基于诱发机理的降雨型滑坡预警研究--以花岗岩风化壳二元结构斜坡为例

RAINFALL-INDUCED MECHANISM BASED EARLY WARNING MODEL FOR SLOPES OF DUALISTIC LAYERS IN WEATHERED GRANITIC AREA

降雨型滑坡是我国主要的滑坡灾害类型,具有区域群集发生的特点,滑坡预警研究是防灾减灾的重要途径。传统的区域降雨型滑坡预报模型多采用统计结果建立降雨参数模型,对降雨诱发机理和斜坡失稳的力学机制考虑不足,预报可靠性和精度有限。本文以花岗岩风化壳地区某典型二元结构斜坡为原型,以实际勘查数据为基础,提取该斜坡结构特征,基于饱和-非饱和渗流理论,分析研究降雨入渗过程和斜坡失稳机制,建立典型斜坡的预警判据。1花岗岩风化壳地区典型二元结构斜坡为类土质斜坡,覆盖土层较厚,剖面上可分为两层,上层为坡积黏性土,土质松散,透水性强,下层为残积黏性土,土质相对致密,透水性较差。2采用不同降雨工况模拟分析降雨入渗过程。以50mm·d^(-1)雨强为例,降雨持时30h以内时,降雨入渗主要集中在上层的坡积黏性土,斜坡前缘优先饱和,滑带开始出现积水现象;降雨持时40~50h时,斜坡表面降水持续入渗,在坡体后缘拉裂缝处,雨水沿着裂缝快速入渗坡体形成静水压力,增加坡体重量,增大下滑力,坡脚渗透路径短,最先饱和破坏,造成斜坡失稳。3监测斜坡不同部位(坡脚、中部、后缘)的孔隙水压力情况,随降雨入渗,斜坡土体孔隙水压力持续增大,由负趋近于零到大于零,斜坡土体由非饱和状态向饱和状态过渡,坡脚最先饱和,中部持续入渗,后缘土体饱和后,裂缝扩大致使大量雨水进入,使本已大量积水的滑带变形错动,斜坡失稳。4模拟分析得到斜坡失稳的不同降雨条件:中雨雨强(10mm·d^(-1)),历时约13d;大雨雨强(25mm·d^(-1)),历时约5d;暴雨雨强(50mm·d^(-1)),历时约2.2d;特大暴雨雨强(100mm·d^(-1)),历时约1.1d。在暴雨雨强时,降雨对该类斜坡的滞后作用约为5h。最后,建立了该类斜坡的临界降雨判据(I-D曲线)。

Rainfall-induced landslides are clustery, recorded by Chinese historic documents. Study of early- warning is a significant way to prevent and reduce damages. Conventional forecasting model is based on statistics of landslides, which is short of inducement mechanism and mechanical mechanism, with limited dependability and precision of forecasting. The typical landslide in weathered granitic crust area is studied in this paper. Based on simplified geological model and saturated - un saturated seepage theory, the process of rainfall infiltration and instability mechanism is studied, then the warning criterion of typical slope is established. （ 1 ） Typical dual-structure slopes in weathered granite crust area are soil-like slopes and covering soil is thick. They are divided into two parts on profile. The upper part is collusive clay with loose soil and high permeability, while the lower part is residual clay with compactness soil and lower permeability. （ 2） Rainfall infiltration under several conditions are simulated. Taking rainfall intensity of 50mm·d^-1 for example, within 30 hours, most rain is in upper collusive clay, causing front edge of slope saturated and sliding zone hydropsy. From 40 to 50 hours, infiltration along with tension cracks around rear edge of slope is faster, and water in cracks may generate hydrostatic increasing weight of soil and sliding power. Meanwhile toe of slope saturates and breaks firstly. Then landslide occurs. （ 3 ） Porewater pressure increases continually from monitoring in three parts of slope （ front, central and rear of slope）. Namely slope soil transforms unsaturated to saturated. The toe of slope gets saturated first, central part under continual infiltration, and upper part saturation cause sliding zone deformation and sliding through hydrostatic pressure. （4） Durations needed when landslides occur under different rainfall conditions are ： 13 days with moderate rain （10mm ·d^-1） , 5 days with heavy rain （ 25mm·d^-1） , 2. 2 days with rainstorm （ 50mm·d^-1） , and 1. 1 days with extraordinary rain storm （100 mm ·d^-1） . Duration of hysteresis is about 5 hours when rainstorm. Finally, critical rainfall criterion （I-D curve） of this kind of slopes is established.