Surcharge Loads On Retaining Walls

A retaining wall holds back soil. The soil pressure is the obvious load and it is rarely the one that causes the failure.

What causes the failure is something placed on the ground behind the wall after it was built: a driveway, a shed, a pool, a stack of material, or a truck.

That is surcharge, and it increases the lateral pressure on the wall by far more than people expect.

Why a small load on top is a large load on the wall

A load applied to the surface behind a wall spreads into the soil and pushes outward as well as downward.

The lateral component is a proportion of the vertical load, and that proportion depends on the soil. In a free-draining granular backfill it might be around a third. In a stiff clay that cannot move, it can approach the full vertical pressure.

Which means parking a vehicle behind a wall does not add a small amount of pressure near the top, it adds a pressure distribution down a significant depth of the wall, and the resulting moment at the base can be a large fraction of the wall's whole capacity.

The surcharges that turn up in practice

Vehicles. A driveway, a parking bay, or simply a truck that reversed up to the wall to unload. The most common surcharge and the one most often not designed for, because the driveway was built after the wall.

Construction plant. An excavator or a compactor working near a wall during backfill applies a load far above anything in the finished condition. Many walls are pushed out of alignment during their own backfilling, before the project is even finished.

A building or footing behind the wall. A shed, a garage, a granny flat or an extension whose footings sit within the zone of influence. This is a structural load transferred into the wall, and it is the condition in [building on the boundary and party walls](/blog/building-on-the-boundary-and-party-walls).

A pool. Water is heavy, and a pool near a wall is both a surcharge and a potential water source. Covered in [building a pool near a house or a boundary](/blog/building-a-pool-near-a-house-or-a-boundary).

Stored material. A soil stockpile, pavers, sand, or landscaping material. Temporary, and the wall does not know that.

Sloping ground above the wall, which is a surcharge in effect because the soil above the wall's top level is pushing down on the retained wedge.

A second wall above, in a terraced arrangement, where the upper wall's footing load becomes a surcharge on the lower one. Terraced walls have to be designed as a system, and spacing them far enough apart that they do not interact is the usual answer.

The zone that matters

Not everything behind a wall is a surcharge on it.

Load applied far enough back has dissipated before it reaches the wall. The rule of thumb is a wedge extending back and up from the base of the wall at an angle related to the soil's friction angle. Inside that zone, a load affects the wall. Outside it, largely not.

Which gives the simplest practical control available: keep the load out of the zone. A driveway set back from the wall, a footing taken down below the influence line, or a stockpile positioned away from it all remove the problem without strengthening anything.

Where the load cannot be moved, the alternative is to take its footings down deep enough that the load is applied below the wall's base rather than behind its stem.

Water makes all of it worse

Surcharge and water are the two loads that break walls, and they frequently arrive together, because a driveway or a paved area behind a wall both surcharges it and directs water into the backfill.

A wall designed for a drained backfill and loaded by both a vehicle and a saturated soil is being loaded well beyond anything it was checked for. The drainage side is the subject of [drainage behind retaining walls](/blog/drainage-behind-retaining-walls), and the two have to be considered as one condition rather than two.

Assessing an existing wall

A wall that is leaning, cracking or rotating has usually had something added behind it, and the investigation starts with what changed rather than with the wall.

The questions: what is on the ground behind it now that was not there originally, how far back is it, how deep do its footings go, and where does the water go. Most leaning walls answer themselves in those four.

The remedies, in order of preference, are to remove or relocate the surcharge, to improve the drainage, to add anchors or soil nails as described in [soil nailing and anchored retaining walls](/blog/soil-nailing-and-anchored-retaining-walls), or to build a new wall in front of the old one.

Strengthening a wall while leaving the surcharge and the water in place is the option that fails twice.

On the drawings

The useful discipline is to state the surcharge the wall was designed for, in numbers, on the drawing.

A wall designed for a light landscaping load and a wall designed for vehicle loading are different walls, and once the drawing is filed, nobody can tell which one was built. Stating it means the next person who wants to park behind it has an answer rather than an assumption, which is the same reasoning behind recording design loads for any element, as in [strengthening an existing slab for a new load](/blog/strengthening-an-existing-slab-for-a-new-load).

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