矿物成分、特征地球化学组分对水在滑带形成中作用的指示意义:以三峡库区大型滑坡为例

Mineral compositions and elements concentrations as indicators for the role of groundwater in the development of landslide slip zones:a case study of large-scale landslides in the Three Gorges area in China

弄清滑坡滑带抗剪强度降低的根本原因是滑坡形成机理研究和滑坡活动趋势预测的主要内容之一。水是滑带形成过程中导致其抗剪强度衰减的最活跃因素之一。三峡库区两个大型滑坡实例分析显示,滑带及其周围岩土矿物成分、地球化学成分的变化特征指示滑带形成与水-岩(土)物理、化学作用的方式及作用程度,从而证实矿物学、地球化学研究方法和理论是揭示滑带抗剪强度降低内在机理的有效途径之一。三峡库区黄土坡滑坡临江I#崩滑体和泄滩滑坡滑带土及其周围岩土矿物学、主量化学元素的含量变化特征指示:前者滑带形成过程中,滑带部位地下水因大气降水补给、地下水的氧化作用活跃,导致滑带土抗剪强度减低,其主要原因是其中泥灰岩碎屑的水解泥化作用、方解石溶解作用和伊利石向伊-蒙混层矿物的转化作用;后者滑带形成过程中,滑带部位地下水与外界水力联系较差、地下水的还原作用强烈,滑带部位长石化学风化、次生粘土矿物增多,可知由伊利石转化的伊-蒙混层矿物增多是导致滑带土抗剪强度衰减的主要原因。

Reduction of shear strength in landslide slip zone is one of the fundamental mechanisms for landsliding. The causes of decrease in shear strength in landslide slip zones are keys to the understanding of landslide mechanism and to the prediction of landsliding events. Groundwater within slope is known to play an important role in reducing shear strength in landslide slip zones during the development of landsliding physical and chemical interactions with solid phases in the slip zones. In this paper, we report the results of a case study that shows the role of groundwater during the development of landsliding. The physical and chemical effects of groundwater in reducing shear strength in these slip zones may be evaluated by examining the variations of mineral compositions and major elements within the slip zones and surrounding areas. The characteristics of mineralogY and geochemistry of two large-scale landslides in the Three Gorges area, China, the No. 1 Linjiang landslide at Huangtupo and the Xietan landslide, reveal that these slip zones developed along pre-existing weak zones within the slopes. Particularly, minerals and elements that are reactive with groundwater within the landslides indicated that groundwater within the slip zone of the No. 1 landslide at Huangtupo is recharged by rainfall, which caused oxidation within the slip zone. The shear strength reduction in the slip zone could have resulted from physical and chemical processes involving hydrolyses, argillation of marls fragments, dissolution of calcite, and transformation of illite to interlayered illite and smectite. In contrast, groundwater in the slip zone of Xietan landslide has little rainfall component. Consequently, reduction is overwhelming, and shear strength decrease in the slip zone during its development was largely caused by chemical weathering of feldspar and transformation of illite to interlayered illite and smectite.