Research On The Damage And Fracture Failure Mechanism Of Dumping Unstable Rocks In Seasonal Frozen Region

The seasonally frozen area accounts for about half of China’s land area,and the rock mass in this area has been subjected to freezing and thawing weathering due to severe temperature changes all the year round,and its mechanical strength has deteriorated significantly,which has caused a lot of geological disasters.Therefore,it is one of the key engineering problems to study the instability and failure mechanism of dumped dangerous rocks under the combined action of freeze-thaw damage and self-weight and frost heaving force,which is of great significance to the stability evaluation of dumped dangerous rocks in seasonal frozen areas.Based on the"Catastrophic Damage Mechanism and Failure Mode of Land Transportation Infrastructure Structure"（a national key research and development project）,this thesis takes a dumping dangerous rock along S209 in Aba Qiang and Tibetan Autonomous Prefecture of Sichuan Province as an example,and studies the damage,fracture and instability failure mechanism of tilting dangerous rock in seasonal frozen area under the action of freeze-thaw cycle through indoor model test,theoretical analysis and numerical simulation.The main research contents and conclusions of this thesis are as follows:（1）Based on fuzzy theory and independent weight coefficient method,a method for evaluating the applicability of similar materials is proposed,which considers the similarity of multiple parameters at the same time and takes the comprehensive fuzzy index(E_CFI)as the evaluation standard.Through indoor orthogonal test,the influence of bone cement ratio,aggregate ratio and cementing material ratio on different parameters is analyzed,and the optimal ratio considering material weight,elastic modulus,tensile strength and compressive strength at the same time is obtained.The corresponding E_CFIis 0.82,which is significantly higher than that corresponding to the optimal ratio of a single evaluation index.（2）Based on fatigue damage principle and effective stress principle,porosityηA prediction model of residual strength of fractured rock mass considering freeze-thaw effect is established for basic parameters;The evolution trend of tensile strength and fracture toughness predicted by the model is roughly the same as the test data,and the percentage difference between the predicted value and the test value is not more than0.54%and 1.56%,respectively.Therefore,the model built in this paper is considered to have good applicability.（3）From the indoor test results,it is known that the deterioration of freeze-thaw damage is manifested in crack propagation,lap joint,penetration（also manifested in the model test as the expansion of the main structural plane）,surface peeling,local rupture,mass loss and so on.Due to the"damage accumulation effect",the crack propagation length and surface peeling area increase gradually with the accumulation of freezing and thawing times,the average mass loss rate of samples increases from0.18%in the first 10 times to 0.87%in the last 80 times of freezing and thawing,and the local fracture is more obvious.The ratio of pore volume to initial pore volume V_f（n）/V_f（0）after n times of freezing and thawing increases exponentially(V_f（n）/V_f（0）=e^0.0095n).（4）Based on MTS criterion and actual working conditions,the crack initiation angleθ₀and the joint stress intensity factor K_eunder the combined action of self-weight and frost heaving force are calculated,and the stability coefficient FS of toppling dangerous rock after different freezing and thawing times under different temperature conditions is calculated by combining the contents of Chapter 3 and the fracture mechanics method.Based on this method,the safety period of accurate dangerous rock is predicted.The reliability of the predicted safety period is verified by indoor model test and numerical simulation.The safety period calculated based on theoretical method is 239 times,the indoor model test is 253 times and the numerical simulation is 241 times,and the ratio of the maximum difference among them to the minimum safety period is less than 5.86%.Based on Griffith’s energy release rate theory,the calculation formula of the end extension length of the main control structure surface under frost heave force is derived.According to the results,the factors that affect the expansion length of the structural plane under the freeze-thaw stress coupling effect are the short axis b at the end of the structural plane,the long axis a at the end of the structural plane,the temperature T in the structural plane,and the number of freeze-thaw cycles,in order of degree of influence.（5）The evolution characteristics of temperature and strain at the end of structural plane during the freezing-thawing cycle of dangerous rock model are monitored by indoor model tests.The evolution of temperature during freezing-thawing process can be roughly divided into eight stages:rapid cooling,cooling rebound,slow cooling,stable fluctuation 1,rapid heating,warming rebound,slow heating and stable fluctuation 2,while the strain evolution at the end of main structural plane can be roughly divided into six stages:freezing contraction,rapid freezing expansion,slow freezing expansion,melting shrinkage and stabilization.The process of temperature conduction is accompanied by heat loss,which leads to the difference of temperature at different positions of dangerous rocks;The damage caused by freeze-thaw process is presented in the form of residual strain.（6）Through model test and numerical simulation,the instability and failure process of dangerous rock mass under the coupling effect of freeze-thaw and stress in natural environment is restored.The displacement of the dangerous rock mass in the free direction presents four stages:small"retraction","surge",slow growth,and stabilization,and the difference of the extension length of main control structure plane between them during freeze-thaw process is less than 6.52%.Among the three principal stresses at the end of the main structural plane,the maximum principal stress is most affected by the frost heaving force,and when the frost heaving force increases from 22.26k Pa to 545.50k Pa,the increase rate is as high as 96.24%,followed by the increase rate of the middle principal stress of 39.73%,and the minimum principal stress has no obvious change.The maximum horizontal displacement occurs at the top of the dangerous rock in the process of failure,and the horizontal displacement reaches5.941×10^-3m;when the dangerous rock is unstable.Comparing the results of calculating the stability of dangerous rocks by fracture mechanics method and limit equilibrium method,the difference between the stability coefficients calculated by the two methods is less than 0.097,and the fracture mechanics method is safer and more reliable.