A Study On Mechanical Properties Of A Thermal-Damaged Granite Under Different Water Contents

As a non-negligible component in the geological environment,water contacts with the rock skeleton through structural planes or rock mass defects,causing complex physical reactions,chemical reactions,and hydraulics effects to induce the damage of rock masses.As a result,many engineering constructions and natural geological disasters happened,seriously threatening the safety of people’s property.Therefore,it is of great theoretical and practical significance to study the mechanical properties of rock under different water contents and corresponding mechanisms of water effect in detail.In this study,Fangshan granite(FG)is selected to investigate the mechanism of water effect on rock’s static and dynamic mechanical properties without the consideration of clay minerals.To set up multiple groups of specimens with different water contents,the thermal treatment of FG is carried out to increase its porosity at first.By multiple microscopic structure detection methods,the optional heat treatment temperature is determined(1000 ℃).After the heat treatment,six groups of specimens with different water contents are prepared by water-saturated and then dried.The static compression experiment under different confining pressures,dynamic compression and dynamic tensile experiments are carried out.The regularity of mechanical properties with water saturation is studied systemically.Finally,the crack propagation model under different loading conditions is deduced based on fracture mechanics,and the water effect on the specimen under different water contents is quantified.In addition,the mechanism of water effect on the multi-crack FG is discussed to give reference for hydraulic engineering,deep underground engineering,and natural disaster prevention.The primary studies and conclusions are as follows:(1)By thin-slices analysis,SEM scanning,and CT technology,the thermally damaged process of FG is studied.The porosity,mineral compositions and contents,and microstructure(cracks)of FG specimens under the thermal-treated temperature from600 ℃ to 1200 ℃ are investigated.The heat treatment temperature of FG specimens is determined to be 1000 ℃.(2)Static compression tests under different confining pressures(0 MPa,5 MPa,10MPa)are carried out on specimens with different water saturations(0%,20%,40%,60%,80% and 100%).The result shows that the uniaxial compressive strength of the specimen increases at first(to 60%)and then decreases with the increase of water saturation.The cohesion shows a similar trend.However,the triaxial compressive strength always shows a downward trend with increasing water saturation.(3)The dynamic compression and dynamic tensile tests under different water saturations are carried out by a split Hopkinson pressure bar(SHPB)device.The result shows that the dynamic compression strength and dynamic tensile strength of the specimen under different water saturations have obvious rate dependence.In the dynamic compression experiment,the strength of dry specimens is higher than that of the fully saturated.The strength of the unsaturated specimen shows an upward trend with the rise of water saturation overall.However,in the dynamic tensile experiment,the dynamic tensile strength of the specimen with water is lower than that of the dry.Specifically,when the loading rate is lower than 200 GPa/s,the strength of the specimen decreases with the increase of water saturation.In addition,the rate dependence of the specimen in the fully saturated state is the most noticeable compared with other water saturation conditions.(4)Based on fracture mechanics,the crack propagation model of FG specimens under different loading rates is derived.The stress-strain curves under different water contents are fitted with good coincidence.Finally,the water effect on the specimen is quantified,and the weakening and strengthening mechanisms of the water effect on the rock are discussed.