Study On The Damage Characteristics Of Granite In Southeast Branch Tunnel Of Seasonal Frozen Area

The area of seasonal frozen area accounts for more than half of China’s land area.With the gradual progress of China’s infrastructure construction,the tunnel projects in seasonal frozen area have been set up and put into operation.The construction of tunnel engineering makes the surrounding rock inside the mountain contact with the external atmosphere,and the surrounding rock and lining are subjected to freeze-thaw cycles,resulting in the internal damage of the surrounding rock and lining.Some tunnel projects constructed in developed countries have been unable to be used normally due to freezing injury,and similar situations have occurred in China.Therefore,the research on the mechanism and evolution law of freeze-thaw damage of tunnel surrounding rock in seasonal frozen area is very important to the design and operation of tunnel construction engineering.Based on Southeast Branch Tunnel project in seasonal frozen area,this paper carries out the following research work according to the fracture fatigue theory 、 fluid mechanics、meso damage mechanics theory and fractal theory as well as the means of the combination of physical and mechanical experiments of the surrounding rock granite of Southeast Branch Tunnel and digital image processing methods.(1)The freeze-thaw cycle test of granite samples from Southeast Branch Tunnel in seasonal frozen area was carried out.The physical parameters include permeability 、porosity 、 P-wave velocity and thermal conductivity of rock samples with different freeze-thaw cycles are obtained by experiments.The mechanical characteristic parameters of rock samples with different freeze-thaw cycles are obtained by uniaxial compression tests.The results show that the physical and mechanical properties of granite deteriorate with the increase of freeze-thaw cycles;(2)The image processing algorithm of matlab software is used to denoise、 sharpen and binarize the enlarged observation photos of rock samples with different freeze-thaw cycles obtained by SEM.Through comparison,the optimal image processing method for SEM image is found.Combined with the region growing algorithm of matlab and the counting processing function of Image Pro Plus,the essence that the development of internal fracture structure of mechanical damage of granite under freeze-thaw cycles is revealed quantitatively according to the theory of solid mechanics,it is concluded that fatigue damage is the essence of freeze-thaw cycle damage to granite;(3)A model for calculating rock permeability parameters based on fractal theory is proposed.The image pro plus counting function is used to process the enlarged observation binary photos of rock samples with different freeze-thaw cycles.The parameters of pore structure are obtained and the permeability and permeability coefficient are calculated by the model,and the results are compared with the indoor permeability measurement test results;on the basis of the above,the interaction between seepage and damage of freeze-thaw rock is analyzed.(4)The paper deduced the fatigue damage model of rock under freeze-thaw cycle.The physical parameters of freeze-thaw granite in the Southeast Branch Tunnel are substituted into the model to obtain the freeze-thaw fatigue damage model of this tunnel granite.