降雨对多年冻土区浅层滑坡失稳的影响研究

Study on influence of rainfall on active layer detachment failure in the permafrost region

近年来青藏高原多年冻土地区降雨量呈增大趋势,导致活动层沿多年冻土层滑脱,诱发的冻土浅层滑坡灾害严重影响区域生态环境和人类活动。冻土浅层滑坡失稳是渗流、温度和应力复杂耦合的过程,明确降雨条件下多年冻土斜坡水热力响应机制,揭示降雨诱发冻土浅层滑坡失稳的机理十分关键。基于冻土水热力耦合数值模拟方法,建立了仅施加气温变化的模型一和在气温变化基础上施加强度为9 mm·d^(-1)、持续降雨18 d的模型二,探讨了低强度、长时间降雨对多年冻土斜坡水热力演化的影响。结果表明:夏季雨水入渗对斜坡浅层温度场产生扰动,进而影响土体冻融过程,活动层以下有形成富水层的可能。雨水入渗导致融土饱和度大幅增加,水分渗流方向由竖直向下逐渐转变为顺坡方向。极限状态下斜坡位移分布在活动层,符合冻土浅层滑坡变形特征,降雨入渗数天后活动层位移有显著增大的趋势,最大位移所在位置向坡脚转移。降雨对斜坡稳定性影响显著,雨水入渗对活动层水热力产生持续影响,斜坡安全系数最小值出现明显滞后。研究结果为青藏地区冻土浅层滑坡灾害防治提供了科学指导。

Under the influence of global climate change,rainfall in the permafrost region of the Qinghai-Tibet Plateau has tended to increase in recent years.The active layer of slope in the permafrost region is melting,a lot of water accumulates at the freezing-thawing interface,and the shear strength of soil decreases rapidly,which caused active layer detachments to occur frequently.Such natural hazards seriously affect the regional ecological environment,and human activities,and restrict social and economic development.Failure of active layer detach⁃ment is a complex coupling process of seepage,temperature,and stress.Driven by climate factors such as rain⁃fall and temperature change,the hydro-thermal-mechanical evolution of the active layer is significant.Most scholars at home and abroad have researched the deformation of permafrost slopes in the process of slope failure and the variation of slope stability under rainfall conditions using model tests,numerical analysis,field monitor⁃ing,etc.Therefore,there is a lack of research on the mechanism of the hydro-thermal-mechanical response of permafrost slopes under rainfall conditions.This paper takes a typical active layer detachment disaster in Qilian County,Qinghai Province as the research object.Based on the numerical simulation method of hydro-thermalmechanical coupling numerical of frozen soil,two models to discuss the effect of low intense and longtime rain⁃fall on the hydro-thermal-mechanical evolution of permafrost slopes were constructed by COMSOL Multiphys⁃ics.Model 1 only considered the variation of temperature,model 2 applied continuous rainfall for 18 days with the intensity of 9 mm·d^(-1) under the condition of air temperature variation.By solving the partial differential equa⁃tions of multiple physical fields,the numerical simulation of the hydro-thermal-mechanical evolution in the tem⁃perature change and rainfall process of permafrost slopes is realized,and the slope stability is calculated by the strength reduction method.The results are as follows:(1)the infiltration of rainwater in summer disturbed the temperature field of the active layer,Furthermore,the freezing and thawing process of soil is affected,even pro⁃moting the melting of fine soil with high ice content below the active layer to form a water-rich layer,and the pore water pressure in the basement of the active layer accumulates,which increases the risk of slope instability.(2)Rainwater infiltration causes a large increase in the saturation of the thawed soil,which increases the bulk density of soil and causes the mechanical property degradation of soil.The direction of the water seepage gradu⁃ally changes from downward to parallel to the slope.It produces a seepage force parallel to the slope,which re⁃sults in increased deformation of the active layer.(3)The displacement of the slope is distributed in the active layer in the limit state,which accords with the deformation characteristics of active layer detachments.The evo⁃lution of soil displacement is closely related to the seepage field formed by rainwater infiltration.The displace⁃ment of the active layer tends to increase significantly after several days of rainfall infiltration,and low intensity and longtime rainfall caused the location of maximum displacement moves towards slope toe.(4)Rainfall has a significant adverse effect on slope stability,the factor of safety of the slope decreases rapidly after rainwater infil⁃tration for a while.Rainwater infiltration has a continuous influence on the thermal-hydro-mechanical evolution process of the active layer,the minimum factor of safety of the slope appeared to be several days lag behind the end of rainfall.The above results of the study provide some theoretical basis for revealing the mechanism of ac⁃tive layer detachments induced by rainfall and provide scientific guidance for the prevention of active layer de⁃tachments in the Qinghai-Tibet Plateau.