| Reasonable exploitation and utilization of mineral resources are important parts in China's domestic policy of sustainable development and implementation of the ?West China Development? strategy.There is a wide distribution of Permian limestone strata with gently-inclined soft interlayers(dip angles of strata and interlayers are less than 25°)in Middle West and Southwest of China,such as the provinces of Hubei,Hunan,Sichuan,Guizhou,Guangxi and Yunnan.These strata possess abundant high-quality limestone resources,which are irreplaceable materials used in infrastructure construction.As mining activities going on,more strata are being revealed and hydrogeological conditions become more complex,which pose greater impact on groundwater and disturbance of original rock masses.The exposure of soft grounds and the increase of free faces of slopes affect the stability and safety of mining slopes seriously.Soft interlayers,for their special properties,often become potential slip surfaces of mine slopes,controlling the stability of slopes both in the process and at final stage.Also,soft interlayers are good seepage channels for rainfall and groundwater.Water-rock interaction aggravates the deterioration of the mechanical properties of soft interlayers,thus making the rheological properties of soft interlayers more distinctive.Therefore,the research on the deterioration law of water on the mechanical parameters of soft interlayers is of vital importance to the safety of construction and stability of mine slopes.In combination with the ?Research on the rheological properties and slide-causing mechanism of soft interlayers with gently-inclined high slopes under repeated actions of rainfall and blasting? –a funded National Natural Science Foundation grant(41672317),and the ?Demonstration of the stability of high and steep slopes in open-pit mining at Esheng stope and research on its optimization? – a scientific research project,this dissertation took the soft interlayer of carbonaceous shale of the Permian Maokou Formation exposed in Esheng mining area,Sichuan as the research object;the hydrogeological ring shear and meteorological conditions of Esheng stope were taken into account to perform conventional ring shear tests and ring shear creep tests.Based on the results of conventional ring shear tests,the law of changes in peak strength and residual strength of the soft interlayer under different water contents was discussed,and the deterioration law of the mechanical parameters of the soft interlayer was analyzed using water content as a damage variable.Ring shear creep tests were performed under different water contents and normal stresses.Based on analysis of the ring shear creep tests results,a new nonlinear viscoelastic-plastic rheological constitutive model(DNMAP model)considering water content as a damage variable was proposed on the basis of fractional calculus.The finite-difference expression of the DNMAP rheological model was derived,and the FLAC3 D secondary development platform was used to achieve secondary development of the new constitutive model through programming in Microsoft Visual Studio 2010,and its feasibility and validity were verified.The deformation body at the 1270 m excavation platform with the soft interlayer of the Permian Maokou Formation as the potential slip surface was taken as the research object to conduct progressive failure analysis and long-term stability prediction,and relevant prevention and control measures were proposed.The following research findings have been achieved:(1)The soft interlayer of carbonaceous shale of the Permian Maokou Formation exposed at the 1270 m excavation platform in Esheng stope was taken as the research object;based on the analysis of its sedimentary and structural environments,the formation process of the soft interlayer was deduced.The mineral,chemical and particle size compositions and other characteristics of the soft interlayer of the Permian Maokou Formation were analyzed by such means as mineral identification,X-ray and laser particle-size distribution,and a series of tests were performed to obtain the basic physical parameters of the soft interlayer.The research revealed that the mineral composition of the soft interlayer of the Permian Maokou Formation mainly included clay minerals and calcites.Since clay minerals are highly hydrophilic,water content has a significant influence on the mechanical properties of the soft interlayer.(2)Conventional ring shear tests were performed on the soft interlayer of the Permian Maokou Formation to study the shear deformation and failure characteristics of the remolded soft interlayer samples under the same density and different water contents.The research showed that normal stress was an important factor affecting the strain hardening and softening behaviors of the soft interlayer.For samples exhibiting strain softening behavior under low normal stresses,the main characteristic of normal displacement during ring shear tests of the samples changed from shear dilatancy to shear shrinkage with the increase of water content and normal stress.Samples exhibiting strain hardening behavior under high normal stresses showed shear dilatancy with the rate tending to be stable.The ring shear strain behaviors were affected by normal stress and water content;with the increase of normal stress and water content,the strain hardening behavior of the samples became more obvious.The law of influence that water content and normal stress have on the peak strength and residual strength of the soft interlayer was analyzed.The test results showed that the peak shear stress,residual shear stress and the corresponding shear displacement of the soft interlayer samples were positively correlated with the water content of the samples and the normal stress.Based on the Mohr-Coulomb strength criterion and the principle of damage mechanics,the damage and deterioration law of water content on the equivalent shear strength parameters of the soft interlayer was analyzed,and the function relational expression of parameter weakening law and water content was obtained.SEM scanning was performed on the ring shear surfaces after ring shear failure of the samples,and the difference in microstructure of the strain-softening and strain-hardening samples was discussed.It was found that the difference in displacement,breakage and directional arrangement of particles during ring shear test was an important factor affecting the macro-mechanical properties of the samples.(3)Based on the results of conventional ring shear tests,ring shear creep tests were performed on the soft interlayer of the Permian Maokou Formation under different water contents and normal stresses.The shear creep deformation and strain rates of the interlayer samples under different water contents were analyzed.Meanwhile,two methods were used to determine the long-term strength of the soft interlayer under different water contents,which was compared with the residual strength.The microcosmic properties of failure surfaces after ring shear creep failure of the samples under different water contents were analyzed using SEM.The test results show that:(1)When the water content was lower than the saturated water content,as the water content increased,the instantaneous shear strain increment increased with the increase of stress level;when the water content was higher than the saturated water content,the instantaneous shear strain increment decreased with the increase of water content,and the maximum increment occurred near the saturated water content.As the water content increased,the cumulative creep increment of the samples increased.From the ratio of deformation to transient elastic strain when the creep curve reached steady-state creep,it could be seen that as the ring shear stress level increased,the ratio of deformation to transient elastic strain during steady-state creep gradually increased.As the stress level increased,the proportion of creep deformation in total deformation increased,and the creep property became more obvious.(2)The influence of water content on the creep deformation rate was mainly reflected as the influence on the duration of creep attenuation.The research showed that the duration of creep attenuation changed with the water content in an apprxomimately linear manner.As the stress level increased,the slope of the straight light fitted under 60% of the residual stress was greater than that under the residual stress.The higher the ring shear stress level was,the slower the increase of the duration of creep attenuation would be.(3)The long-term strength determined by the isochronous stress-strain curve method was higher than that determined by the steady-state creep rate method,but the magnitude difference was not obvious.Fitting analysis of the correlation between long-term strength and water content of the interlayer under different water contents showed that the long-term strength was negatively correlated with the water content;a higher the water content would lead to a lower long-term strength of the soft interlayer.(4)The samples with different water contents exhibited significant difference in microcosmic properties after creep failure.For samples with low water contents,their failure surfaces were smooth and the deformation scratch was evident;the sample particles were obviously flake-shaped.As the water content increased,the samples were gradually softened.Since the samples contain expansive clay minerals,the particles were arranged more densely due to the increase of expansibility,and the isotropy was evident.As a result of the increase of deformation before sample failure,numerous gaps and cracks could be found on the failure surfaces which exhibited the ductile failure characteristic.(4)Several common definitions in fractional calculus were introduced in detail,and several existing rheological component models commonly seen were improved based on fractional calculus to propose the respective creep constitutive equation.By fitting the test curve before creep failure of the soft interlayer samples in saturated state,with a comprehensive consideration to various factors,the Maxwell component model improved based on fractional calculus was chosen as the viscoelastic part of the new creep constitutive model.Through analysis,it was found that the Abel dashpot with non-steady parameters could describe the characteristics of the accelerated creep failure segment.The viscoplastic body combined in parallel with the plastic component was connected in series with the improved Maxwell component model to establish a new non-linear shear creep constitutive model(NMAP model),and its creep constitutive equation was derived.Fitting of the accelerated creep failure segment in ring shear tests of the samples showed that the new NMAP component constitutive model could greatly fit the whole-process creep test curve.(5)Based on the damage theory,water content was introduced into the NMAP model as a damage variable to establish a non-linear viscoelastic-plastic rheological constitutive model(DNMAP model)considering the damage under water content.Fitting of the curve of ring shear creep test results showed that the DNMAP model could well fit the whole creep process of the samples under different water contents,especially the accelerated creep stage.(6)The equation for the new DNMAP model was derived in detail,and discretization of the three-dimensional difference expression was carried out for its creep constitutive equation,thus obtaining the difference expression of stress increment.Using the FLAC3D500VS2010 Addin.msi plug-in in FLAC3 D,development of the new creep constitutive model based on fractional calculus was achieved in Visual Studio 2010.Also,a three-dimensional numerical calculation model was established for calculation of the results of uniaxial compression creep tests.The rationality of the new DNMAP model was demonstrated,and its validity in the secondary development realization process in FLAC3 D was also proven.(7)Based on the introduction of the basic geological conditions and geological environments of “1270-1380m”deformation bodies in Esheng mining area,progressive failure of the deformation bodies was analyzed considering the strain softening behavior,and the long-term stability of the deformation bodies was predicted considering the rheological behavior.During progressive failure analysis of the deformation bodies,the limit equilibrium method and the gravity increase method based on the strain softening model were respectively applied to the numerical simulation and calculation.The results indicated that the steady-state safety factor calculated by the gravity increase method was slightly larger than that calculated by the limit equilibrium method using the residual strength.The trend of change in shear strain increment distribution at different time steps in the process of numerical calculation by the gravity increase method was used to express the progressive failure development process of the deformation bodies.The shear strain increment experienced the process from occurrence,extension to stabilization.When the time step calculated reached a certain value,the shear strain increment distribution tended to be stable,and the slope safety factor no longer changed.Based on the slope displacement back analysis method,inversion of parameters of the rheological model was conducted using genetic algorithm and neural network.Through comparison between the measured and calculated values,the rationality of the inversion parameters and the validity of the DNMAP model proposed were proved..The stability development trend of dangerous rock is predicted by calculating the displacement change and shear strain increment of deformation body. |
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