Effect Of Heterogeneity Of Brittle Rock On Fracture Mechanism Under Thermal-mechanical Coupling Effect

A variety of construction are strongly linked to the long-large-deep tunnels in the western part of China,especially with the process of West Development.Under the thermal-mechanical coupling effect,the geo-hazards in tunnels become significantly dangerous to the projects and workers.So it is extremely urgent to analysis the fracture and rock burst of brittle rock in this environment,thus researching on the micro-structure of rock could be a very good way to reveal the core theory of fracturing in brittle rock.We are based on the railway tunnel project from Dali to Ruili of Gaoligongshan,perform systematic experiments and numerical simulations to investigate the micro-heterogeneity,also discuss the mechanism of fracturing in brittle rock by numerical test.Micro-heterogeneity is the emphasis of this research,it contains microcontact heterogeneity,microgeometric heterogeneity and grain-scale elastic heterogeneity.Based on the pre-existing results of mechanical experiments of Gaoligongshan Granit,we simulate the micro-structure of brittle rock by PFC-GBM model and experiments of mineral composition analysis,also perform the numerical experiments to discuss the effect of micro-heterogeneity.The main understandings show as follow:(1)The main mineral compositions of Gaoligongshan Granite are biotite,quartz,K-Feldspar and plagioclase.The most mineral,feldspar,has larger abundant grains,could form the dominate load-bearing skeleton.The mechanical properties of quartz put a beneficial effect on the load-bearing of granite mass,while the biotite may be ruptured first in the load test.(2)The comparative research of homogenous model,shape-heterogeneity model and total-heterogeneity model show that higher homogeneity leads to a higher strength and more tend to elastic fracture,the curve of kinetic energy tends to display a,also have a higher rate to generate rockburst;On the other hand,heterogeneity model shows that the weak contacts break first,the relative energy released in this process,the curve of kinetic energy tends to be.But the inhomogeneous effect is impaired under triaxial compression test.(3)The numerical experiments show that the crack initiation stress decrease,thenumber of transgranular fracture increase and ratio of tension/shear fracture increase with the increasing of temperature;the peak strength increase,transgranular fracture increase while ratio of tension/shear fracture decrease with the increasing of confinement pressure.In general,the effect of confining pressure is stronger than the effect of temperature.(4)The most significant factor of heterogeneity is the microcontact heterogeneity.The contacts of minerals are the weak parts in the granite mass,they will soon be broke under the loading.In the PFC-GBM,the properties of contacts are relative to the properties of minerals,to be more accurate,the stiffness of microcontacts are closely bond with the elasticity modulus of minerals.The weaker contacts will occur tensile failure at first time,and be the origin of the crack coalescence,then as the microcracks connecting,the fine contacts may break or some compression cracks occur shear failure.(5)Another important factor of micro-heterogeneity is microgeometric heterogeneity.The main rupture of rock is intergranular fractures,these fractures' path are determined by the microgeometry of minerals.The larger abundant grains could form the dominate load-bearing skeleton and dominate the stress development in the region.(6)The properties of minerals partly determine the properties of microcontacts,also is one part of micro-heterogeneity.The transgranular failures are scarce under axial-loading test,so the grain-scale elastic heterogeneity is just a peripheral factor;While under the triaxial-loading test,many transgranular failures are generated,therefore,the grain-scale elastic heterogeneity is a dominant factor.In the crack coalescence,the transgranular failures keep the cracks out the control of microgeometry.In the investigation,the weak grain may break firstly under the loading,but more we found is that in the interactive region of weak and strong grains,the compression of these two kinds of grains leads to more transgranular failures.(7)Numerical experiments of different contents of minerals show that contents of mineral also play a significant effect on the macroscopic response,increasing quartz and decreasing biotite is a effective way to improve the peak strength and elasticity modulus of the granite mass under axial and triaxial loading test.Otherwise,the quartz-increased model under triaxial-loading test show that the failure mode appears a vibrating when attaching peak strength,and major energy releasing lag behind peak strength.