Study Of Rock Fracture Mechanism Induced By Thermal Gradient

Rock cracking induced by temperature gradient is widespread in nature,and with the increase of human disturbance in underground space in the past 100 years,it has penetrated into most underground engineering structures and become a common rock fracture phenomenon.It is of great significance to the research on the mechanism of this phenomenon and the process of crack propagation.The mechanism of any rock fracture induced by temperature gradient is common.However,the each occurrence of rock fracture induced by temperature gradient corresponds to different rock scales,geometric characteristics and constraints,and the resulting crack propagation process also has its own characteristics.In this paper,the mechanism of rock fracture induced by temperature gradient and the process of crack propagation are taken as the main line of study,and comprehensively discussed from the perspectives of four thermal loading methods:heating,thermal shock,cold shock and cooling.The main work and research results are as follows:(1)Starting from the scale effect,the rock fracture mechanism induced by heating was systematically studied.The results show that the maximum tensile stress of the small-scale model appears at the outer boundary,leading to unloading at the same time of crack initiation,which is in good agreement with the experimental results.The maximum tensile stress of the large-scale model was transferred to the vicinity of the circular hole.The internal stress could not be unloaded and the second crack initiation resulted in multi-area failure.Under the coupling effect of in-situ stress and heating,tensile stress may rise near the circular hole of the engineering-scale model,affect stability of surrounding rock,cause engineering disaster.(2)From the aspect of scale and heating rate interaction,the understanding of the nature of rock fracture mechanism induced by heating is strengthened.The results show that the increase of the heating rate can also lead to the transfer of the maximum tensile stress from the outer boundary to the vicinity of the circular hole.The relative relationship between the heating rate and the model scale is the essential reason for this shift.In addition,the effect of decreasing the heating rate and increasing the model size on the crack initiation location is the same,and vice versa.It is assumed that the two models have the same parameters and boundary conditions,when the ratio of the rate of warming is equal to the inverse of the square of the model scale;the normalized crack initiation location in the two models is the same,which is defined as“scale-heating rate equivalent effect”.(3)Based on the buckling theory,the rock fracture mechanism induced by thermal shock is discussed.The results show that the single crack propagates in a direction parallel to the model boundary in the high compressive stress,while the region from the crack tip to the model surface expands,causing shear slip and eventual detachment,which is in good agreement with the experimental results.The results of the non-preflawed model show that the thermal spallation process of the dense rock needs to be triggered by tensile failure induced by temperature gradient.And the spallation mode can be divided into single crack spallation mode and multi-crack through spallation mode.The destruction of tunnel wall by tunnel fire is the result of thermal spallation and in-situ stress,which may lead to tunnel instability and large engineering disaster(4)Through experimental study,the cold shock effect of water and liquid nitrogen on rock samples was compared,and the possibility of secondary cracks in geothermal reservoir was analyzed.The results showed that water and liquid nitrogen had a very obvious cold shock effect on rock samples above 620?,water cooling mainly played a role in the opening of the original crack,while liquid nitrogen cooling showed an excellent ability to fracture.Liquid nitrogen is better than water in reducing strength and increasing permeability coefficient,and in-situ cold shock must be better than rock sample cold shock.Therefore,thermal stimulation using liquid nitrogen may be better in geothermal reservoir modification.(5)The mechanism of rock fracture induced by cold shock is discussed from the model of reservoir reconstruction.The surface tensile stress decreases with the decrease of crack spacing,resulting in the saturation of the crack.The tensile stress at the specific crack tip decreases with the increase of the crack length on both sides,leading to the crack classification.The secondary crack morphology is affected the surrounding rock temperature and constraint state.And secondary cracks between multiple main cracks can be connected,which is conducive to the production of a denser fracture network in geothermal reservoirs.(6)The mechanism of rock fracture induced by cooling is analyzed with the background of high temperature tunnel of Sichuan-Tibet railway.The results show that under the effect of ventilation,failure surface of tunnel sidewall lamellar spallation is reduced,proportion of shear failure is decreased,and probability of rock burst is reduced.The main reason is that large temperature gradient near the tunnel surface leads to the decrease of maximum principal stress.During excavation of the unusually high temperature section of Sichuan-Tibet railway tunnel,it is necessary to reduce excavation speed and delay release speed of surrounding rock stress,and try to take more time to vent the surrounding rock.The effect of long-term ventilation cooling causes stress concentration around the tunnel to transfer to the deep surrounding rock,which contributes to the long-term stability of the tunnel.