Study On Freeze-thaw Damage Mechanism Of Different Lithologic Rocks With Fractures

Under the call of “the Great Western Development Strategy”,“the One Belt and Road Initiative” and other major strategic plans,a large number of major infrastructure projects such as railways,highways,water conservancy,mines,and civil engineering construction have emerged in the western region of China.Due to the geological,hydrological and natural conditions unique to the high-cold mountainous areas in the west,these major projects to be built or completed have suffered the adverse effects of repeated freezing and thawing,and even suffered from disasters caused by freeze-thaw.There are macroscopic or microscopic fractures or joints contained in rock masses in cold areas.Therefore,on the one hand,freezing-thawing action results in water-ice phase transitions in the fractures and generates frost heaving forces,which drives the development of primary fractures;on the other hand,for the ice melts into water again and distribute in the fractures,repeated freezing-thawing cycles result in weakened rock masses,increased deformation,and evolution of the fracture network,eventually leading to direct freezing-thawing damage to the rock mass.Therefore,in this paper,by conducting a geological survey of the studied cold mountainous area,namely Zhedou Mountain,the intact rock samples in the survey area have been selected to study the response characteristics of rocks to frost heave under different temperature drop rates.Combining with the actual fracture condition of the slope(i.e.,rock mass with unloaded fractures at the bottom and saturated water in the fracture at the top),four types of rocks in the survey area were used to prefabricate this fracture pattern.Then,a series of freezing-thawing cycles tests and mechanical tests were performed for different lithologic rocks with pre-fractures,supplemented by strain testing and acoustic emission(AE)monitoring.Based on comparative analysis methods,the related deformation,AE,mechanical properties and macro-damage characteristics of different lithologic rocks with pre-fractures under freezing-thawing conditions were studied,and the selected rock samples were numerically simulated and analyzed,as well.Ultimately,the frost heaving effect and fracture mechanism of rock masses with fractures in alpine mountainous area under the condition of repeated freezing-thawing cycles are discussed.Based on the above study,the following conclusions have been drawn as follows:(1)The collapse-slip hazards in the study area are analyzed,which mainly included four kinds of deformation damage types: erosion type,slip type,dump type,and fall type.Based on comparison and analysis,it is found that The deformation induced factors and developmental evolution mechanisms of the four kinds of collapse-slip disasters are common,that is,they are all firstly formed initial fractures under the action of strong geological tectonic activities,such as internal and external geological forces,and then the upper fractures are fully stored water under the action of rainwater and surface water,which results in deterioration of rock mass damage under freezing-thawing cycles.Finally,the fractures develop gradually and eventually lead to instability failure of the slope.(2)The freezing-thawing cycle tests of dry and saturated limestones under different temperature drop rates were carried out to obtain corresponding strain curves.The response characteristics of dry and saturated limestones to different temperature drop rates were analyzed.The results show that there is no significant difference in the micro-strain of dry limestone at different temperature drop rates because the lack of participation in water.However,the faster the rate of temperature drop,the greater the rate of increase in the micro-strain of saturated limestone and the greater the resulting frost heave force as the number of freeze-thaw cycles increases.(3)Combined with frost heaving analysis and fracture mechanics theory of water ice medium in fractured rock,the freeze-thaw failure effect of fissured red sandstone and limestone is revealed,and the numerical simulation of thermal-mechanical coupling is carried out by selecting slate sample as representative.The numerical simulation results are analyzed by using the distribution process diagram of crack failure zone and the incremental distribution process diagram of volume variation.The simulation results show that the cracks of slate model begin at the top of the rock bridge and propagate along the vertical direction of the rock bridge.The simulation results are in good agreement with the results of freeze-thaw test.(4)By analyzing the stress-strain and AE characteristics of rock samples with fractures after freezing-thawing cycles under uniaxial compression tests,comparing with fractures characteristics,the following research results are obtained: The uniaxial compressive strengths of the four kinds of lithologic rock samples all decreased significantly with the increase of freezing-thawing cycle times.In particular,due to the development of lamellar joints in slates,the peak strain values of the slate samples are very different at different freezing-thawing cycle times,while the peak strain values of the other three lithologic rock samples are similar at different freezing-thawing cycle times.With the increase of freezing-thawing times,the same lithologic rock samples have relatively more acoustic emissions before the peak strength.The peak values of AE count rates appeared at post-peak stress drop points,and the corresponding times of peak values of the AE counts are advanced.In the early stage,the AE cumulative energy is less,and it reaches the peak value at the peak strengths or post-peak stress drop points,at which time the cumulative energy is suddenly released,indicating that the rock sample fractures are expanded and destroyed.