高寒山地分布着广泛的天然冻结层理岩石，其断裂力学特性受到冻结作用的显著影响。随着高寒山区工程建设的增多，冻结层理岩石的的I-II复合型断裂力学特性的研究对高寒区工程建设具有重要的指导意义。然而，目前对于高寒区冻结层理岩石的I-II复合型断裂特性研究较少，冻结作用对层理岩石断裂特性的影响机制尚不清楚。

 本文以不同裂缝倾角的层理砂岩为研究对象，在-20℃条件下开展不同初始饱和度岩石三点弯曲试验，测定其I-II复合型断裂韧度，采用超声波速测试测定其各向异性弹性常数，利用有限元模型计算无量纲物理因子Y_I和Y_II，总结了初始饱和度与裂缝倾角因素对断裂韧度的影响规律；借助声发射（AE）与数字图像相关法（DIC）两种监测手段，分析了断裂破坏过程中声发射特征与裂缝尖端水平应变场受初始饱和度与裂缝倾角的影响规律；揭示了层理与含水量对冻结岩石I-II复合型断裂韧度的影响机制。主要得到以下几方面结论：

（1）冻结层理岩石有效断裂韧度由无量纲物理因子Y_I和Y_II和峰值荷载决定，前者主要受裂缝倾角控制，后者受到裂缝倾角与初始饱和度的共同控制。初始饱和度影响下断裂韧度以20%和60%为界呈现“快速上升-缓慢上升-缓慢下降”的三阶段变化，裂缝倾角影响下断裂韧度以45°为界呈现先减小后增大的两阶段变化。

（2）冻结层理岩石I-II复合型断裂破坏特征中的声发射特征主要受初始饱和度影响，随着初始饱和度的增加，振铃计数峰值降低，峰值出现前的振铃计数连续增长现象逐渐消失；岩石加载过程中声发射事件规模减小，大尺度的破裂事件增加；裂纹扩展过程中的张拉裂纹占比逐渐降低，剪切裂纹占比逐渐上升。表面应变场特征则同时受初始饱和裂缝倾角影响，随着两因素的增加，裂缝尖端断裂过程区越来越偏离预制裂缝平面，并且其应变值大小逐渐降低。

（3）冻结岩石内部含有孔隙冰，孔隙冰在不同外荷载作用下对冻结岩石力学性质产生三种强化效应：1）充填作用；2）支撑作用；3）胶结作用，以及由于自身性质引发冻胀损伤产生弱化效应。含水量的增加令岩石冻结时内部的孔隙冰含量上升，并且低含水量时主要由孔隙冰的强化效应主导，断裂韧度增加；高含水量下由冻胀损伤作用主导，断裂韧度降低。层理方向与预制裂缝方向夹角的改变使孔隙冰分别受到纯张拉荷载，拉剪荷载和压剪荷载，因此不同荷载形式下占据主导的孔隙冰强化效应不同，进而改变了冻结岩石断裂韧度。

There are a wide range of natural frozen bedding rocks in the alpine mountains, and their fracture mechanical properties are significantly affected by freezing. With the increase of engineering construction in alpine mountainous area, the research of I-II mixed mode fracture mechanics characteristics of frozen bedding rock has important guiding significance for engineering construction in alpine mountainous area. However, at present, there are few studies on I-II mixed-mode fracture characteristics of frozen stratified rocks in high cold region, and the mechanism of freezing effect on fracture characteristics of bedding rocks is still unclear.

In this paper, bedding sandstones with different crack inclination angles are taken as the research object, and three-point bending tests are carried out under the condition of -20℃ for rocks with different initial saturation to determine their I-II mixed-mode fracture toughness. Ultrasonic wave velocity test is used to determine their anisotropic elastic constants, and dimensionless physical factors Y_I and Y_II are calculated by finite element model. The effects of initial saturation and crack inclination on fracture toughness are summarized. By means of acoustic emission (AE) and digital image correlation (DIC) monitoring methods, the effects of acoustic emission characteristics and horizontal strain field of fracture tip on initial saturation and crack inclination during fracture failure are analyzed. The influence mechanism of bedding and water content on I-II fracture toughness of frozen rock is revealed. The main conclusions are as follows:

(1) The effective fracture toughness of frozen bedding rocks is determined by the dimensionless physical factors Y_I and Y_II and the peak load, the former is mainly controlled by the crack inclination angle, and the latter is controlled by the crack inclination angle and the initial saturation degree. Under the influence of initial saturation degree, the fracture toughness shows a three-stage change of "rapid increase-slow increase-slow decrease" with the boundaries of 20% and 60%, and under the influence of crack inclination angle, the fracture toughness shows a two-stage change of decreasing and then increasing with the boundaries of 45°.

(2) The acoustic emission characteristics of I-II mixed-mode fracture damage in frozen bedding rocks are mainly affected by the initial saturation degree, with the increase of initial saturation degree, the peak value of ringing counts decreases, and the phenomenon of continuous growth of ringing counts before the peak value disappears; the scale of the acoustic emission events decreases in the process of rock loading, and the large-scale rupture events increase; the percentage of tensile cracks in the process of crack expansion decreases, and the percentage of shear cracks increases gradually. The surface strain field characteristics are also affected by the initial saturation degree and crack inclination angle, and with the increase of the two factors, the fracture process zone at the crack tip deviates more and more from the prefabricated crack plane, and the magnitude of its strain value decreases gradually.

(3) Frozen rock contains pore ice, which has three kinds of strengthening effects on the mechanical properties of frozen rock under different external loads: 1) filling, 2) supporting, 3) cementing, and weakening effects due to the freezing damage induced by its own properties. The increase in water content increases the pore ice content in the frozen rock, and the fracture toughness increases at low water content, which is dominated by the strengthening effect of pore ice, and decreases at high water content, which is dominated by the effect of frost heave damage. The change of the angle between the bedding direction and the prefabricated crack direction subjected the pore ice to pure tensile, tensile-shear and compressive-shear loads, respectively, so that the dominant pore-ice strengthening effect differed among the different loading forms, which in turn altered the fracture toughness of the frozen rock.