Reinforced Soil Walls On Development Sites

Reinforced soil walls hold up a face of fill by building strength into the fill itself. Horizontal layers of polymer grid or steel strips are laid between layers of compacted fill and tied to a facing at the front, so the whole reinforced zone acts as one heavy block holding back the ground behind it.

It is one of the most economical ways to retain a tall face of fill. Most of the wall is buried where nobody looks at it, though, which makes it one of the easier walls to get wrong.

Where it suits

A reinforced soil wall suits a site being built up. Raising a warehouse pad, carrying a road along a slope or forming a level platform above a boundary are the classic uses, because the fill is going in anyway and the reinforcement goes in with it.

It suits a cut far less. Retaining an excavation this way means digging out the ground where the reinforcement has to go and then putting it back, and on a tight Sydney boundary that space isn't there. For a basement, a cut in clay or a face beside a neighbour, the answer is usually shoring, anchors or soil nails, compared in [soil nailing and anchored retaining walls](/blog/soil-nailing-and-anchored-retaining-walls).

The room behind the face

The reinforcement runs back into the fill a long way, often most of the wall's height, and further where heavy loads sit on top. A wall a few metres high therefore needs a strip of land behind it nearly as deep as the reinforcement is long.

That strip has to sit inside the site, or be covered by an agreement with the neighbour. It also has to stay clear for the life of the wall, which shapes what can be built on top of it.

The facing

The facing holds the fill in at the front and gives the wall its look. Low and medium walls often use dry-stacked concrete blocks, with the grid clamped between the courses. Taller walls, and walls along roads, often use precast concrete panels with strips or grid fixed to the back of each panel.

The facing isn't the structure on its own. What matters is the connection between the facing and the reinforcement, because a wall that loses that connection pushes its face out even while the fill behind it is sound.

What the design checks

AS 4678, the standard for earth-retaining structures, covers reinforced soil walls. The design checks the reinforced block as a whole for sliding, overturning, bearing on the ground below and the stability of the slope it sits on. Then it checks the inside, with the reinforcement assessed for breaking, for pulling out of the fill and for its connection to the face.

Polymer grid slowly loses strength when it carries a load for years, and steel strips corrode. The design allows for both over the wall's design life, so the product on the specification has to be the one the calculation assumed, not a cheaper substitute picked up on site.

Fill quality

Reinforced soil walls are built with select fill, usually a free-draining granular material, placed in thin layers and compacted to the specification. The reinforcement grips the fill by friction, and clay or wet material doesn't give it the same hold.

Each layer gets tested as it goes in, the same discipline as [controlled fill and compaction testing](/blog/controlled-fill-and-compaction-testing) under a slab. Skipping that testing to keep the crew moving puts the whole wall on trust.

Water

Water behind a retaining wall adds pressure the design never counted on. A reinforced soil wall needs a drain at the base of the fill, a drainage layer behind the reinforced block where the ground behind it is wet, and surface water led away from the top of the wall rather than over it.

The principles are the same as for any retaining wall, set out in [drainage behind retaining walls](/blog/drainage-behind-retaining-walls).

Loads on top, and digging later

Anything placed near the top of the wall loads the reinforced block. A driveway, a parked truck, a building slab or a stockpile all add to the pressure, and the design has to know about each of them, as covered in [surcharge loads on retaining walls](/blog/surcharge-loads-on-retaining-walls).

Digging is the other risk. A stormwater trench, a fence post or a light pole footing drilled into the reinforced zone cuts the grid. So services get planned into the fill as it is placed, and the drawings show where nobody should excavate later.

How we approach it

We design reinforced soil walls together with the earthworks, so the wall height, the platform level and the source of the fill get settled at the same time. Reinforced soil structures are part of the expertise of Dr Le Duc Ta, our Principal Geotechnical Engineer, along with shoring and slope stability, so each wall is checked on the same ground model as the slope it holds.

A reinforced soil wall is only as good as the layers nobody will ever see again.

ACSES provides geotechnical engineering, including retaining wall design, across Sydney. Talk to us about a project.

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