10.4 Major factors affecting landslides
Factors causing landslides to occur fall into two categories: 1) things increasing driving forces, and 2) things reducing resisting forces.
Factors increasing driving forces:
1. Steepening the slope
2. Adding weight to (loading) the slope, especially the upper parts
3. Increasing the height of a slope (either by human or natural downcutting)
3. Seismic shaking
Factor reducing resisting forces:
1. Adding water to the slope --> causes increased pore pressure --> reduces frictional strength
2. Steepening the slope --> reduces normal stress, and thus reduces internal friction
3. Bedding, jointing, or foliation parallel to slope or dipping out of slope --> these discontinuities are low-strength zones along which the rock can fail and slide out of the slope
4. Intrinsically weak materials (e.g., deeply weathered, sheared, unconsolidated, or clay-rich materials)
5. Undercutting the slope --> reduces support
6. Removing vegetation, especially trees --> loss of root strength, also increased water in soil due to reduced evaporation losses
7. Seismic shaking
Most of the factors causing landslides are similar to slope failure, however the two most significant factors that are most critical for landslides are the rainfall and seismicity. The following discussion explains their mechanism in the context of landslides.
Majority of landslides are triggered by heavy rainfall. This is because the rainfall drives an increase in pore water pressures within the soil. The Figure 17 illustrates the forces acting on an unstable block on a slope. Movement is driven by shear stress, which is generated by the mass of the block acting under gravity down the slope. Resistance to movement is the result of the normal load. When the slope fills with water, the fluid pressure provides the block with buoyancy, reducing the resistance to movement. In addition, in some cases fluid pressures can act down the slope as a result of groundwater flow to provide a hydraulic push to the landslide that further decreases the stability. Whilst the example given in Figures 17 and 18 is clearly an artificial situation, the mechanics are essentially as per a real landslide
Figure 17: Diagram illustrating the resistance to, and causes of, movement in a slope system consisting of an unstable block
Figure 18: Diagram illustrating the resistance to, and causes of, movement in a slope system consisting of an unstable block
• Loss of
suction forces in silty materials, leading to generally shallow failures (this
may be an important mechanism in residual soils in tropical areas following
• Undercutting of the toe of the slope through river erosion.
In many cold mountain areas, snowmelt can be a key mechanism by which landslide initiation can occur. This can be especially significant when sudden increases in temperature lead to rapid melting of the snow pack. This water can then infiltrate into the ground, which may have impermeable layers below the surface due to still-frozen soil or rock, leading to rapid increases in pore water pressure, and resultant landslide activity. This effect can be especially serious when the warmer weather is accompanied by precipitation, which both adds to the groundwater and accelerates the rate of thawing.
Rapid changes in the groundwater level along a slope can also trigger landslides. This is often the case where a slope is adjacent to a water body or a river. When the water level adjacent to the slope falls rapidly, the groundwater level frequently cannot dissipate quickly enough, leaving an artificially high water table. This subjects the slope to higher than normal shear stresses, leading to potential instability. This is probably the most important mechanism by which river bank materials fail, being significant after a flood as the river level declines.
Figure 19: Groundwater conditions when the river level is stable
Figure 20: Groundwater conditions on the falling limb of the hydrograph.
In some cases, failures are triggered as a result of undercutting of the slope by a river, especially during a flood. This undercutting serves both to increase the gradient of the slope, reducing stability, and to remove toe weighting, which also decreases stability. Subsurface water, if present in a slope or if it could develop during the life of a project, should be considered in slope stability analyses. The presence of subsurface water in a slope can reduce effective stresses when positive pore-water pressures develop, causing a reduction in shear resistance. Subsurface water can also increase de-stabilizing forces in the slope via the additional weight associated with a moist slide mass or via seepage forces.
The second major factor in the triggering of landslides is seismicity. Landslides occur during earthquakes as a result of two separate but interconnected processes: seismic shaking and pore water pressure generation. The passage of the earthquake waves through the rock and soil produces a complex set of accelerations that effectively act to change the gravitational load on the slope. So, for example, vertical accelerations successively increase and decrease the normal load acting on the slope. Similarly, horizontal accelerations induce a shearing force due to the inertia of the landslide mass during the accelerations. These processes are complex, but can be sufficient to induce failure of the slope. These processes can be much more serious in mountainous areas in which the seismic waves interact with the terrain to produce increases in the magnitude of the ground accelerations. This process is termed 'topographic amplification'. The maximum acceleration is usually seen at the crest of the slope or along the ridge line, meaning that it is a characteristic of seismically triggered landslides that they extend to the top of the slope.