Study On The Stability Induced By Stepped Excavations And Rheological Effect Of Underground Cavern Groups In High In-Situ Stress Areas

To encourage scientific innovation,the Chinese government has formulated a Mid- to Long-Term Plan for Development of Science and Technology(2006-2020),which highlights research in the basic science and frontier technologies,with priority given to energy,water resources and environmental protection.As for the exploitation of energy and water resources,it is concretely embodied in the several dozens of world-class hydroelectric power stations in southwestern areas of China.The underground cavern groups are mostly adopted in these projects.Moreover,these underground caverns are generally located in the high mountain-canyon areas which give rise to high in-situ stresses in vicinity of the underground cavern groups.The high in-situ stresses are also intensely influenced by the terrains and slope gradients. Under the complicated circumstances,splitting or spalling failures just like falling masses and rockburst frequently appear in the construction process,especially near the side walls of underground caverns in the actual projects just like Yuzixi Hydropower Station,Ertan Hydropower Station,Laxiwa Hydropower Station and Pubugou Hydropower Station.Therefore,it is very significant and necessary to study the stability of surrounding rock masses in the process of stepped excavations. Secondly,the long-term stability of underground cavern groups is also highlighted since the deformation and crippling failures in most projects are not completed in a short time.The rheological phenomena can be found everywhere,for instance, sliding,cracking,even collapse and other failures often occur in the rock masses due to the long-term action of high in-situ stress.In this dissertation,taking two underground cavern groups which are under construction as the engineering background,the laboratory experiments,large-scale geomechanical model test, numerical simulations and theory innovations are comprehensively applied to study the stepped-excavation and long-term stability.In this dissertation,the main investigation work focuses on the following.(1) Using the servo-controlled testing machine developed by Geotechnical & Structural Engineering Research Center of Shandong University,uniaxial compressive experiments are performed on the granite and marble specimens.Each stage of the complete stress-strain curve,deformation characteristics,strength characteristics and failure principles are analyzed.The physico-mechanical parameters of rock masses are also obtained,which provide the accurate parameters for the rheological tests and numerical simulations.During the study on the uniaxial compressive rheological tests of marble specimens,a newly apparatus is developed and the uniaxial compressive rheological tests are performed(including a dry specimen and a water-saturated specimen).The relationship curves between axial/circumferential strain and stress levels are obtained,and therefore the variation curves of Poisson's ratio are also obtained.According to the curves,the related rheological parameters are also obtained through inversion analyses on a basis of Nishihara model,which can provide the accurate parameters for long-term stability analyses of the actual projects.(2) Prior to the large-scale geomechanical model test,a newly developed analogous material is developed to simulate most rock masses.After conducting literature reviews on the analogous materials all over the world,a new analogous material called IBSCM is developed.This is made from iron mineral powder,barite powder,quartz powder and alcoholic solution with rosin.Some tests such as uniaxial compressive test,quasi triaxial test and Brazilian test have been performed to obtain the physico-mechanical parameters of the composite material under different proportional mixtures.IBSCM has been successfully applied in several large-scale geomechanical model tests including the one which will be introduced in this dissertation.(3) In the study on the stability of stepped excavation,taking the underground cavern group of Shuangjiangkou(SJK) Hydropower Station as an engineering background,a large-scale quasi-three geomechanical model test and numerical simulations are performed.The developed steel structure can simulate the complicated circumstances just like high in-situ stress and high overburden depth.It also can apply the true 3D loading on six surfaces of the physical model.An automatic hydraulic loading system is also developed.Transparent windows are installed in the steel frame to monitor the cracking extension on the model surfaces. During the whole physical model test,a few advanced deformation monitoring techniques are applied together firstly,which realize the accurate real-time monitoring in the process of excavations.A unique grouting technique and prestressed cables are adopted in the model test.Through the overloading test,the whole process from micro cracks,crack initiation,crack extension to wide-range failures is monitored.The opening displacements induced by splitting failures are further studied through a proposed displacement prediction formula.The results of test,numerical simulation and theoretical derivation are comparatively analyzed and some principles are obtained,which have made certain guiding significance to the actual projects and future geomechanical model test.(4) In the study on the long-term stability of underground cavern groups,a non-constant-parameter damage rheological constitutive model for jointed rock masses is put forward based on the in-situ geological data and uniaxial compressive rheological test results.In this constitutive model,a second-order damage tensor is applied to describe the initial geometry damage of jointed rock masses,thereinto the major influencing factors are joint plane areas,directions,spacing and density.Then, taking the Nishihara model as the basic model to study the time-dependent characteristics of rock masses,its one-dimensional constitutive equations are transformed into three-dimensional forms.The three-dimensional visco-elastic-plastic damage constitutive equations are deduced by introducing the second-order damage tensor into the rheological constitutive equations on a basis of the equivalent strain hypothesis.Next,the related parameters are substituted by the nonlinear expressions of the rheological parameters obtained from the uniaxial compressive rheological tests.Eventually,taking FLAC^3D Version 3.0 as the development platform,the non-constant-parameter damage rheological constitutive model for jointed rock masses are compiled in the environment of VC++7.0,which realize the secondary development of FLAC^3D.Experimental results and this new constitutive model are applied to the engineering practice of rock mass engineering in the study on long-term stability of underground cavern groups in SJK Hydropower Station and Jinping First Stage Hydropower Station.The long-term stability of rock engineering under complicated circumstances is predicted and compared with the elasto-plastic analyses,which brings forward reasonable evaluation and suggestion for long-term stability and safety of rock engineering.