Study On Mechanical Chatacteristics And Grouting Durability Of Yungang Sandstone

Yungang Grottoes has experienced 1,500 years of vicissitudes,including the change of dynasties,the burning of wars,the natural environment,man-made damage and the geological problems caused by the modern coal mine development.A variety of disasters produce a large number of alternating loads,which makes the weathering condition inside and outside the grottoes more serious.The most significant feature of the weathered rock is that a large number of cracks are produced and the strength is reduced.Grouting is a good method to strengthen the strength of crack lifting.However,the reinforced rock mass will still be exposed to the natural environment and continue to be affected by the external environment.Therefore,based on the rescue and reinforcement project of Yungang Grottoes,this thesis explores the influence of alternating load on rock mass strength,and adopts fractured rock mass to replace the weathered Yungang sandstone with a large number of cracks and strength reduction,so as to study the influence of cracks on rock mass performance.Through investigation,it is found that the most important factors affecting weathering in natural environment are freeze-thaw damage caused by seasonal replacement and large temperature difference between day and night,and then continue to explore the influence of freeze-thaw on the long-term stability of rock mass after grouting.The main conclusions are as follows:(1)The classification and cyclic loading and unloading of sandstone are carried out,and the acoustic emission system is loaded at the same time.The mechanical parameters such as peak strength,elastic modulus and AE parameters such as frequency,b value and damage location of sandstone were studied under the condition of classification and circulation.The strength of sandstone increases by 4.1%,8.9%and 18.9% respectively under graded loading and unloading.The difference of lower limit of unloading has a great influence on the internal structural activities of sandstone,resulting in delayed failure of Kaiser effect.Under cyclic loading and unloading,the strength of rock samples decreases by 1.7%,6.2% and 10.3%respectively.The cyclic interval has a great influence on the internal structural activities of sandstone,which gradually reduces the response ratio to a fixed value.The overall b value of cyclic loading and unloading is one order of magnitude lower than that of graded loading and unloading.The "calm period" and "sudden drop period" can provide early warning signals for sandstone failure.Frequency distribution,damage location signal distribution and proportion also provide guidance for sandstone failure.(2)By replacing weathered sandstone with fissure rock mass,the influence of fissure on rock mass performance is studied,and the relationship between crack initiation stress,crack initiation Angle and crack is studied,and the mechanical mechanism of crack initiation and crack closure is revealed.The initiation stress of open crack is the minimum at 45°,and the initiation Angle decreases gradually with the increase of prefabricated dip Angle.The theoretical value is in agreement with the experimental value.The minimum peak strength of closed crack is 60°,and the coincidence of theoretical value and experimental value is high.The relationship curve between stress and ringing count of fractured rock mass was constructed,and the correlation model between prefabricated Angle and acoustic emission RA-AF parameters was established to explain the influence of fractured rock mass.(3)The freeze-thaw cycle test is carried out on the rock mass after grouting,to explore the mechanical parameters,water absorption,frost resistance and other damage changes of the grouting rock mass after freeze-thaw.The freeze-thaw + load rock mass damage model and crack + freeze-thaw + load rock mass damage model are constructed to interpret the nonlinear damage characteristics of rock mass under the coupling action of crack,freeze thaw and load.