<p>寒区冻融作用使岩石细观结构改变，导致宏观力学性能劣化，严重威胁岩体工程的安全稳定。因而，研究冻融条件下岩石物理力学特性和破坏机理，对寒区工程建设及安全服役具有重要的理论及工程意义。本文以寒区岩体工程为背景，采用室内试验、数值模拟和理论分析相结合的方法，系统的研究冻融作用下干燥和饱和红砂岩宏细观特性及损伤演化规律。主要研究成果如下：</p> <p>（1）开展红砂岩冻融循环试验，研究对比质量、体积和纵波波速等宏观物理参量随冻融循环次数的变化规律。结果表明，随冻融循环次数的增加，干燥红砂岩质量和体积加速减小，其变化率均呈现下凹型上升趋势；饱和红砂岩质量和体积表现出先增加后减少的规律；干燥和饱和红砂岩纵波波速持续减小，其变化率均逐渐增大。在相同冻融次数下，饱和红砂岩的3种宏观物理参量在降低过程中的速率明显大于干燥红砂岩，表明孔隙水的存在导致红砂岩受冻融作用影响较大，内部结构劣化加剧。</p> <p>（2）对冻融红砂岩进行核磁共振测试，获得T₂谱分布曲线和谱面积，研究细观结构特征随冻融循环次数的变化规律。结果表明，冻融作用使红砂岩孔隙结构重新分布，束缚水减少，孔隙数量增加。冻融初期，干燥红砂岩主要表现为孔隙之间的扩展，大孔隙生成，饱和红砂岩以小孔隙产生和扩展为主。随冻融循环次数的增加，干燥红砂岩内部各类孔隙增加，而大孔隙增加速率相对缓慢；饱和红砂岩小孔隙产生速度减缓，大孔隙产生和发育趋势得到发展。干燥和饱和红砂岩分形维数均随着冻融循环次数的增加而增加，结合孔隙数量的变化特征可知，饱和红砂岩受冻融作用影响较大，内部孔隙发育更快，孔隙结构更为复杂。</p> <p>（3）对冻融红砂岩开展力学特性试验，分析宏观力学性能与冻融循环次数、围压之间的关系，定量描述细观结构变化与宏观力学响应的关联性。结果表明，冻融循环作用引发内部细观结构发育，宏观上表现为岩样承载能力降低，变形增加；围压作用抑制了内部损伤且限制了侧向变形，宏观上表现为红砂岩抵抗破坏和变形的能力增强。基于PFC^2D模拟结果表明，采用细观力学参数变化等效替代冻融效应可有效模拟红砂岩变形破坏过程；随冻融循环次数的增加，裂纹演化速率降低，表明红砂岩压缩性增强，脆性特征减弱。综合对比宏观力学参量与细观裂纹的变化规律可知，相同冻融循环次数和围压下，干燥红砂岩展现出裂纹扩展速率更快，抗压强度较高，变形减小的特点。</p> <p>（4）考虑初始损伤、冻融及荷载等因素的影响，将岩石微元总面积抽象为未损伤、初始损伤、冻融损伤、受荷损伤和耦合损伤5部分。根据有效承载面积折减对各损伤的影响，确定总损伤变量；基于细观结构的扩展特性，引入孔隙度和分形维数，确定初始及冻融损伤变量；考虑已有损伤对未损伤部分持续转化为损伤的动态演化过程的影响，确定受荷损伤变量。假定损伤部分不承受力，建立了单向应力状态下冻融岩石损伤本构模型，通过试验结果验证了模型的合理性，理论表征了冻融作用下红砂岩受荷损伤特性及宏观力学行为。结果表明，该模型既可反映冻融作用下岩石峰前的应力-应变行为，又可较好的描述峰后脆性破坏特征。</p> <p>（5）通过宏观特征参量的变化定量表征围压对损伤的抑制作用，对细观损伤变量进行修正，确定岩石的有效初始、冻融损伤变量。假定损伤部分承受残余应力，未损伤部分承担有效应力，通过总损伤效应，建立了三向应力状态下冻融岩石损伤本构模型，通过试验结果验证了模型的合理性。结果表明，本构模型可较好的描述岩石变形破坏全过程，反映了冻融循环和围压对岩石力学特性的影响；围压不变时，随着冻融次数的增加，红砂岩损伤加剧，表现为抗压强度降低，塑性增强；冻融次数一定时，随着围压的增加，损伤抑制越发显著，表现为刚度和强度等宏观特征参量增大，塑性特性渐次增强。通过确定力学参数与冻融循环次数、围压之间的关系，减少了本构模型确定过程中所需的力学参数数据，使得模型适用性更广。</p>

<p>The freeze-thaw action in cold region changes the mesoscopic&nbsp;structure of rock, which leads to the deterioration of macroscopic mechanical properties and seriously threatens the safety and stability of rock engineering. Therefore, it is of great theoretical and engineering significance to study the physical and mechanical properties and failure mechanism of rock under freeze-thaw conditions for engineering construction and safe service in cold regions. Based on the background of rock mass engineering in cold region, this paper systematically studies the macroscopic and mesoscopic characteristics and damage evolution law of dry and saturated red sandstone under freeze-thaw action by means of laboratory test, numerical simulation and theoretical analysis.&nbsp;The main research results are as follows:</p> <p>(1)&nbsp;The freeze-thaw cycle test of red sandstone was&nbsp;carried out, and the variation of mass, volume and longitudinal wave velocity with the number of freeze-thaw cycles was studied and compared. The results show that with the increase of freeze-thaw cycles, the mass and volume of dry red sandstone decrease rapidly, and the change rate shows a concave upward trend. The mass and volume of saturated red sandstone increase first and then decrease. The longitudinal wave velocity of dry and saturated red sandstone continues to decrease, and its change rate gradually increases. Under the same number of freeze-thaw cycles, the rate of three macroscopic&nbsp;physical parameters of saturated red sandstone in the reduction process is significantly greater than that of dry red sandstone, indicating that the presence of pore water causes red sandstone to be greatly affected by freeze-thaw action,&nbsp;and the deterioration of internal structure is aggravated.</p> <p>(2) The nuclear magnetic resonance test of red sandstone after freeze-thaw cycles was carried out, and the T₂&nbsp;spectrum distribution curve and spectrum area were obtained. The variation of mesoscopic&nbsp;structure characteristics with the number of freeze-thaw cycles was studied.&nbsp;The results show that the freeze-thaw action makes the pore structure of red sandstone&nbsp;redistribute, resulting in a decrease in bound water and an increase in the number of pores.&nbsp;At the initial stage of freeze-thaw, the dry red sandstone mainly shows&nbsp;the expansion between pores and the generation of large pores, and&nbsp;the saturated red sandstone mainly shows&nbsp;the generation and expansion of small pores. With the increase of freeze-thaw cycles, all kinds of pores in dry red sandstone increases, while the increase rate of large pores is relatively slow. The generation rate of small pores in saturated red sandstone slows down, and&nbsp;the generation and development trend of large&nbsp;pores is developed.&nbsp;The fractal dimension of dry and saturated red sandstone increases with the increase of freeze-thaw cycles.&nbsp;Combined with the variation characteristics of the number of pores, the saturated red sandstone is greatly affected by freeze-thaw&nbsp;action, the internal pores develop faster and the pore structure is more complex.</p> <p>(3) The mechanical properties test of red sandstone after freeze-thaw cycles was carried out. The relationship between macro mechanical properties and freeze-thaw cycles and confining pressure was analyzed, and the correlation between mesoscopic&nbsp;structure change and macro mechanical response was quantitatively described. The results show that the freeze-thaw cycles cause the development of internal mesoscopic&nbsp;structure, which is shown in the macroscopic&nbsp;view as the decrease of the bearing capacity and the increase of deformation&nbsp;of red sandstone. The confining pressure inhibits the internal damage and restricts the lateral deformation, which is shown in the macroscopic&nbsp;view as&nbsp;the red sandstone resistances to damage and deformation ability is enhanced. Based on PFC^2D&nbsp;simulation results, it is shown that the process of deformation and failure of red sandstone can be effectively simulated by using the equivalent substitution of mesoscopic mechanical parameters for freeze-thaw effect. With the increase of freeze-thaw cycles, the crack evolution rate decreases, indicating that the compressibility of red sandstone is enhanced and the brittleness is weakened. According to the comprehensive comparison of the variation of macroscopic mechanical parameters and mesoscopic&nbsp;cracks, dry red sandstone shows the characteristics of faster crack propagation rate, higher compressive strength and lower deformation.</p> <p>(4) Considering the influence of initial damage, freeze-thaw and load factors, the total area of rock micro elements is abstracted into five parts: non-damage, initial damage, freeze-thaw damage, load damage and coupled damage. According to the influence of effective bearing area reduction on each damage, the total damage variable is determined. Based on the expansion characteristics of mesoscopic structure, porosity and fractal dimension are introduced to determine the initial and freeze-thaw damage variables. Considering the influence of the existing damage on the continuous transformation of non-damage part into damage dynamic evolution process, the load damage variable is determined. Assuming that the damage part does not bear the force, a damage constitutive model of freeze-thaw rock under unidirectional stress state is established. The rationality of the model is verified by test results, and the damage characteristics and macro mechanical behavior of red sandstone under freeze-thaw and load are theoretically characterized. The results show that the model can not only reflect the pre-peak stress-strain behavior of rock under freeze-thaw action, but also describe the post-peak brittle failure characteristics.</p> <p>(5) The inhibition effect of confining pressure on damage is quantitatively characterized by the change of macroscopic&nbsp;characteristic parameters, and the effective initial and freeze-thaw damage variables of rock are determined by modifying the mesoscopic damage variables. Assuming that damage parts bear residual stress and non-damage part bears effective stress, a damage constitutive model of freeze-thaw rock under triaxial stress state is established through the total damage effect. The rationality of the model is verified by test results. The results show that the model can better describe the whole process of rock deformation and failure, and reflect the influence of freeze-thaw cycle and confining pressure on the mechanical properties of rock. When the confining pressure is constant, with the increase of freeze-thaw cycles, the damage of red sandstone is intensified, which shows that the compressive strength decreases and the plasticity increases. When the freeze-thaw cycle is constant, with the increase of confining pressure, the damage inhibition of red sandstone is more significant, which is manifested by the increase of stiffness and strength, and the plastic characteristics are gradually enhanced.&nbsp;the expressions between&nbsp;the number of freeze-thaw cycles, confining pressure and mechanical parameters are determined, the mechanical parameter data required in the process of determining the constitutive model is reduced, making the model more applicable.</p>