Underpinning for Residential Construction in Australia

What it is

Underpinning is a remediation method that transfers the load of an existing building from a footing that is failing onto a new footing taken down to competent ground. It is not a fresh build foundation, it is a repair to a structure that has already moved. Subsidence cracks, sticking doors, separated cornices and gaps between the slab and external paving are the usual signs that bring an engineer to inspect.

In residential work across Australia, underpinning is required when a footing has lost bearing. The common causes are tree root drying of reactive clay, leaking stormwater or sewer pipes, fill that was never engineered, cut and fill on sloping blocks and adjacent excavation by a neighbour. The structural engineer determines the cause first, because underpinning a building above an active leak is wasted money.

When underpinning is required

A geotechnical and structural assessment under AS 2870 and AS 2159 sits behind every underpinning design. The engineer needs:

• A measured settlement survey of the affected walls.
• Bore logs to the proposed founding stratum.
• A moisture profile through the active zone of any reactive clay.
• Confirmation that the cause of movement has been removed.

If those four items are not in the report, the underpinning design has no defensible basis and the inspector should reject the system before any excavation starts.

Mass concrete versus mini pile

Mass concrete underpinning

Mass concrete is the traditional method. Sections of footing one metre long are hand dug to the new founding depth in a hit and miss sequence. Each pit is poured with low slump concrete, dry packed at the top to lock against the existing footing once cured. Mass concrete suits shallow founding depths up to about three metres and stable ground that holds an open pit overnight. It is labour intensive and slow but the equipment footprint is small, which matters on tight inner city blocks.

Mini pile underpinning

Mini piles are small diameter steel or concrete piles installed through or beside the existing footing using a low headroom rig. Capacity is taken to a deeper competent layer than mass concrete can practically reach. Mini piles suit collapsing soils where an open pit is unsafe, deep founding strata beyond five metres and buildings where occupancy must continue during the works. The pile head is then connected to the existing footing through a needle beam or a bracket detail.

The choice between methods is engineering driven and not a contractor preference. An inspector should see a written justification for the chosen method in the design report.

Consent, licensing and insurance

Underpinning is licensable work in every state. In NSW it falls under the underpinning and piering work category administered by NSW Fair Trading. In Queensland it is captured by the QBCC licence classes for foundation work. In Victoria the Victorian Building Authority registers practitioners who carry out structural work on existing buildings. The principal builder must hold the correct class and the underpinning subcontractor must hold it too if engaged direct.

Domestic Building Insurance under the Victorian scheme administered by the Victorian Managed Insurance Authority applies to underpinning contracts above the state threshold. Equivalent statutory home warranty cover applies in NSW under icare and in Queensland under the QBCC Home Warranty Scheme. The work is high risk and the policies are scrutinised closely, so the paperwork has to be straight before site mobilisation.

Inspection hold points for TradeLens

Four hold points cover the critical path on a typical underpinning job:

1. Pre dig inspection: cause of movement confirmed as remediated, services located, dilapidation report of the building and the neighbour signed off.
2. First pit or first pile inspection: depth, founding stratum and reinforcement before concrete or grout placement.
3. Dry pack inspection on mass concrete or needle beam connection on mini piles, witnessed before any backfill.
4. Post complete level survey, repeated at one and three months, to confirm the building is stable.

Common defects

Recurring defects include pits dug deeper than the shoring allows leading to face collapse, dry pack omitted or done with the wrong mix so the new footing never picks up the load, mini piles installed off line so the eccentricity overloads the bracket, the cause of movement left active so the building keeps settling around the repair and missing notification to the adjoining owner where party wall obligations apply.