Research On Microwave Irradiation Rock Fracture Mechanism And Strength Deterioration Law

TBM has incomparable advantages in long and large tunnels.It has a high degree of automation and a fast construction speed.However,when encountering harder rocks,its tool wear speed will increase and the tunneling speed will slow down,which greatly affects the construction cost and efficiency.The "microwave+mechanical" rock breaking method can solve this problem well.The internal factors of rock that affect the effect of microwave rock breaking mainly include rock type,mineral composition,distribution method,particle size and mineral particle boundary contour,etc.The external factors mainly include microwave irradiation method and ground stress.Microwave-assisted rock breaking is manifested as the formation and propagation of microcracks in the rock under microwave irradiation.Therefore,this paper uses discrete element software PFC to simulate the microwave irradiation of four representative rocks to study the rock-induced stresses under different confining pressures and powers.The mechanics test of the rock before and after the microwave irradiation was carried out,and the connection and penetration mechanism of the microwave irradiation crack and the change rule of mechanical parameters were studied under the action of external load.Using physical tests,the influence of microwave irradiation on rock strength and wear resistance was further studied.The main conclusions are as follows:(1)The microwave irradiation rock analysis model was established using PFC,and the crack propagation law of four representative rocks under microwave irradiation was studied when the power density Pd=1 × 1010W/m3 and Pd=1×1011 W/m3.Pegmatite cracks from inside quartz and orthoclase.As the irradiation time increases,the cracks further expand in the orthoclase in a network shape;the main minerals contained in granite and pegmatite are roughly the same,showing similar laws;Basalt cracks from the inside of plagioclase and pyroxene.With the increase of irradiation time,the cracks expand inside the plagioclase and pyroxene;limestone cracks from inside the magnetite,and with the increase of irradiation time.the cracks are inside the magnetite and at the grain boundary.When high-power irradiation,the cracks produced are more messy,and more and shorter cracks are produced at the grain boundaries.(2)When the lower power is irradiated,the confining pressure has a restrictive effect on the crack propagation in the rock.The greater the confining pressure,the tension cracks are greatly reduced,and the shear cracks are slightly increased.With the increase of confining pressure,the cracks produced by pegmatite gradually concentrated at the junction of quartz,plagioclase and orthoclase;granite cracks gradually gathered at the junction of quartz,albite and biotite;basalt cracks also gradually concentrated there.Contains the junction of the three minerals;the limestone is concentrated around the magnetite.During high-power irradiation,as the irradiation time increases,the greater the confining pressure,the more cracks in the rock.(3)Under microwave irradiation,the number of cracks in minerals and grain boundaries of rocks varies with confining pressure.At lower power irradiation,the cracks in pegmatite are reduced with increasing confining pressure at chlorite,orthoclase,quartz and its grain boundaries,granite orthoclase,quartz,orthoclase-biotite and orthoclase-quartz,basalt plagioclase,dolomite-calcite,apatite-calcite,magnetite-calcite and magnetite-apatite;The cracks in other parts of pegmatite,pyroxene,pyroxene-plagioclase and other parts of limestone increase first and then decrease with the increase of confining pressure,and the other parts of granite increase with the increase of confining pressure.When exposed to high power,the cracks in quartz and plagioclase-northoclase in pegmatite decrease first and then increase with the increase of confining pressure;in other parts of pegmatite,the number of cracks in orthoclase,biotite,albite,orthoclase-biotite and orthoclase-sodium feldspar in basalt decreases gradually and increases gradually with the increase of confining pressure in short time,and then increases gradually;in limestone,dolomite,calcite,magnetite,dolomite-magnetite and apatite-calcite increase with the increase of confining pressure.(4)In the rock samples before and after microwave irradiation are simulated by conventional mechanical tests.As the microwave irradiation time increases,most of the cracks generated in the mechanical test start from the crack tip generated by the microwave irradiation.After 0.1s of microwave irradiation,the uniaxial compressive strength of pegmatite decreased by 68.52%,granite decreased by 47.33%,basalt decreased by 49.61%,and limestone decreased by 71.12%;the tensile strength of pegmatite decreased by 69.53%,and granite decreased by 69.53%.The highest reduction is 68.30%,the highest reduction of basalt is 80.50%,and the highest reduction of limestone is 87.65%.During biaxial compression,with the increase of confining pressure,the reduction of biaxial compressive strength of each rock gradually decreases.(5)In the physical test,the temperature of granite is low.Small cracks begin to appear in the microscopic view at 15 minutes,the color of the sample turns yellow,the temperature of basalt is the highest and the heating rate is fast,cracks appear on the surface at 5 minutes,and begin to melt at 25 minutes.The temperature is followed by obvious decomposition of calcium carbonate in the rock at 25 minutes.After 30 minutes of irradiation,the point load strength of granite has the highest reduction rate,which is manifested as a radial fracture from the center of the sample.The initial strength of limestone is low and the reduction is the smallest.(6)After microwave irradiation,the CAI value of the granite abrasion resistance index is significantly reduced,and the basalt has little change.After the limestone is irradiated,the scratches are deeper,and more powder is scraped during the sliding process.The mass loss of granite and limestone increases sharply within 30 minutes.Since the cracks appear earlier,there is a greater quality loss at 5 minutes.The point load strength of granite is linearly related to the CAI value,the limestone is exponentially related,and the basalt has little correlation.