流变性是岩石重要的力学特性，为探究寒区裂隙岩体在长期外部荷载作用下的流变力学特性及破坏机理，对含不同倾角的预制裂隙砂岩开展-10℃冻结环境下的三轴力学试验，得到了裂隙倾角对岩样物理力学参数及破坏模式的影响规律。之后又开展了分级加载蠕变试验，分析了不同岩样在蠕变三阶段的变形及损伤演化规律。最后建立了裂隙砂岩的非线性黏弹塑性本构模型，并对蠕变特征参数进行拟合，得出以下结论：

（1）裂隙倾角对加载过程中砂岩应力应变曲线发展有较大影响。应力应变曲线初始段呈“上凸型”特征，是由于该阶段岩体主要由岩样孔隙内部冰体承担荷载的结果。砂岩峰值强度、弹性模量、抗剪强度参数与裂隙倾角呈线性正相关；泊松比、体积应变与裂隙倾角呈非线性负相关关系，但变化幅度不明显；应用体积应变法并结合Hook定律建立考虑裂隙砂岩强度和变形的损伤演化方程发现，随裂隙倾角增大，砂岩起裂应力水平、扩容应力水平均逐渐增大，而峰后应力应变曲线跌落状态越明显，说明应力应变曲线弹性阶段经历时间增加，而塑性阶段相对变短，砂岩脆性增强，抵抗塑性变形能力下降。另外，峰后细观扫描显示低温冻结增强了砂岩内部颗粒间的胶结度，使得不同裂隙倾角砂岩均呈剪切破坏模式，破坏面较均匀，未出现明显的次生裂纹，翼裂纹起始于预制裂隙两侧端部向试样上下端面发展；随裂隙倾角增大，不同扫描高度下裂纹表面积波动范围减小，但破裂面裂纹体积呈增大趋势。研究结果可以对含裂隙岩层的冻结法施工及为后续开展低温冻结环境下的分级加载蠕变试验提供理论参考。

（2）分析了不同岩样在蠕变三阶段的变形及损伤演化规律。不同应力等级下，岩石稳态蠕变持续时间随裂隙角度增大逐渐增加，蠕变速率则呈非线性减小，另一方面，随应力等级的升高，瞬时应变及蠕变应变量也呈增大趋势。采用等时曲线簇法及改进的稳态蠕变速率法得到了砂岩蠕变长期强度发现，随裂隙倾角增大砂岩长期强度也显著增大。对比岩石破坏模式发现，主破坏面与水平方向夹角呈先减小后增大的U型变化趋势，其中0°、15°倾角岩石表现为整体剪切贯通破坏模式，75°、90°倾角岩石表现为拉剪贯通破坏，且岩样内部无微裂纹及次生裂纹产生。

（3）根据分级加载蠕变试验结果引入含分数阶微积分的软体元件和改进的非线性黏缸，建立非线性黏弹塑性本构模型，同时利用1stOpt软件进行参数识别和验证发现，所提出的蠕变本构模型可以较为准确地描述低温冻结环境下裂隙砂岩的分级加载蠕变曲线，尤其是在不稳定蠕变阶段。同时又分析了加载应力等级和裂隙倾角对蠕变模型参数的影响，最后对蠕变特征参数进行拟合，验证了蠕变模型的准确性和合理性。

Rheology is an important mechanical property of rock. In order to explore the rheological mechanical property and failure mechanism of fractured rock mass in cold region under long-term external load, triaxial mechanical tests were carried out on saturated sandstone with different fracture dip angles in freezing environment at -10℃, and the influence law of fracture geometry characteristics on physical and mechanical parameters and failure modes of rock samples was obtained. Then, triaxial graded loading creep test was carried out, and the deformation and damage evolution law of different rock samples in three stages of creep was analyzed. Finally, the nonlinear viscoelastic-plastic constitutive model of fractured sandstone is established, and the creep characteristic parameters are fitted, and the following conclusions are drawn:

（1）The fracture dip angle has a great influence on the development of stress-strain curves of sandstone during loading. The initial segment of the stress-strain curve is "convex", which is due to the fact that the rock mass is mainly loaded by ice bodies in pores and fissures. The peak strength, elastic modulus and shear strength parameters of sandstone are linearly positively correlated with fracture dip angle, while Poisson's ratio and volume strain are negatively correlated with fracture dip angle, but the variation range is not obvious. The damage evolution equation considering the strength and deformation of fractured sandstone is established by volume strain method combined with Hook's law. It is found that with the increase of fracture dip angle, the crack initiation and dilatation stresses of the sandstone increase gradually, while the drop state of post-peak stress-strain curve is more obvious, indicating that the elastic stage of stress-strain curve becomes longer, the plastic stage becomes shorter, the brittleness of sandstone increases, and the ability to resist plastic deformation decreases. In addition, the post-peak mesoscopic scan showed that saturated freezing enhanced the degree of cementation between the particles in the sandstone, making the sandstones with different fracture dip angles present a shear failure mode, and the failure surface was more uniform. With the increase of fracture dip angle, the fluctuation range of crack surface area decreases at different scanning heights, but the crack volume on fracture surface increases. The research results can provide theoretical reference for the freezing construction of fractured rock strata and for the follow-up multi-stage loading creep test under low temperature freezing environment.

（2）The deformation and damage evolution of different rock samples in the three stages of creep are analyzed. Under different stress levels, the steady creep duration of rock increases gradually with the increase of crack angle, while the creep rate decreases nonlinearly. On the other hand, with the increase of stress grade, the instantaneous deformation and creep deformation also increase. The long-term creep strength of sandstone is obtained by isochronal curve method and steady-state creep rate method. It is found that the long-term strength of sandstone increases significantly with the increase of fracture inclination. Compared with the rock failure mode, it is found that the angle between the main failure surface and the horizontal direction first decreases and then increases. Among them, the rock with 0° and 15° dip angle shows the overall shear through failure mode, and the rock with 75° and 90° dip angle shows tension-shear through failure, and there are no micro-cracks and secondary cracks in the rock sample.

（3）according to the creep test results of graded loading, the software element with fractional calculus and the improved nonlinear viscous cylinder are introduced to establish the nonlinear viscoelastic-plastic constitutive model. At the same time, the parameters are identified and verified by 1stOpt software. It is found that the proposed creep constitutive model can accurately describe the graded loading creep curve of fractured sandstone under low temperature freezing environment, especially in the unstable creep stage. At the same time, the effects of loading stress level and crack inclination angle on the parameters of the creep model are analyzed. finally, the creep characteristic parameters are fitted to verify the accuracy and rationality of the creep model.

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