NZS 3604-type pile foundations
Generally, a pile foundation must be in good condition to properly support the structure. Piles that are misaligned, undermined by subfloor excavations, missing or irregularly spaced or not in full contact with the bearers should be repaired or replaced. Concrete piles that are cracked or crumbling and timber piles that are split, rotting or showing other signs of damage should be also repaired or replaced.
If all piles are leaning in the same direction by more than 15 mm per 1 m of height, this is an indication that they may have been stressed in an earthquake.
The overall bracing capacity of an existing suspended timber floor foundation should be reviewed against the provisions of NZS 3604:2011 Timber-framed buildings. This is to determine whether it is potentially weaker than a new design would be.
New residential properties built according to NZS 3604:2011 must be constructed with specific connections joining the foundation elements. In the case of concrete or timber pile foundations, each pile must be securely tied to the bearer it supports. This ensures the structure does not fall off the piles when exposed to lateral seismic forces.
In an existing building, a pile foundation may not be tied to the frame or the connections may have deteriorated or been removed. New connections can be retrofitted between the pile and bearers or floor joists. The pile-to-bearer connections should be either:
- corrosion-resistant wire fixed to the pile and bearer with staples (concrete piles), or
- Z-nails (wire dogs) and skewed nails (timber piles).
There are also proprietary fixings available to retrofit to existing timber piles, or new galvanised wires, appropriately stapled to the bearers, can be fixed to disconnected concrete piles.
Typical pile-to-bearer connections with concrete and timber piles.
State of foundations in Wellington
A 2007 study carried out by Victoria University of Wellington assessed the adequacy of 80 dwellings’ foundations in Wellington against the foundation requirements of NZS 3604:1999 Timber-framed buildings. The study reported that:
- 39 percent had inadequate subfloor bracing
- 16 percent relied solely on the strength of ordinary piles
- 11 percent relied entirely on large concrete anchors, such as concrete steps and chimney foundations
- 76 percent had some form of fixing deficiency, ranging from degradation to incorrect or non-existent fixings.