A driveway out at Terrace End had a grid of hairline cracks running in two parallel lines the exact width of a tractor’s rear wheels, right where the owner had been backing a loaded trailer in and parking it for the winter. The slab was standard 100 mm residential concrete, poured to a spec that assumes cars and the occasional light trailer, not a few tonnes sitting on two narrow contact points every day for months. That is the gap that catches a lot of Palmerston North sections out: a driveway built to ordinary domestic loading, then asked to carry farm machinery or a heavy trailer because the section backs onto paddock or a rural block.
What “heavy vehicle” actually means for a slab
A standard passenger car spreads its weight across four reasonably wide tyre contact patches and rarely exceeds about 1.5 to 2 tonnes. A loaded box trailer, a small digger, a tractor or a truck concentrates far more weight onto much smaller contact areas, sometimes with a single axle carrying several tonnes. The concrete does not fail from the total weight so much as from the load concentrated at a point, repeated in the same spot week after week, which is exactly what produces that parallel cracking pattern rather than one clean fracture.
Thickness and reinforcing for the load, not just the vehicle
For ordinary car use, 100 mm of concrete over a compacted base is the usual residential standard. Once a driveway needs to carry regular heavy vehicle traffic, whether that is a truck delivering firewood, a tractor, or machinery moving between a rural block and the section, the slab generally needs to go to 125 to 150 mm, with reinforcing mesh through the middle of the pour rather than left out, and in some cases thickened edge beams where a wheel path runs close to the edge of the slab. On the alluvial flat ground around Terrace End, where the underlying soil is generally reasonable but not rock-hard, that extra thickness and reinforcing matters more than it would on a firmer gravel base, because a thin slab flexes slightly under a heavy point load and repeated flexing is what starts the cracking.
The base course matters as much as the slab on top
A thicker slab poured over an under-compacted base does not solve the problem; it just delays it. On flat river-terrace ground like this, a base of at least 150 mm of well-compacted metal, properly rolled rather than just spread and left, gives the slab even support across its width. Where the ground has any soft patches, especially near an old drainage line or a filled-in ditch, it is worth having those probed and properly re-compacted before the pour rather than assuming the metal base alone will bridge over a soft spot.
Working out where the heavy loading will actually sit
It is worth mapping out, before the forms go in, exactly where a trailer will be parked, where a truck will manoeuvre, or where machinery will cross the driveway regularly, because a targeted thickened strip along that specific path is considerably cheaper than upgrading the whole slab to the heaviest possible spec. A full driveway built to handle regular heavy loading generally runs somewhere between $220 and $420 per square metre once the extra thickness, mesh and base depth are priced in, against $180 to $320 for a standard car-only driveway of the same size.
Getting the thickness, reinforcing and base matched to what will actually cross the driveway, rather than to a generic residential spec, is worth raising directly with a Palmerston North driveway contractor at the quoting stage, since it is far cheaper to build in the extra strength than to break out and repour a cracked slab later.
Frequently asked questions
How thick should a driveway be for a tractor or heavy trailer? Generally 125 to 150 mm with reinforcing mesh, compared with the standard 100 mm used for ordinary car traffic.
Why does a driveway crack in parallel lines rather than randomly? That pattern usually follows a repeated wheel or load path, where concentrated point loading has flexed the same strip of concrete over and over.
Does the base course need to change for heavy vehicle loading? Yes, a deeper, well-compacted base of at least 150 mm of metal is needed to support a thicker slab evenly, especially over flat alluvial ground.