stability of anti-dip layered rock slope with bending-based toppling deformation.4)The safety factor of rock mass from the aspect of critical moment and bending angle is proposed,this method assumes that the instability of rock mass is because the bending moment and bending angle of rock mass exceed their limit values,which is more suitable for evaluating the stability of anti-dip layered rock slope with bending-based toppling deformation.5)It is assumed that the rock mass experiences combined bendingfracture-based toppling failure,and the deformation area is divided into fracture and shear areas.The mechanical parameters of rock mass are degraded several times according to a certain gradient,and the stability state of each rock layer is judged by residual sliding force.Assuming that the frequency of instability is regarded as the failure probability of each rock layer,then the failure probability reflects the deterioration process of rock mass and can be used as an evaluation method for the long-term failure probability of anti-dip layered rock slope.The above stability evaluation methods related to anti-dip layered rock slope show good application effects.