A study of the pro-water pressure for initiation of a large landslide triggered by a strong earthquake based on fluid-structure coupling
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摘要:
2008年汶川Ms8.0级强震触发了体积近12×108 m3的大光包滑坡。该滑坡发生于古生代碳酸盐岩地层，滑带地质背景为斜坡内部深埋400 m、最大厚度达5 m的先期层间构造错动带。最新调查表明，该错动带是斜坡内部地下水通道，错动带岩体处于饱和状态。为揭示强震过程与地下水相关的大光包滑坡启动机制，提出了一种具有软弱层带的硬质碳酸盐岩边坡简化模型，将层间构造错动带概化为碳酸盐岩硬层内部软弱层带，采用FLAC3D程序中的流固耦合算法模拟了模型的响应特性。研究结果表明：强震过程中软弱层带上下碳酸盐岩硬层的变形响应时间、波型、大小出现明显差异，上硬层相对于下硬层产生了张离、压缩和剪切3种非协调变形模式，由此对软弱层带产生了振动冲压-张拉和振动剪切动力学行为，饱水软弱层带形成了具有瞬间放大和累积增涨特征的超孔隙水压力。这里将上下硬层差异性变形称为非协调变形，认为非协调变形是软弱层带应力放大成因，推测软弱层带应力瞬间放大以及放大应力长持时作用下的岩体致损是超孔隙水压力激发和累积的成因；强震过程软弱层带超孔隙水压力导致其内有效应力快速降低，使得斜坡前部锁固段应力快速集中，而后被突然剪断，滑坡骤然启动，揭示了强震过程中超孔隙水压力是大光包滑坡启动的主要原因。
Abstract:The Daguangbao landslide is the largest co-seismic landslide with a volume of approximately 12×108 m3, which triggered by the Wenchuan earthquake (Ms 8.0) occurred on May 12, 2008. The slide zone is located in the Paleozoic carbonate strata and it is an interlayer structural dislocation zone with a depth of 400 m and a maximum thickness of 5 m inside the slope. A previous study concluded that the shear failure of the Daguangbao landslide developed within the bedding fault. Our latest investigation showed that this bedding fault was under the water table, and its rock mass was saturated before the landslide. To reveal the initiation mechanism of the Daguangbao landslide related to the groundwater, a simplified model of a hard carbonate slope with an inter-weak layer is given. The response characteristics of the model are stimulated by a fluid-structure coupling algorithm using the FLAC3D. The results show that significant differences in deformation patterns during the earthquake, such as response time, wave shape, and amplitude exist between the upper and lower hard layers. Three uncoordinated deformation patterns, i.e., tension, compress, and shearing coming from the upper layer to the weak layer, are identified. The deformation pattern results in the amplification of compress and shear forces within the weak layer. The stress amplification subsequently induces excess pore water pressure with instant amplification and accumulation features. It is reasonably inferred that the excess pore water pressure essentially reduces the effective stress of the bedding fault. Our simulation confirms that the excess pore-water pressure can be generated within a saturated bedding fault deep situated in a slope during a strong earthquake. We propose that the excess pore water pressure within the saturated fault at 400 m underground is the cause of the Daguangbao landslide.
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表 1 数值模拟参数与本构模型综合选取表
Table 1. Comprehensive selection of the numerical simulation parameters and constitutive model
岩层 本构模型 干密度/（kg·m−3） 内聚力/Pa 内摩擦角/（°） 体积模量/Pa 剪切模量/Pa 抗拉强度/Pa 孔隙率/% 渗透系数/（cm·s−1） 白云岩 弹性 2700 1.8×107 42 3×1010 2.6×1010 1×107 2 1×10−12 软弱层带 摩尔-库伦；
Finn（Byrne）2300 3×105 25 1.1×108 3.5×107 1×106 20 5.9×10−7 
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表 2 模拟工况一览表
Table 2. Test of the numerical simulation conditions
水位 垂直向振动 水平向振动 双向振动
（垂直与水平向同时振动）无水位 √ √ √ 有水位 √ √ √
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