Landslide Hazard Report_compressed

                        AUSTRALIAN GEOGUIDE LR6 (RETAINING WALLS)



RETAINING WALLS
Retaining walls are used to support cuts and fills. Some are built in the open and backfill is placed behind them (gravity
walls). Others are inserted into the ground (cast in situ or driven piles) and the ground is subsequently excavated on one
side. Retaining walls, like all man-made structures, have a finite life. Properly engineered walls should last 50 years, or
more, without needing significant repairs. However, not all walls fit this category. Some, particularly those built by
inexperienced tradesmen without engineering input, can deflect and even fail because they are unable to withstand the
pressures that develop in the ground around them or because the materials from which they are built deteriorate with
time. Design of retaining walls more than 900mm high should be undertaken by a geotechnical practitioner or
structural engineer and normally require local council approval.
Retaining walls have to withstand the weight of the ground on the high side, any water pressure forces that develop, any
additional load (surcharge) on the ground surface and sometimes swelling pressures from expansive clays. These
forces are resisted by the wall itself and the ground on the low side. Engineers calculate the forces that the retained
ground, the water, and the surcharge impose on a wall (the disturbing force) as well as the maximum force that the wall
and ground on the low side can provide to resist them (the restoring force). The ratio of the restoring force to the
disturbing force is called the "factor of safety" (GeoGuide LR1). Permanent retaining walls designed in accordance with
accepted engineering standards will normally have a factor of safety in the range 1.5 to 2.
Never add surcharge to the high side of a wall (e.g. place fill, erect a structure, stockpile bulk materials, or park vehicles)
unless you know the wall has been designed with that purpose in mind.
Never more than lightly water plants on the high side of a retaining wall.
Never excavate at the toe of a retaining wall.
Any of these actions will reduce the factor of safety of the wall and could
lead to failure. If in doubt about any aspect of an existing retaining wall, or
changes you would like to make near one, seek advice from a
geotechnical practitioner, or a structural engineer. This GeoGuide sets out
basic inspection requirements for retaining walls and identifies some
common signs that might indicate all is not well. GeoGuide LR11
provides information about records that should be kept.
GRAVITY WALLS
Gravity walls are so called because they rely on their own weight (the
force of gravity) to hold the ground behind in place.
Formed concrete and reinforced blockwork walls (Figure 1) - should                Figure 1- Typical formed concrete wall
be built so the backfill can drain. They should be inspected at least once
a year. Look for signs of tilting, bulging, cracking, or a drop in ground
level on the high side, as any of these may indicate that the wall has
started to fail. Look for rust staining, which may indicate that the steel
reinforcement is deteriorating and the wall is losing structural strength
("concrete cancer"). Ensure that weep holes are clear and that water is
able to drain at all times, as high water pressures behind the wall can lead
to sudden and catastrophic failure.
Concrete “crib” walls (Figure 2) - should be filled with clean gravel, or
"blue metal" with a nominated grading. Sometimes soil is used to reduce
cost, but this is undesirable, from an engineering perspective, unless
internal drainage is incorporated in the wall's construction. Without
backfill drainage, a soil filled crib wall is likely to have a lower factor of
safety than is required. Crib walls should be inspected as for formed                   Figure 2 -Typical crib
concrete walls. In addition, you should check that material is not being lost
through the structure of the wall, which has large gaps through it.
Timber “crib” walls - should be checked as for concrete crib walls. In
addition, check the condition of the timber. Once individual elements
show signs of rotting, it is necessary to have the wall replaced. If you are
uncertain seek advice from a geotechnical practitioner, or a structural
engineer.
Masonry walls: natural stone, brick, or interlocking blocks (Figure 3) -
more than about 1m high, should be wider at the bottom than at the top
and include specific measures to permit drainage of the backfill. They
should be checked as for formed concrete walls. Natural stone walls
should be inspected for signs of deterioration of the individual blocks:
strength loss, corners becoming rounded, cracks appearing, or debris                   Figure 3 -Typical masonry wall
from the blocks collecting at the foot of the wall.



170                         Australian Geomechanics Vol 42 No 1 March 2007