Slope Stability - lindsays

AUSTRALIAN GEOGUIDE LR5 (WATER & DRAINAGE)

One way or another, water usually plays a critical part in initiating a landslide (GeoGuide LR2). For this reason, itis a
key factor to be controlled on sites with more than a low landslide risk (GeoGuide LR7).

Groundwater and Groundwater Flow

The ground is permeable and water flows through it as illustrated in Figure 1. When rain falls on the ground, some of it
tuns along the surface ("surface water run-off") and some soaks in, becoming groundwater. Groundwater seeps
downwards along any path it can find until it meets the water table: the local level below which the ground is saturated. If
it reaches the water table, groundwater either comes to a halt in what is effectively underground storage, or it continues
to flow downwards, often towards a spring where it can seep out and become surface water again. Above the water
table the ground is said to be "partially saturated’, because it contains both water and. air. Suctions can develop in the
partially saturated zone which have the effect of holding the ground together and reducing the risk of a landslide.
Vegetation and trees in particular draw large quantities of water out of the ground on a daily basis from the partially
saturated zone. This lowers the water table and increases suctions, both of which reduce the likelihood of a landslide
occurring.

earr’ ——-\__ Weaste water and effluent disposal
v “augment natural inflows

      

Vegetation extracts water
and lowers water table

Surface water
run-off

Figure 1 - Groundwater flow

Groundwater Flow and Landslides

The landslide risk in a hillside can be affected by increase in soak-away drainage or the construction of retaining walls
which inhibit groundwater flow. The groundwater is likely to rise after heavy rain, but it can also rise when human
interference upsets the delicate natural balance. Activities such as felling trees and earthworks can lead to:

a reduction in the beneficial suctions in the partially saturated zone above the water table.

increased static water pressures below the water table,

increased hydraulic pressures due to groundwater flow,

loss of strength, or softening, of clay rich strata,

loss of natural cementing in some strata,

e — transportation of soil particles.

Any of these effects, or a combination of them, can lead to landslides like those illustrated in GeoGuides LR2, LR3 and
LR4.

Limiting the Effect of Water

Site clearance and construction must be carefully considered if changes in groundwater conditions are to be limited.
GeoGuide LR8 considers good and poor development practices. Not surprisingly much of the advice relates to sensible
treatment of water and is not repeated here. Adoption of appropriate techniques should make it possible to either
maintain the current ground water table, or even cause it to drop, by limiting inflow to the ground.

If drainage measures and surface protection are relied on to keep the risk of a landslide to a tolerable level, it is important
that they are inspected routinely and maintained (GeoGuide LR11).

The following techniques may be considered to limit the destabilising effects of rising groundwater due to development
and are illustrated in Figure 2.

168 Australian Geomechanics Vol 42 No 1 March 2007