Evolution mechanism of rainstorm-induced shallow landslides on slopes covered by arbors considering the influence of wind-induced vibration
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摘要: 强对流或台风等极端天气下乔木坡地发生浅层滑坡灾害往往是暴雨和强风共同作用的结果。以皖南山区一处暴雨型浅层滑坡——畈章组滑坡为例, 通过现场调查和气象资料的分析表明, 除暴雨外风荷载也有可能促进滑坡的启动。为揭示该滑坡启动与破坏后这一完整运动过程的演化机制, 首先基于无限斜坡模型分析了实际降雨条件下的滑坡稳定性, 然后对取自于滑坡体内乔木根系周围和滑动面附近的两种土样利用DPRI型环剪仪, 分别开展了不排水循环剪切试验和自然排水残余剪切试验。结果表明: ①降雨入渗引起滑动面孔隙水压力的上升, 并导致稳定性的降低是畈章组滑坡启动的直接原因; ②乔木根系周围的饱和土在风振作用产生的动剪切荷载下易形成高的超孔隙水压力, 并导致浅表层的局部失稳滑动, 增加了畈章组滑坡整体破坏的可能性; ③滑动面土体的残余强度具有强烈的"正速率效应", 从而控制了畈章组滑坡启动后不会表现出高速远程的运动特征, 与现场调查结论一致。研究结果可以为暴雨协同风振作用下富乔木坡地浅层滑坡的预警预报研究提供参考。Abstract: Shallow landslides on slopes covered by trees are often the result of the coaction of heavy rainfall and strong winds under extreme weather, such as severe convection or typhoons. The Fanzhangzu landslide, a rainstorm-induced shallow landslide in the mountainous area in southern Anhui, was taken as a case study. Through field investigation and meteorological data analysis, it is considered that in addition to rainstorms, wind loads may also promote landslide initiation. To reveal the mechanism of the landslide evolution process, including initiation and postfailure movement, the stability under actual rainfall conditions was first analysed based on the infinite slope model. Then, by using a DPRI ring shear apparatus, undrained cyclic shear tests and natural drainage residual shear tests were carried out on two kinds of soil samples taken from the surroundings of the arbor roots and the sliding surface, respectively. The results show that ① The increase in porewater pressure in the sliding surface results from rainfall infiltration, and the subsequent decrease in stability is the direct reason for the initiation of the Fanzhangzu landslide during rainstorms. ② For the saturated soils around the arbor roots, high excess porewater pressure can be built up under cyclic shear loading resulting from wind-induced vibration, which leads to local failure in the shallow layer and elevates the potential for instability of the Fanzhangzu landslide. ③ The residual strength of the sliding surface soil has a significant positive shear rate effect, so the Fanzhangzu landslide does not show the characteristics of high-speed and long-distance movement in the postfailure stage, which is consistent with the field investigation.
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图 1 强降雨协同风振诱发浅层滑坡示意
Figure 1. Schematic illustration of a shallow landslide triggered by heavy rainfall combined with wind-induced vibration
图 2 畈章组滑坡工程地质背景及取样位置
Figure 2. Engineering geological setting of the Fanzhangzu landslide and the location of samples
图 4 畈章组滑坡处最大风速与风向
Figure 4. Maximum wind velocity and wind direction on the Fanzhangzu landslide
图 6 试样FZZ-1黏土矿物的X射线粉晶衍射
Figure 6. X-ray powder diffraction analysis for the clay minerals of sample FZZ-1
图 8 畈章组滑坡滑动面孔隙水压力及稳定系数变化
Figure 8. Variation in the pore-water pressure on the sliding surface and the factor of safety of the Fanzhangzu landslide
图 9 DPRI-5型环剪仪构造示意图
Figure 9. Schematic diagram of structure representation of the DPRI-5 ring shear apparatus
图 10 饱和试样FZZ-2不排水循环剪切试验(初始静剪应力τ0=35 kPa)
Figure 10. Undrained cyclic ring-shear test on saturated sample FZZ-2 at the initial static shear stress (τ0) of 35 kPa
图 11 饱和试样FZZ-2不排水循环剪切试验(预设静剪应力τ=60 kPa)
Figure 11. Undrained cyclic ring-shear test on saturated sample FZZ-2 at the preset static shear stress (τ) of 60 kPa
图 13 不同剪切速率下试样FZZ-1自然排水环剪试验(σ=260 kPa)
Figure 13. Ring shear test on sample FZZ-1 with different shear displacement rates under naturally drained conditions at a total normal stress of 260 kPa
图 15 试验后试样FZZ-1剪切带粒度成分
Figure 15. Grain size distribution of the shear zone of sample FZZ-1 before and after ring shear tests
表 1 试样基本物理性质指标
Table 1. Physical properties of the samples
试样 ρd/(g·cm-3) Gs wL/% wP/% IP Ks/(cm·s-1) FZZ-1 1.37 2.73 42.19 24.33 17.86 1.95×10-5 FZZ-2 1.30 2.70 38.13 25.01 13.12 5.22×10-5 注：ρd为干密度；Gs为土粒相对密度；wL为液限含水率；wP为塑限含水率；IP为塑性指数；Ks为饱和渗透系数 
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