Influence of rainfall on sliding modes of cutting slope of weathered granite stratum: Taking Yunxiao section in the Yunping freeway in Fujian for example
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摘要:
降雨引起路堑边坡的浅层局部滑动是工程中常见的现象，但其内在原因的研究尚不充分。为此，以福建省云平高速云霄段花岗岩风化层中的某两级路堑边坡为例，采用饱和和非饱和土抗剪强度理论，按照强度折减有限元方法分析了该边坡在一般工况、地下水渗流工况和降雨入渗工况下的滑动模式。结果表明：一般工况下该边坡滑动模式为深层整体滑动；地下水渗流作用可使边坡由深层滑动模式向浅层滑动模式转化；长时小或中等降雨作用下坡体内出现三道滑动面，且一级边坡内滑动面最先贯通，滑坡模式由深层整体滑动向浅层局部滑动演化。短时强降雨作用下仅在一级边坡内出现贯通的滑动面，即由整体滑动模式完全转变为浅层局部滑动模式。降雨工况越不利表现越为明显，与实际滑坡现象相吻合，且安全系数不满足规范要求，从而从理论上验证了该路堑边坡发生浅层局部滑动的必然性。此外，为避免花岗岩风化层路堑边坡在降雨入渗下发生浅层局部滑动，对假设的不同放坡方案进行了数值模拟分析。结果表明，适当放缓坡率可使一级边坡内不出现滑动面，滑坡模式仍为较深位置的整体滑动，且安全系数可得到有效提高，使其满足规范要求。因此，实践中，采用文中方法对设计的类似地层中的路堑边坡事先进行降雨入渗影响下的模拟分析极为必要。将使放坡坡率更为科学合理，可在确保边坡整体稳定性的同时，极大降低其发生浅层局部滑动的可能性。
Abstract:It is a common phenomenon that rainfall may cause locally shallow sliding of cutting slopes, but the study on the immanent cause is still insufficient. Take a two-grade cutting slope in the weathered granite stratum in Yunxiao section in the Yunping freeway in Fujian as an example, the shear strength theories of saturated soil and unsaturated soil and the strength reduction method of finite element were adopted to study the sliding modes of cutting slope under general working condition, groundwater seepage, and rainfall infiltration. The results show that the sliding mode of the slope exhibits the globally deep sliding under the general working condition. The sliding mode turns to be the shallow sliding from the deep sliding under groundwater seepage. Three slip surfaces may develop in the slope after a long-term light and/or moderate rainfalls, and the slip surface within the first-grade slope runs through firstly so that the sliding mode evolves into the locally shallow sliding from the globally deep sliding. A run-through slip surface only occurs within the first-grade slope after short-term strong rainfalls, i.e., the sliding mode completely turns to be the locally shallow one from the globally sliding one, and the more unfavorable the working condition of a rainfall is, the more remarkable it exhibits. It also agrees well with the phenomenon of the practical landslide, and the safety factor cannot meet the requirement of the specification, and thereby the inevitability of the locally shallow sliding of the cutting slope was theoretically verified. In addition, to avoid the occurrence of the locally shallow sliding of cutting slopes in the weathered granite stratum under the infiltration of rainfalls, various hypothetical schemes were numerically modeled. The results show that no slip surface will occur within the first-grade slope if it is sloped at an appropriate gentle slope, and as a result, the sliding mode is still a globally sliding in a deeper location and the safety factor may be effectively enhanced to meet the requirement of the specification. Therefore, in practice, it is extremely necessary to carry out the simulation analysis of a designed cutting slope in similar stratum under the infiltration of rainfall beforehand. Thus, the gradient of sloping will be more scientific and reasonable, so probability of the occurrence of the locally shallow sliding may be significantly reduced.
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表 1 土工参数
Table 1. Geotechnical properties of soils
土层编号 γ/(kN·m−3) γd/(kN·m−3) E/kPa υ c/kPa 
/(°)c'/kPa 
/(°)7-210 19.5 15.6 8000 0.28 24.9 16.8 22.4 20.6 7-211 20.0 16.5 10 000 0.30 28.9 17.8 24.6 22.2 
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表 2 不同降雨工况边坡稳定安全系数
Table 2. Safety factors of slope under different rainfall conditions
工况 降雨强度/(mm·h−1） 降雨历时/h 安全系数 1 20 4 1.152 2 30 4 1.113 3 40 4 1.091 4 40 3 1.106 5 40 2 1.140
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