Study On Instability Mechanism Of Rock Slopes Under Freezing And Thawing Cycle In Cold Region

Compared with the slopes at normal temperature,the slopes in the cold region are subject to day-night freeze-thaw caused by the temperature difference between day and night and the four-season freeze-thaw caused by the temperature difference due to its unique natural geographical environment and climatic conditions.The change of the physical and mechanical properties of rocks under the action of freeze-thaw cycles is the basic condition for the instability of slope.Based on the field monitoring data of a mine slope in Mozhugongka County,Tibet,this paper analyzes the influencing factors of rock slope stability in cold areas;through rock laboratory experiments,studies the evolution characteristics of the physical and mechanical characteristics of rocks under repeated freezing and thawing;combined with numerical simulation,To build a model of rock slope instability under repeated freeze-thaw action and seismic waves;Finally,the analytic hierarchy process(AHP)is used to establish a model for analyzing and evaluating the stability of rock slopes in cold areas under multi-factor effects,and to provide safety measures for cold areas The prediction of rock slope stability provides theoretical and experimental data.The main conclusions of this paper include the following:(1)Based on field monitoring data for a full year from 2016 to 2017,it is found that:Every year from November to April is a freezing period.May to November is a melting period.The annual negative temperature is 207 days,the normal temperature is 158 days,the annual minimum temperature is-18°C,and the annual maximum temperature is 26°C.The freezing-thawing cycle caused by seasonal positive and negative temperature fluctuations is one of the important factors for the instability of rock slopes in cold regions.(2)Through a series of experiments of limestone with different moisture content under different freezing-thawing cycles,it is found that damage and deterioration of limestone internal structure induced by temperature alternations and crack propagation development are one of the main reasons for rock failure.The process of freezing-thawing damage degradation of limestone is the appearance of small crack--small crack--long crack--penetrating crack--rock fracture.The experimental results of the change of wave velocity show that it begins to show an increasing trend,and then begins to show a decreasing trend when it reaches a certain critical point,which may be caused by the change of rock internal structure.The results of strength loss show that as the number of freezing-thawing increases,the peak compressive strength decreases and the freezing-thawing coefficient decreases.The experimental results of thermal conductivity measurement show that the thermal conductivity of limestone is significantly higher in saturated state than in dry state.Rock failure mechanism under the freeze-thaw cycle is mainly water,ice and rock three-phase medium,have different sensitive response to freezing and thawing,when frozen,water ice phase volume expansion than grain structure can withstand the limiting case,rock deformation is caused by frost heave force produced by local linear elastic deformation,the rock damage is irreversible.(3)Based on the data of field monitoring and laboratory experiment,the evaluation model of rock slope stability under the condition of multi-physical field coupling is established,and the evolution rule of rock slope stability under the condition of different freezing time and seismic wave is studied.The results showed that with the increase of freezing-thawing days,because the freezing speed was greater than the melting speed,the freezing depth increased continuously.On the 20 th day under the action of natural temperature difference,the freezing depth is 2m,on the 20 th day under the action of limit temperature difference,the freezing depth is 3.3m.With the increase of the freezing depth,the mechanical characteristics of the slope surface are changed,so that the safety factor decreases,and the decrease in the early stage is greater than that in the later stage.With the increase of freezing depth,the relative displacement of the slope roof is the largest under the action of seismic wave.(4)Through analytic hierarchy process(AHP),a multi-factor evaluation model for the stability of rock slope in cold region is established.The calculated results show that the outside climate change and the stratum structure are the two main factors causing the instability of the rock slope in the cold region,accounting for 0.1967 and 0.1940.In view of the instability of the slope in the cold region,the measures given are that the high rock slope can be protected flexibly and the unstable structure in the slope can be sprayed with concrete to block the freezing-thawing effect.