| China’s development and utilization of mineral resources and the construction of underground projects have begun to develop to the deep part of the earth,and the main position of resources in the future will be underground.The complex environment of underground rock mass and the complex structure of rock mass itself make us encounter many safety problems in underground actual engineering,especially the long-term stability of rock mass in the multi-field coupling environment of stress field and seepage field.Although many achievements have been made in the study of the mechanical properties of underground rock masses,due to the complexity of rock mass structure and occurrence environment,the study of the temporal behavior of rock mass with complex fractures under stress-seepage coupling conditions is still in the stage of understanding and exploration.Therefore,the indoor mechanically controlled loading method is used to carry out triaxial compression tests on the complete limestone to simulate the complex fractured rock mass generated in the natural state,and the MTS815 rock mechanics test system is used to carry out a series of stress-seepage coupling tests and seepage-creep coupling tests on the obtained fractured rock masses,revealing the temporal behavior and permeability variation law under the stress-seepage coupling of complex fractured limestone.Finally,based on the combination theory of elements,using fractional calculus and Hausdorff derivative theory,considering the influence of internal damage and seepage factors of fractured rock mass,a creep model that can describe the whole process of creep of fractured rock mass under stress-seepage coupling conditions is established on the basis of Nishihara model,in order to provide a theoretical basis for the development and utilization of underground resources and the actual underground engineering.The main research results of this paper are summarized as follows:(1)Through the conventional triaxial compression test of intact limestone and fractured limestone in the saturated state,the influence of fracture on the basic mechanical properties of limestone was revealed.The pre-peak stage of intact limestone and fractured limestone is basically the same in the triaxial compression process,which is divided into compaction stage,linear elastic deformation stage,and stable fracture development stage,and complete limestone also includes unstable fracture development stage and plastic flow stage.The deformation parameters and compressive strength of intact limestone and fractured limestone increased with the increase of confining pressure,and fractured limestone was more affected by confining pressure,and confining pressure could effectively improve the mechanical properties of fractured rock mass.(2)Through the stress-seepage coupling test of fractured limestone,the influence of different confining pressure and water pressure on the basic mechanical properties of fractured limestone and the evolution law of permeability were revealed.The deformation of fractured limestone under stress-seepage coupling showed stage characteristics: pore closure stage,linear elastic deformation stage,stable development stage of new fracture and unstable development stage of new fracture.The mechanical properties of fractured limestone have significant confining pressure strengthening effect and water pressure weakening effect,and the deformation modulus and Poisson’s ratio increase linearly with the increase of confining pressure,while the deformation modulus decreases linearly with the increase of water pressure.The compressive strength is linearly related to the confining pressure and water pressure,which increases with the increase of confining pressure and decreases with the increase of water pressure.The permeability of fractured limestone generally decreases with the increase of confining pressure and increases with the increase of water pressure.(3)Through the seepage-creep coupling test of fractured limestone,the creep deformation failure characteristics,long-term strength characteristics and permeability evolution characteristics of fractured limestone under stress-seepage coupling conditions were revealed.The creep curve of fractured limestone shows the characteristics of three stages of typical creep of rock.When the confining pressure is the same,the instantaneous deformation of fractured limestone increases linearly with the level of partial stress,and the creep deformation increases exponentially with the level of partial stress.Both the instantaneous deformation and creep deformation decrease with the increase of confining pressure.The creep failure characteristics of fractured limestone are significantly affected by confining pressure and water pressure,which shows that the angle between the distribution of fracture surface and the axial direction decreases with the increase of water pressure and increases with the increase of confining pressure,and the long-term strength of fractured limestone also has a significant water pressure weakening effect.The change of permeability in the creep process showed a phased law that was more consistent with creep deformation,and the increase of permeability with time did not fluctuate much in the deceleration and steady-state creep stages.In the accelerated creep phase,the increase in penetration rate with time shows an increasing trend.(4)On the basis of the Nishihara model,a nonlinear creep model describing the creep process of fractured limestone under stress-seepage coupling is established considering the creep characteristics of creep and the influence of osmotic water pressure.A Scott-Blair element with a coefficient of change based on creep damage is introduced,and a nonlinear creep model that can describe the accelerated creep stage is established according to the component combination theory.Considering the influence of seepage,the concept of effective stress is introduced,the influence of osmotic water pressure on rock mass is transformed into a weakening effect on confining pressure,the water pressure weakening coefficient is introduced to redefine the stress of rock mass,and the nonlinear creep model that can describe the whole process of creep of fractured rock mass under stress-seepage coupling conditions is obtained by substituting it into a nonlinear model. |
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