Considerations in engineering large-scale retaining wall structures

Engineers and officials onsite at the collapse of concrete on the Darlington upgrade. Image credit: Tricia Watkinson.

How a segmental retaining wall may prevent disasters like the recent embankment collapse on the Darlington Upgrade Project in Adelaide.

The recent collapse of sections of a retaining wall onto a new motorway being built as part of Adelaide’s North-South Corridor has brought the structural integrity of this type of retaining wall into question.
Water build-up behind the 220-metre-long embankment is being blamed for two sections of shotcrete crumbling onto the road. South Australia’s Minister for Transport, Stephan Knoll, announced that damage to the $620 million Darlington Upgrade Project may have been caused by heavy rain saturating the soil behind the concrete wall and causing it to move.
Regardless of the wall’s construction materials, its design should be influenced by the behaviour and strength of soil within and adjacent to it. This includes the soil’s natural angle of repose and migration of water through the soil skeleton. The soil around the problematic Darlington Upgrade Project is clay, which has a higher angle of repose than soil, with a higher gravel or sand make-up. Clay also has affinity towards water – it doesn’t let water pass through it and, once it has absorbed water, it doesn’t let it go easily. In strict engineering terms, clay has undesirable “shrink and swell” properties, with varying moisture levels.

Surrounding soil should be an important consideration in retaining wall design and construction. Image credit: Brickworks.

Austral Masonry Sales Engineer Anas Ajaz says clay soils behave like plastic when mixed with water, exerting more pressure than other soils on a retaining wall after rain.
“Often when these walls fail, they fail due to water build-up that fills the void in the soil, making the soil heavier. The failure often follows heavy rainfall, with water slowly percolating into the system until the failure point is reached.”
Geotechnical engineers typically test the surrounding soil ahead of the design of a retaining wall, along with assessing the location, topography and rainfall to determine design and drainage requirements.
Retaining walls are designed to withstand retained soil pressure and surcharges, not hydrostatic pressure. “Build-up of water is unacceptable,” explains Ajaz, “so one of the main considerations is to calculate the amount of water that may enter the retaining-wall system in a worst-case scenario and then to make sure it drains out adequately.”
Unlike reinforced concrete, which is impermeable, segmental concrete blocks allow water to drain. Austral Masonry specialises in segmental retaining walls.
“With segmental concrete blocks, the magic happens behind the wall façade,” says Ajaz. “Walls more than five metres high are usually designed with a free-draining aggregate column behind the blocks, and reinforcement measures in the form of geogrids to ensure adequate structural integrity over the long-term. A wall of about 10 metres high can extend as deep as 10 metres into the retained soil.”
The segmental blocks that make up the wall façade and supporting geogrids behind it aren’t bound together by mortar, but are interlocked using either fibreglass pins or concrete lugs. The result is a monolithic structure with gaps in between the blocks allowing it to flex independently to a degree and for excess water to drain.

A small section of the completed retaining wall at Oakdale South. Image credit: Brickworks.

Austral Masonry’s civil retaining wall masonry blocks have been chosen for several large-scale commercial projects (including Sydney’s Oakdale South industrial estate and the North Connex motorway link) not just for their structural integrity and durability, but also for their cost-effectiveness and aesthetics.
A variety of wall textures and colours can be achieved as there is a wide range of face unit options available, all with the same dry-stacked flexible system to better tolerate soil movement and heavy rainfall. The smaller size and weight of each segmental block can also make it easier and less expensive to construct walls in locations that have access difficulties, without the need for specialised equipment.

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