Temporary confined water responsible for triggering the landslide of a piedmont gentle slope in Ningzhen 1 Area , China 2 3

19 The traditional viewpoint is that a piedmont gentle slope is conducive to the overall stability of a mountain, which in 20 turn has a counterpressure effect on the whole mountain. However, in recent years, with the increase in extreme 21 heavy rainfall, some landslides have occurred in the piedmont gentle slopes in Ningzhen area. On July 6, 2016, a 22 landslide occurred on the P0 slope of Paomashan Mountain, which is a typical piedmont gentle slope. After field 23 investigation and analysis of monitoring data, we found that it was an intermittent creeping landslide, staged by 24 initial deformation, isokinetic deformation and accelerated deformation. Survey data show that the slope has a very 25 special stratum structure, that is, clay in the surface, coarse-grained soil or weathered rock in the middle, and 26 bedrock in the bottom. In addition, the permeability of each layer is weak-strong-weak from top to bottom. 27 According to the monitoring data, we found that temporary confined water (confined water formed and dissipated in 28 https://doi.org/10.5194/nhess-2019-404 Preprint. Discussion started: 8 January 2020 c © Author(s) 2020. CC BY 4.0 License.


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According to the traditional view, a piedmont gentle slope is beneficial to the overall stability of a mountain 39 (

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Confined water is formed between two impermeable aquifers and has confined properties. If the surface of 55 the slope is an impermeable layer and the middle is a permeable layer, high-intensity rainfall may not allow the 56 https://doi.org/10.5194/nhess-2019-404 Preprint. Discussion started: 8 January 2020 c Author(s) 2020. CC BY 4.0 License. converged groundwater to be discharged immediately along the interface, thus forming a "confined basin" (Huang et Fig. 1, the mountains in the Ningzhen area show a 58 special characteristic: The upper part of the mountain is mainly exposed bedrock with fracture development, with a 59 steep slope, generally between 30° and 50°; the lower part of the mountain is composed of Quaternary gravelly soil 60 and loose sediments with slope angles mostly between 8° and 15°, which we refer to as a piedmont gentle slope. The 61 piedmont gentle slope in the Ningzhen area has the topographic structure of a steep upward and gentle downward 62 with a stratum structure: clay in the surface, coarse-grained soil or weathered rock in the middle and bedrock in the 63 bottom, which forms a special "binary stratigraphic structure" (Yan et al. 2019). The permeability of the strata is 64 weak-strong-weak from top to bottom. Additionally, the upper part of the slope is usually rock with weathered 65 fissures, which is convenient for rainfall infiltration. This slope structure is conducive to the formation of confined        In addition, we use the limit equilibrium theory to analyze the stability of the P0 slope, and the results are 224 shown in Fig. 11. The stability coefficient of the slope is far greater than 1 when there is no strong rainfall, and the 225 slope is in a relatively stable state, and this indicates that strong rainfall is a necessary condition to induce such slope

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For the upper part of the slope: as a rainfall infiltration channel, most of the rainwater penetrates into the 238 slope through this place. Intercepting peripheral water and drainage ditches can be set on the stable slope surface 5 239 m away from the exposed part of the gravel soil layer to prevent rainfall from converging on the slope surface; in 240 addition, the gravel soil exposed on the upper part of the slope can be replaced with clay and other materials with 241 poor permeability to weaken the permeability of this part and reduce rainwater infiltration.

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For the middle part of the slope: temporary confined water in this part will produce an uplift effect 243 perpendicular to the bottom of the aquiclude. To weaken the effect of confined water, a dewatering well should be 244 installed in the middle of the slope. During continuous heavy rainfall, the groundwater level is first monitored by the 245 dewatering well. When the pressure head in the monitoring well reaches a predetermined dangerous value (the value 246 can be calculated by the slope stability), the slope will be dewatered by the dewatering well so that the groundwater 247 level in the monitoring well will always be kept within a certain safety range. Paomashan Mountain in Zhenjiang City as an example, the sliding mechanism was studied. The following 258 conclusions can be drawn:

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(1) The piedmont gentle slope in the Ningzhen area has a special stratum structure, that is, clay in the 260 surface, coarse-grained soil or weathered rock in the middle, and bedrock in the bottom, and the 261 permeability of the layers is weak-strong-weak from top to bottom.

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(2) The landslide of P0 is an intermittent creeping landslide, which can be divided into three stages: initial 263 deformation, isokinetic deformation and accelerated deformation. Temporary confined water is a 264 necessary condition to trigger this type of landslide, and the slope stability coefficient will decrease 265 gradually with the increase in the confined water level.

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(3) According to the simulation results, when the rainfall intensity is 150 mm/d, the rainfall duration is 267 approximately 2.0 d, and the permeability coefficient of the middle layer is between 0.0001 m/s and 268 0.0003 m/s, which is beneficial to the formation of confined water.

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(4) For the prevention and control of landslides on gentle slopes in front of mountains in the Ningzhen 270 area, the principles of "water control" and "engineering prevention and control" should be followed.