Study On Compressed Fracture Mechanism And Fracture Dilatation Of Brittle Surrounding Rock Of Large Underground Cavern

The excavation of underground caverns often causes local compressive stress concentration in surrounding rock,and the mainly failure mode of surrounding rock under compressive stress is tensile brittle fracture.Brittle fractures appear as spalling and rock bursts on the surface of the surrounding rock,while opening cracks are formed within a certain depth of the surrounding rock which causes dilatancy deformation.Especially for large-scale underground projects,continuous change of the stress caused by the layered excavation make the problem of fracture damage and dilatancy deformation becomes more prominent.In present thesis,the compression fracture mechanism and fracture delatation effect of brittle rock are studied in terms of the brittle rock deformation and failure,the propagation of cracks and fractures and the multi-fissured rock mass deforamtion.Taking the underground powerhouse of Baihetan Hydropower Station as a study case,the fracture of brittle rock and dilatancy deformation during excavation were investigated and analyzed.The main research contents,methods and conclusions of this article are described as follows.(1)Case study of brittle rock fracture and dilatancy deformation in large underground cavern: During the layered excavation of the underground powerhouse on the left bank of the Baihetan Hydropower Station,large deformation of the roof arch caused the anchor cable to be overloaded or even falure.Through the surrounding rock borehole imaging,wave velocity test and displacement monitoring data,it was proved that a large number of tensile fractures are the main reason for the deformation of the roof surrounding rock.Preliminary analysis shows that the concentration of compressive stress in the surrounding rock during excavation is the main driving force lead to compressive deformation of the roof arch.The compressive deformation of brittle surrounding rock include the fracture of brittle rock,the propagation of tensile cracks and the fracture dilatation of rock mass.The dilatation mechanism of rock-mass under compressive stress conditions is summarized,including brittle rock fracture,joint fracture and rock block dislocation uplift.(2)Mechanical properties and micro-fracture analysis of brittle rock:Taking Baihetan basalt as a typical brittle rock sample,triaxial test and acoustic emission test were carried out to analyze the stress-strain characteristics and micro-fracture characteristics of brittle rock under low confining pressure and fracture evolution process.The acoustic emission waveform parameters(average frequency AF,rising angle RA)were used to analyze the microfracture modes(shear fracture & tensile fracture).It is shown that the failure of brittle rock under low confining pressure is dominated by tensile microfracture,and confining pressure has a significant constraint effect on tensile fracture.A finite element model was established by combining Voronoi polygons and Cohesive elements,and numerically simulated the process of rock micro-fractures.The development process of micro-fractures in the rock was explained: The failure of the rock first started with local shear cracks,tensile fractures occur at both ends of the cracks,and eventually the tensile fractures coalescence cause the rock to failure.By changing the parameters of cohesive element,the influence of microcrack on rock strength is discussed.(3)Theoretical analysis of compression fracture mechanism: Shearing Induced Tensile Fracture was adopted as the basic fracture mode of tensile fracture in compressed rock.Based on linear elastic fracture mechanics,the fracture driving force a closed inclined crack was deduced,and the initiation criterion of the closed shear crack was derived according to the maximum circumferential stress.The stress intensity factor(SIF)at the crack tip of the wing was calculated by using the equivalent linear crack model and the Weight function.The propagation feature of wing crack under axial loading and lateral unloading was analyzed.Based on the displacement function of the crack tip,the opening volume of the wing crack was caculated,and the influencing factors of the opening volume were analyzed.The extended finite element(XFEM)method was used to numerically simulate the compressive fracture process of the closed crack,the path of the wing crack propagation was obtained and the theoretical analysis results were verified.Finally,based on the representive volume element(RVE)method,a theoretical model of the multi-cracked rock mass is established,and the plastic deformation and dilatancy characteristics of the rock mass caused by the cracks and openings are analyzed.(4)Experimental research on brittle rock mass fracture and delatacy:Similar materials of brittle rock were prepared,and fractured rock mass models with different crack densities were made.Using observation methods such as PIV particle velocimetry,acoustic emission monitor,laser rangefinder,fracture mechanism and deformation characteristics of rock mass were studied.There are two dilatancy modes of rock mass:(1)Pre-peak dilatancy: The expansion of rock mass before peak stress is mainly caused by the opening of wing cracks.Since wing cracks are stable cracks,more load is required to generate further crack propagation,rock appeared strain hardening characteristics.(2)Post-peak dilatancy: After the stress reaches the peak strength,the rock mass is fragmented,and the dilatantion is caused by the sliding and uplift of the broken rock blocks.As the deformation increases,the strength of the friction surfaces decrease,and the stress-strain curve gradually decreases,which is a typical strain softening process.(5)Fracture dilatation model for brittle rock mass: On the assumption that the size of the rock mass is much larger than the crack length,rock mass can be equivalent to a continuous medium with the same deformation characteristics,and the deformation caused by the fracture can be regarded as irreversible plastic deformation,a fracture dilatation model is established based on the theoretical framework of incremental plastic constitutive.According to the shape of the open wing crack,the lateral plastic strain is selected as the internal variable and the corresponding non-correlated flow law is established to describe the plastic expansion deformation,and the yield function and subsequent yield function are derived.Finally,the fracture dilatation model was used to simulate the deformation of surrounding rock during the excavation of a large underground cavern(Baihetan hydropower station),and the simulation results are in good agreement with the monitoring data,which indicates that the model is suitable for modeling the dilatancy deformation characteristics of brittle rock.