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AUSTRALIAN GEOGUIDE LRG (RETAINING WALLS)

 

 

RETAINING WALLS

 

Retaining walls are used to support cuts and fills. Some are built in the open and back®ll 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 fad because they are unable to withstand the
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time. Design of retaining walls more than 900mm high should be undertaken by a geotechnical practitioner or
structural ineer and normal ire local council ‘oval.

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 a5 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 normaly have a factor of safety in the range 1.5 to 2.

Never ado surcharge to the high side of a wall (e.g. place fil, 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. es ee

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. fin 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 mdicate 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 ther own weight (the
force of gravity) to hold the ground behind in place.

Formed concrete and reinforced blockwork wails (Figure 1) - should
be built so the backfill can drain. They should be inspected at least once
@ 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 j— tnekeod draivage layer
Started to fal. Look for rust staining, which may indicate that the steel tented a
reinforcement is deteriorating and the wall is losing structural strength
(concrete cancer”). Ensure that weep holes are clear and that water is St, $$ Fras dvadeing rater

 

 

able to drain at all times, as high water pressures behind the wall can lead Seana on Peaea

to sudden and catastrophic failure. a]

Concrete “crib” walls (Figure 2) - should be filled with clean gravel, or S aa
“plue metal with a nominated grading. Sometimes soil is used to reduce Ie
cost, but this is undesirable, from an engineering perspective. unless a

intemal drainage is incorporated in the wall's construction. Without “= erateedina nee
backfill drainage, a soil filled crib wall is likely to have a lower factor of Al
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 [— Orinerage tiryier
|

Timber “crib” walls - should be checked as for concrete crib walls. in {
addition, check the condition of the timber. Once individual e@ements
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, comers becoming rounded, cracks appearing, or debris
from the blocks collecting at the foot of the wail.

   

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Li a6 for concrete wal

Figure 3 -Typical masonry wall

170 Australian Geomechanics Vol 42 No 1 March 2007