Slope Stability - lindsays

AUSTRALIAN GEOGUIDE LR5 (WATER & DRAINAGE)

        
  
 

a Retain trees wherever possible.
bl can cause the water
le

Surface protection and
contour drains reduce
inflow and increase run-off

oa Sub-soil drains
| Pop . intercept

groundwater
Water table drops”

Surface water drains (dish drains, or table drains) - are often used to prevent scour and limit inflow to a slope, Other
than in rock, they are relatively ineffective unless they have an impermeable lining. You should clear them regularly, and
as required, and not less than once a year. If you live in an area with seasonal rainfall, it is best to do this near the end
of the dry season. If you notice that soil or rock debris is falling from the slope above, determine the source and take
appropriate action. This may mean you have to seek advice from a geotechnical practitioner.

Surface protection - is sometimes used in addition to surface water drainage to prevent scour and minimise water
inflow to a slope. You should inspect concrete, shotcrete or stone pitching for cracking and other signs of deterioration at
least once a year. Make sure that weepholes are free of obstructions and able to drain. If the protection is deteriorating,
you should seek advice from a geotechnical practitioner.

Sub-soil drains - are often constructed behind retaining walls and on hillsides to intercept groundwater. Their function is
to remove water from the ground through an appropriate outlet. It is important that subsoil drains are designed to
complement other measures being used. They should be laid in a sand, or gravel, bed and protected with a graded
stone or geotextile filter to reduce the chance of clogging. Sub-soil drains should always be laid to a fall of at least 4
vertical on 100 horizontal. Ideally the high end should be brought to the surface, so it can be flushed with water from
time to time as part of routine maintenance procedures.

Deep, underground drains - are usually only used in extreme circumstances, where the landslide risk is assessed as
not being tolerable and other stabilisation measures are considered to be impractical. They work by permanently
lowering the water table in a slope. They are not often used in domestic scale developments, but if you have any on your
site be aware that professional maintenance is essential. If they are not maintained and stop working, the water table will
rise and a landslide may even occur during normal weather conditions. Both an increase or a reduction in the normal
flow from deep drains could indicate a problem if it appears to be unrelated to recent rainfall. If changes of this sort are
observed, you should have the drains and your site checked by a geotechnical practitioner.

Documentation - design drawings and specifications for geotechnical measures intended to minimise landslide risk can
be of great assistance to a geotechnical specialist, or structural engineer, called in to inspect and report on them. Copies
of available documentation should be retained and passed to the new owner when the property is sold (GeoGuide
LR11). You should also request details of an appropriate maintenance program for drainage works from the designer
and keep that information with other relevant documentation and maintenance records.

More information relevant to your particular situation may be found in other Australian GeoGuides:

GeoGuide LR1  - Introduction GeoGuide LR7 - Landslide Risk

GeoGuide LR2 - Landslides GeoGuide LR8 = - Hillside Construction

GeoGuide LR3 - Landslides in Soil GeoGuide LR9 - Effluent & Surface Water Disposal
GeoGuide LR4 - Landslides in Rock GeoGuide LR10 - Coastal Landslides

GeoGuide LR6 - Retaining Walls GeoGuide LR11 - Record Keeping

The Australian GeoGuides (LR series) are a set of publications intended for properly owners; local councils; planning authorities;
developers; insurers; lawyers and, in fact, anyone who lives with, or has an interest in, a natural or engineered slope, a cutting, or an
excavation, They are intended to help you understand why slopes and retaining structures can be a hazard and what can be done with
appropriate professional advice and local council approval (if required) to remove, reduce, or minimise the risk they represent. The
GeoGuides have been prepared by the Australian Geomechanics Society, a specialist technical society within Engineers Australia, the
national peak body for all engineering disciplines in Australia, whose members are professional geotechnical engineers and engineering
geologists with a particular interest in ground engineering. The GeoGuides have been funded under the Australian governments’
National Disaster Mitigation Program.

 

Australian Geomechanics Vol 42 No 1 March 2007 169