基于冻融交界面直剪试验的冻土斜坡失稳过程研究

Study on the instability process of slopes in permafrost regions by direct shear test of freezing-thawing interface

为了探讨多年冻土区自然斜坡失稳机制,开展了不同含水率黏土、粉土、砂土的土-冰交界面直接剪切试验和相应融土的直接剪切试验。结果表明,砂土和砂土-冰冻融交界面剪切应力-变形特性主要表现为弹性变形,且剪应力存在明显峰值;粉土、黏土及相应的冻融交界面在很小的变形范围内表现为塑性变形,且剪应力无峰值。水分对砂土活动层抗剪强度影响较弱,表现为水分增高,内摩擦角小幅降低。水分对粉黏土活动层抗剪强度影响剧烈,表现为水分增高,粉黏土黏聚力急剧减小。研究发现,冻土区斜坡失稳更易发生于细颗粒粉黏土中。相对于粉土,粉土-冰冻融交界面抵抗剪切变形的能力更强,粉土斜坡潜在滑动面更易发育在冻融交界面上层附近;相对于黏土,黏土-冰冻融交界面抵抗剪切变形的能力更弱,黏土斜坡更易在冻融交界面处发生滑动。同时,细粒土斜坡极易在达到最大融化深度前提前失稳,斜坡坡度越高,失稳时间越提前。融化期活动层水分增多导致潜在滑动面黏聚力降低是细粒土冻土斜坡失稳的最主要原因,孔隙水压对冻土斜坡具有一定影响,在稳定性评价时要考虑活动层水位的影响。

To study the effect of mechanical property of freezing-thawing interface on slope stability, we carried out a series of direct shear tests of soils and ice-soil interfaces for saturated or nearly saturated gravel soil, silt, and clay under different normal stresses. Results show that the shear stress-deformation behaviors of gravel soil and corresponding thawing-freezing interface are all elastic deformation with clear peak shear stress. Silt, clay and corresponding thawing-freezing interfaces have plastic deformation within a small range, and there is no peak shear stress. Moisture content has little effect on shear strength of gravel soil in active layer, with little decreasing of the friction angles of gravel soil and ice-gravel soil interface with the increasing of moisture content. But for silt clay and clay soil, the effect of moisture on strength shows great decreasing of the cohesive force with the increasing of moisture content. We find that slope instability occurred more likely in the fine particle soil slope. Compared to silt soil, the corresponding thawing-freezing interface has a stronger resisting shear deformation ability, and the sliding slope will be in thawing soil layer above the interface, but the opposite the case for the clay soil. At the same time, the fine-grained soil slope tends to slide before reaching its maximum thawing depth. The higher the slope gradient, the earlier the time of instability. The main reason of slope failures in permafrost regions contributes to the lower cohesive force of sliding surface resulted from the higher moisture contents in active layers and the pore water pressure can affect the slope stability, and the influence of depth of water layer need to be taken into account.