冻土区浅表层土石混合体受冻融作用影响，内部冰晶反复消融重聚、孔隙结构重组诱发其强度折减，极易引起工程病害，往往是寒区致灾、诱灾的发源处。探明冻融作用下土石混合体力学特性及强度劣化机制是寒区工程灾变防控的关键科学问题之一。论文以不同含石率、冻融次数、温度下土石混合体为研究对象，开展宏细观试验、数值模拟及理论研究。利用核磁共振（NMR）对土石混合体在冻融作用下孔隙结构演化及未冻水含量进行识别；开展高低温三轴试验探明土石混合体宏观力学行为；结合数值模拟建立土石混合体三轴压缩过程变形破坏特征与宏观力学行为的对应关系；首次建立考虑冰水比例变化的冻结土石混合体细观计算模型，并揭示出冻融作用下土石混合体强度劣化机制。文章内容为寒区土石混合体灾害防治提供理论参考。
       针对冻融过程中土石混合体孔隙结构变化及冰水相变问题，采用NMR技术获取了不同冻融次数试样孔隙分布、连通性等演化特征及不同冻融次数后试样冰水相变过程融冰、持水特性。研究发现冻融后高含石率试样T2谱形态变化差异较为明显，主要体现在大、中孔孔隙分布。-15℃~-5℃区间试样内部大孔隙中的冰最先消融，而相变区间不同温度下T2曲线差异性明显，有更多小孔隙内的冰在该阶段消融。冻融循环后由于试样孔隙孔径增大及连通性的增强导致融冰速率增加。
       针对冻融环境下土石混合体宏观力学特性，通过三轴试验明确了土石混合体力学行为随冻融循环次数及温度的变化规律。结果表明，随冻融次数增加，土石混合体强度参数均呈现递减趋势，且在第1次冻融后衰减率最大。3、15、20次冻融后的试样受应力集中影响发生非对称破坏。不同负温下试样应力应变曲线的形态差异明显：-10℃以下呈脆性破坏特征，-5℃时曲线具有良好的线性关系，而-3℃时曲线出现了“二次升高”的现象。除含石率为35%的试样，其余含石率的试样的应力应变曲线在冻融后均发生更加显著的跳跃、波动，黏聚力及内摩擦角的冻融损失率均在含石率为35%时出现拐点。
       针对冻融作用下土石混合体受荷变形破坏特征，利用PFC2D及COMSOL软件获取了试样裂隙、颗粒旋转等细观结构演化过程，明确了受荷下冻结土石混合体应力分布、变形等特征。研究发现未经历冻融及10次冻融后试样最终为鼓胀变形破坏，1、6、15次冻融后试样发生单侧偏心破坏。冻融后45%、55%含石率试样破坏后形成贯通剪切带而65%含石率试样发生单侧偏心破坏。在-5℃~0℃区间应力集中分布明显增强，损伤破坏的贯通性明显。达到正温后试样内部仅有部分区域出现应力集中且整体应力值较小，损伤破坏主要产生于土石界面区。
       针对冻融作用下土石混合体强度劣化机制，基于考虑冰水比例变化的冻结土石混合体细观计算模型以及孔隙结构与强度关联变化，明确了冻融作用下土石混合体强度劣化机制。研究表明，-3℃以下试样宏观强度主要来源于颗粒间的摩擦力，随温度升高强度逐渐受颗粒间咬合力控制。土石混合体强度衰减快慢主要取决于其内部水分迁移通路的通畅性及孔隙大小、数目。低含石率试样中小孔隙数目衰减率的降低是其黏聚力和剪切强度衰减逐渐趋于平缓的原因，而高含石率试样强度的劣化主要是由于大孔孔隙形态变化引起骨架效应衰减所导致。以上研究成果不仅对于认知寒区土石混合体孕灾机制及针对性防控具有重要指导价值，而且是对土石混合体研究体系的前瞻探索与有益补充。

      Affected by freezing and thawing, the shallow soil-rock mixture in cold regions is subject to strength reduction induced by repeated melting and regrouping of internal ice crystals and pore structure reorganization, which is very easy to cause engineering diseases, and is often the source of disasters in cold regions. It is one of the key scientific problems in the prevention and control of engineering disasters in cold regions to explore the mechanical properties and strength deterioration mechanism of soil-rock mixtures under the action of freezing and thawing. This paper took the soil-rock mixture under different rock content, freeze–thaw cycle times and temperature as the research object, carried out macro and meso test, numerical simulation and theoretical research. Nuclear magnetic resonance (NMR) was used to identify the evolution of pore structure and unfrozen water content of soil-rock mixture under freeze–thaw cycle. Carried out high and low temperature triaxial test to explore the macro mechanical behavior of soil-rock mixture. Numerical simulation was used to establish the corresponding relationship between deformation and failure characteristics during triaxial compression and macro mechanical behavior of soil-rock mixture. The meso calculation model of frozen soil-rock mixture considering the change of ice water ratio was established for the first time, and the strength deterioration mechanism of soil-rock mixture under freeze–thaw was revealed. The content of this paper provides a theoretical reference for the disaster prevention of soil-rock mixture in cold regions.
      Aiming at the change of pore structure and ice water phase transition of soil-rock mixture during freezing and thawing, the evolution characteristics of pore distribution and connectivity of samples with different freeze–thaw cycle times were obtained by NMR technology. At the same time, the ice melting and water holding characteristics of the sample in the ice water phase change process after different freeze–thaw cycle times were obtained. It is found that, the morphological change difference of high rock content samples is obvious after freeze–thaw cycles, which is mainly reflected in the distribution of large and medium pores. The ice in the sample’s large pore in the range of –15℃ ~ –5℃ melts first. In the phase transition range, the T2 curve is obviously different at different temperature, more ice in the small pore melts at this stage. After freeze–thaw cycles, the ice melting rate of the sample increases due to the increase of pore size and the change of connectivity.
      Aiming at the macro mechanical characteristics of soil-rock mixture under freeze–thaw environment, through the triaxial test, the change law of mechanical behavior of soil-rock mixture with the number of freeze–thaw cycles and temperature was clarified. The results show that with the increase of freeze–thaw cycles, the strength parameters show a decreasing trend, and the loss rate is the largest after the first freeze–thaw cycle. The samples after 3, 15 and 20 times of freeze–thaw cycle are asymmetrically damaged under the influence of stress concentration. The shape of stress–strain curves of samples at different negative temperatures is obviously different: The curves below –10℃ show the characteristics of brittle failure, the curves at –5℃ have a good linear relationship, and the curves at –3℃ show the phenomenon of "secondary rise". Except for the samples with rock content of 35%, the stress–strain curves of samples with other rock content have more significant jump and fluctuation after freeze–thaw cycles. The loss rates of cohesion and internal friction angle have inflection points when the rock content is 35%.
      Aiming at the load deformation and failure characteristics of soil-rock mixture under freezing and thawing, using PFC2D and COMSOL software, the meso structure evolution processes such as crack and particle rotation of the sample were obtained, and the stress distribution and deformation characteristics of the frozen soil-rock mixture under load were determined. The results show that the samples without and 10 times freeze–thaw cycles were finally bulging deformation failure, and the samples have unilateral eccentric failure after 1, 6 and 15 times of freeze–thaw. After freeze–thaw cycles, the samples with 45% and 55% rock content broke through the shear band, while the sample with 65% rock content had unilateral eccentric failure. In the range of –5℃~0℃, the stress concentration distribution is obviously enhanced, and the transfixion of damage failure is obvious. After reaching the positive temperature, only some areas of the sample appear stress concentration and the overall stress value is small, and the damage mainly occurs in the soil-rock interface area.
      Aiming at the strength deterioration mechanism of soil-rock mixture under freezing–thawing, based on the meso calculation model of frozen soil-rock mixture considering the change of ice water ratio and the correlation between pore structure and strength, the strength degradation mechanism of soil-rock mixture under freeze-thaw was clarified. The results show that the macroscopic strength of the sample below –3℃ mainly comes from the friction between particles. With the increasing of temperature, the strength is gradually controlled by the biting force of the particle. The strength decay rate of soil-rock mixture mainly depends on the smoothness of its internal water migration path and the size and number of pores. For the samples with low rock content, the decrease of the decay rate of the number of small pores is the reason for the gradual flattening of the decay of cohesion and shear strength, and the degradation of high rock content samples is mainly due to the weakening of the skeleton effect caused by big pore morphology changes. The above research results not only have important guiding value for understanding the mechanism of soil-rock mixture disaster pregnancy and targeted prevention and control in cold regions, but also provide a forward-looking exploration and beneficial supplement for the research system of soil-rock mixture.

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