Soil Nailing And Anchored Retaining Walls

On a tight Sydney site, a deep excavation frequently has no room for a propped wall and no room for the batter a cantilever would need.

The support then has to come from the ground behind the wall, and there are two ways to get it.

Soil nailing

Steel bars installed into the face of an excavation as it is cut down, grouted into drilled holes, with a facing applied over them.

The nails are passive. They are not tensioned at installation. They develop force as the ground behind tries to move, and they hold it by friction along their length.

The method suits ground that can stand unsupported for a short period while each lift is cut and nailed. Cemented sands, stiff clays and weathered rock generally do. Loose sands and soft clays generally do not, because the face collapses before the nail is in.

The great advantage is that it is built top down with the excavation, so there is no separate wall to install first and no props across the site.

Ground anchors

A tendon installed through a wall into a bond zone well behind the failure surface, then stressed against the wall and locked off.

Anchors are active. They apply load immediately, which means the wall moves less, and on a site with structures close behind that matters more than anything.

They are used with contiguous pile walls, secant walls and diaphragm walls, where the wall is installed first and then excavated in front of in stages, anchoring at each level.

Choosing between them

Ground conditions decide most of it. If the face will not stand, soil nailing is out.

Movement tolerance. Where a neighbouring building sits close to the excavation, the smaller movements that come with stressed anchors are frequently the deciding factor. That interacts directly with the [excavation near existing structures](/blog/excavation-near-existing-structures-best-practices) question.

Water. A high water table generally pushes the answer toward a wall system with anchors rather than a nailed face, and it brings the [hydrostatic uplift](/blog/basement-tanking-and-hydrostatic-uplift) question with it.

Permanence. A temporary works solution and a permanent one carry different durability requirements. A permanent anchor needs double corrosion protection and a permanent nail needs its own protection, which changes the cost a great deal.

Both go outside the boundary

This is the practical issue that catches developers.

Nails and anchors extend well beyond the wall, which means they extend under the neighbouring property or under the road.

That requires an easement, a licence or a formal agreement with the adjoining owner or the road authority before installation. It is negotiated, it takes time, and occasionally it is refused.

Where it is refused, the design has to change to something self-supporting, which is usually a propped wall or a thicker cantilever, and that decision is far cheaper made at design stage than after the piling rig is on site.

Temporary anchors are sometimes destressed and abandoned in place at completion, and whether that is acceptable to the neighbour is part of the same agreement.

Testing is not optional

Anchors are proof tested. A proportion are tested to a load above their working load, and every anchor is stressed and locked off to a recorded value.

Soil nails are also tested, on sacrificial nails installed for that purpose, to confirm the bond assumed in design actually exists in the ground on this site.

Design values for ground bond come from correlations and from experience. Testing is what converts them into something you can rely on, and it is why the [geotechnical investigation](/blog/understanding-geotechnical-investigation-reports) has to be adequate before any of this is designed.

Monitoring during construction

Survey targets on the wall and on any structure behind it, read at a set frequency, against trigger levels agreed before work starts.

Movement is expected. Movement beyond the trigger is a stop and review. Setting those levels in advance is what makes the decision straightforward on the day.

ACSES provides geotechnical engineering and temporary works design for excavation and retention across Sydney. Talk to us about a site.