青藏高原丘陵山地多年冻土活动层滑脱形成机理研究

The permafrost on the Qinghai-Tibet Plateau has been degrading continuously under the combined influences of persistent climatic warming and steadily increasing human activities, and showing as the increasing of mean annual ground temperatures and active-layer thicknesses. Permarost gradation and some other factors would induce the occurrence of active-layer detanchment in hilly slope regions where massive ground-ice exists. Its development would not only result in the permaforst environment deterioration in the active-layer detachment areas and nearby areas, but also seriously affect the safety and stability of the neighbouring infrastructures and the future engineering planning. At present, active-layer detachment failures are wildely spread in permafrost regions of the Qinghai-Tibet Plateau, but its forming mechanism and factors has not yet been completely ascertained. Aiming at its forming mechanism discussion and engineering requirements, the active-layer detachment failures developed in the hill and mountain areas around the Beiluhe basin are selected to analyze its distribution characteristics and development conditions of topography, geology, hydrology and permafrost. In adition, based on ground temperature and moisture monitoring, field tests at a typical active-layer detachment site, and numerical simulation, the forming process and mechanism of active-layer detachment will be clarified. This study will have an important significance for engineering planning and disaster precaution.

受气候变暖和人类活动的影响，青藏高原多年冻土正处于持续的退化状态，突出的体现在年平均地温升高和活动层厚度增加。在冻土厚层地下冰发育的丘陵斜坡地带，多年冻土退化及其他因素的共同作用会诱发冻土活动层滑脱，其快速发育一方面会导致滑脱区及周边冻土环境恶化，另一方面会严重影响滑脱区临近工程的安全稳定及未来工程规划。目前，活动层滑脱现象在青藏高原多年冻土区普遍发育，但其形成机理及诱发因素尚未彻底查明。针对这一机理探讨和工程防灾需求，本申请选取青藏高原北麓河盆地周边丘陵山地区域发育的冻土活动层滑脱为研究对象，通过野外调查和遥感解译，分析活动层滑脱的分布特征及其发育的地形、地质、水文和冻土条件；基于典型活动层滑脱区的水热监测、现场试验以及数值模拟，揭示活动层滑脱的形成过程和机理。研究成果对冻土斜坡区工程规划和防灾减灾具有指导意义。

本项目选取青藏高原北麓河地区发育的冻土活动层滑脱为研究对象，首先基于高分辨率遥感解译及现场调查分析了北麓河地区冻土活动层滑脱的分布特征及演化过程，其次通过现场地温、变形、孔隙水压力及水分监测明确了活动层滑脱的发育条件，最后结合冰-土界面现场直剪试验以及斜坡稳定性分析，揭示了活动层滑脱的形成过程和发育机理。本项目得出的重要结论包括：(1)北麓河地区的冻土活动层滑脱主要分布在坡度为4-6°的阴坡区域，并且这些活动层滑脱在过去10年呈显著增加的趋势，其数量和面积分别增加了253%和617%；(2)冻土活动层滑脱的发生与地质条件、冻土条件、气候条件及人类活动具有一定的相关性，但夏季的异常高温和强降水是引起北麓河地区冻土活动层滑脱发生的主要因素；(3)现场监测表明，冻土活动层滑脱的后期变形主要发生在滑脱发生后的3-4年之内，并且活动层滑脱的发生明显改变了活脱发生区域及其附近土体的地温状况和水分条件；(4)数值模拟结果显示，随着降雨强度和降雨持续时间的增加，坡体内的孔隙水压力逐渐升高，斜坡的安全系数逐渐减小，地震发生过程中坡体内各节点的位移、速度和加速度明显升高，并且当水位高度大于1.42m或地震加速度大于0.03g时，斜坡将处于失稳状态。本项目的研究成果对多年冻土斜坡区工程规划和防灾减灾具有重要的指导意义。