富冰碎屑流启动敏感因素及运移特性研究

Permafrost is sensitive to clime changing. Global climate warming in recent years results in a large number of slope failures in glaciated high mountain area, among which the debris flow is the common and hazardous one. The debris flow process in permafrost region is still poorly understood up to now. The basic structural, physical and mechanical parameters of slope deposits in periglacial region are studied in this project, and the thermal regime and permafrost degradation of these deposits are calculated using the climate response model. Physical scale models are established in laboratory, and the climate and rainfall response experiments of slope deposits are carried out, the initiation and slide processes of the debris flows are recorded and analyzed. The samples with different ice contents are prepared and the dynamic characteristics of the debris flows is studied. The dynamic frictional characteristics of the debris flows are obtained during the experiments. A real two-phase mechanical model for ice-rich debris flows is presented based on the experimental results and the mechanical model of ice-free debris flow. The proposed model can consider the internal mass and momentum exchanges between the phases and the dynamic strength weakening of the material of debris flow. The stability of typical debris flow origination deposit and the dynamic characteristics of debris flow are simulated using the proposed mechanical model. The results will throw light on the prevention and mitigation of debris flow hazard in permafrost region.

多年冻土对气候升温非常敏感，全球气候变暖导致寒区碎屑流灾害频发。然而到目前为止，对于寒区碎屑流的启动和运移特性仍然缺乏足够的了解。本项目从青藏高原冰缘区碎屑流源堆积体入手，获得堆积体的基本特征参数，运用数值模型预测其在气候升温条件下的热状态变化；建立室内尺寸模型，模拟斜坡堆积体在气候升温和地表水入渗条件下的破坏过程，评价碎屑流对环境条件的敏感性；制备不同含冰量碎屑流试样，模拟碎屑流的运移过程，建立碎屑流的摩擦系数与运移过程动态物理力学参数的关系；在实验结果的基础上，对现有碎屑流两相耦合力学模型进行扩充，建立考虑地下冰相变和碎屑流强度随过程弱化的富冰碎屑流两相耦合力学模型。对典型富冰碎屑流源区堆积体失稳和运移过程进行预测和模拟，为多年冻土区碎屑流灾害的预防和治理提供参考和建议。

以青藏高原昆仑山口附近多年冻土区土石堆积体为研究对象，进行了详细的现场调查，获得了寒区堆积体的组成成分，结构特征，温度和含冰（水）量等相关参数。调查显示寒区土石堆积体具有独特的三层结构：块碎石层，土石混合层和富冰层；建立了寒区堆积体边坡物理试验模型，模拟气候变化条件下边坡内部的含水（冰）量变化和冻融变形情况，通过数值模型试验来预测边坡的长期稳定性。结果显示外界温度变化引起边坡内部水分重分布，水分在上限以上聚集，为碎屑流灾害的发生提供了便利；通过室内试验和数值模拟相结合的手段，对外部降雨入渗条件下，碎屑堆积体边坡的含水量和变形情况进行了研究，结果显示随着降雨次数的增加，表面细小颗粒随雨水逐渐流失，形成表层松散碎石结构。降雨使得边坡上限以上含水量剧增，边坡冻融变形增大，在上限附近形成连续贯通的塑性区，极有可能引发碎屑流灾害的发生；通过室内力学试验，研究了土石混合体在不同含水量、不同温度条件下的强度和力学参数变化情况。结果显示，土石混合体在冻结状态下强度较大，但在完全融化以后，强度大大下降。在负温情况下，土石混合体的强度随着温度的降低迅速增加，但当温度低于-5℃时，强度曲线趋于平缓。土石混合体的强度在负温条件随着含水量的增大先增加，当含水量超过临界值时，其强度又会减少，本项目土石混合体的临界含水量处于10%-12之间；通过物理模型试验，研究了含冰碎屑流的运移特性，结果显示随着碎屑流的运移，其运移状态逐渐由颗粒滚动状态向整体的塑性流动状态转变，其粘滞性增大而摩擦系数减小；建立了富冰碎屑流运移数值模型，对研究区的碎屑流源区堆积体边坡进行模拟，分析了气候升温条件下，富冰碎屑堆积体的失稳和碎屑流启动、运移过程。结果显示边坡在冻结时由于具有较高强度而不会发生破坏。在气候升温条件下，融化的土石混合体在重力的作用下沿坡面向下运移，形成碎屑流灾害。碎屑流经历加速和减速稳定两个过程，最后在坡脚平缓处堆积。