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Landslips
A landslip develops when the forces holding the land onto
the slope become less than the gravitational forces pulling
it down the slope. Most landslips on farmland are triggered
by heavy rainfall. Waterlogging increases the weight on the
slope and may reduce the shear strength by increasing the
pore water pressure. This water pressure forces the soil layers
apart allowing the topsoil to slide like a fluid down the
slope.
The potential for trees to stabilise and control landslips
is largely dependent on how ‘deep-seated’ the slip
is. There are reports of large landslips occurring in forested
areas. In these cases the point of failure is generally below
the rooting depth of the trees suggesting that there was little
potential for trees to anchor the base of the slip into firm
ground. In fact, the presence of large trees may actually
increase the risk of such slips by increasing the weight of
the slope and causing soil movements during strong winds.
Where it is clear that the depth of the failure is greater
than the rooting depth of the trees, alternative control options
may need to be considered such as deep drainage or structural
support.
Shallow-seated slips, less than about two metres, are unlikely
to occur in forested areas and can mostly be controlled or
prevented by dense tree planting. The trees not only anchor
soil layers, but also reduce waterlogging. By drying the soil
to depth (sometimes to 6 metres or more) during the growing
seasons, the risk of waterlogging during heavy rains is low.
A dense cover of trees may also intercept up to 15% of rainfall
preventing it even reaching the soil surface. Trees should
be planted well up the slope from the slip as well as over
the effected area to prevent waterlogging. Trees may be used
in conjunction with other means of reducing waterlogging,
such as spoon drains above the area and sub surface drains
through the site.
Landslips can also occur as a result of undermining of the
slope. This is commonly seen where road construction or erosion
occurs at the base of the slope. It may be possible to establish
a thick row of trees along the base of the slope to act as
a retaining wall. Tolerant tree species that are able to grow
close together and form a fence, such as pines or other softwoods,
should be used.
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