Paneles, monitores y Prevención de amenazas

Determinant for the groundwater ow pattern in landslides is the relative and not the absolute permeability. The permeability contrast controls preferential ow and the formation of positive pore water pressure. These are both important processes for the triggering of landslides. There- fore the permeability contrast and not the absolute permeability is used in the hydrogeological classication. Because of the horizontal layering, the vertical permeability contrast is assumed to be dominant and the lateral permeability contrast is neglected in the classication. On Fig- ure 3.1 the contrast of permeability is displayed with a signature for high permeable layers. A permeability contrast of two orders of magnitude (or larger) that occur abruptly (in between less than one meter of prole) is signicant (Rogers and Selby, 1980; Carvalho Vieira and Fer- reira Fernandes, 2004).

If two layers are assumed, four dierent combinations of high and low permeable are possible (classes 1-4 in Figure 3.1). In a three-layer situation, it would be a total of eight combinations5_.

Nevertheless, the three-layer cases can be reduced to two classes (class 5 and class 6 in Figure 3.1)). The other three-layer combinations are already taken into account by the two-layer cases. (For example if the unconsolidated sediment and the weathered bedrock are high permeable over low permeable bedrock, the implications on the landslide triggering are similar to the case where there is only one high permeable layer over low permeable bedrock). The six classes are:

1 ) High permeable top layer over low permeable bottom layer:

• Example: Horizontal beds of permeable ash overlying impermeable mudow deposits, Nishigo Village, Japan (Chigira, 2002). Often, the top soil is more permeable due to macro pores than the subsoil and thus plays an important role for the inltration and lateral drainage of the soil (Van Asch et al., 1999).

• Hydrogeology: Rainwater inltrates in the unconsolidated sediment and percolates downwards to the impeding layer where a groundwater table is formed. Mainly in the saturated zone, water is owing parallel to the slope towards the toe of the slope. The subsurface storm ow theory (see Section 2.2.2) is based on the assumption of a permeable layer over one or more impeding layers or a progressive decrease in per- meability with depth (Kirkby, 1978). Depending on the rain intensity, the saturation can occur bottom up or, by transient water-table perching, top down (Iverson et al., 1997; Lourenco et al., 2006; Kienzler, 2007).

• Landslides: The situation of permeable unconsolidated sediment over an impermeable substratum is very frequent in the case of shallow landslides. Convergent topography (hollows) favour saturated ow and make a slope additionally susceptible for the local accumulation of groundwater and thus for landsliding (Iverson et al., 1997; Chigira, 2002; Montgomery et al., 2002).

2 ) Low permeable top layer over low permeable bottom layer:

• Example: Fine grained moraine covering fresh granite or clayey silt covering Flysch (Trisenberg landslide, see Section 8.3).

• Hydrogeology: The in-situ inltration capacity of the low permeable top layer is small and water percolates slowly as subsurface stormow in the top layer. Long-term rainfall is needed to saturate the unconsolidated sediment. During precipitation, the saturation of the soil can occur top-down and perched groundwater tables are formed inside the low permeable top layer (Iverson et al., 1997; Lourenco et al., 2006).

3.3. PERMEABILITY

• Landslides: In low permeable unconsolidated sediment, especially when high plastic, landslides are rather continuous and slowly creeping than rapid movements.

3 ) Low permeable top layer over high permeable bottom layer:

• Example: Morainic material over a relatively high permeable dolomite (Mikos et al., 2004).

• Hydrogeology: The capacity of in-situ inltration is small and little water percolates through the low permeable top layer. If the bottom layer is unsaturated, a capillary barrier may form at the transition between the two layers (Lu, 2010) and only little water inltrates into the bottom layer. If the bottom layer is saturated a conned or an artesian aquifer can be formed, which increases the pore water pressure on the potential landslide slip surface. If the bottom layer is semi-conned, little water exltrates into the top layer (aquitard). This is common for example in deposits of river deltas or coastal plains, where low permeable clay overlies high permeable sand or gravel beds. In this case, the ow in the aquitard is mainly vertical (Oosterbaan and Nijland, 1994). The bottom layer may be fed further upslope by inltration of rain water and it is likely that the hydrogeological catchment is larger than the hydrological catchment.

• Landslides: When the bottom layer is semi-conned, conned or artesian, a very unfavourable situation for the triggering of landslides can develop. High porewater pressure and seepage forces from below can lead to inner erosion and liquefaction and a hydraulic shear failure can occur.

4 ) High permeable top layer over high permeable bottom layer:

• Examples: Silty little clayey sand over Molasse sandstone (Bollinger et al., 2000). • Hydrogeology: Water inltrates easily at the surface and percolates trough the un-

consolidated sediment. If the bottom layer is saturated, groundwater can exltrate into the top layer. If it is unsaturated, it acts as a sink and water inltrates. But no conned groundwater can be formed.

• Landslides: If the hydraulic head in the bottom layer is higher than in the top layer, groundwater ow is upward which is unfavourable for the triggering of landslides. Inner erosion and liquefaction may occur.

5 ) Low permeable top layer and high permeable middle layer over low permeable bottom layer:

• Example: Low permeable colluvium material and volcanic ash (silty clay) over strongly weathered greywacke with jointing pattern (sandy silt, silty sand) over fresh low per- meable greywacke. (Rogers and Selby, 1980).

• Hydrogeology: Water inltrates (for example through cracks in the top layer, at the top of the slope) into the middle layer which has considerably higher hydraulic conductivities than the overlying horizons. Flow takes place through the permeable middle layer. This can lead to semi-conned or artesian aquifers, especially if the bedrock shows an irregular topography and hollow structures (Figure 3.3) or at the toe of a landslide.

• Landslides: If the lower soil horizon is more permeable than the surface horizon, an upward pressure may develop. This favours sallow landsliding (Rogers and Selby, 1980). Water penetrating downwards in the permeable horizon can create upthrust and buoyancy eects. The formation of conned aquifer and preferential water ow in

3.4. SATURATION