The crack in the garage floor was only 4 mm wide, but it ran arrow-straight from the roller door corner back toward the house, and it lined up almost exactly with a line I could see in the section’s old survey plan — the point where the original ground had been cut back half a metre to level the building pad. The agent wanted to call it shrinkage. I’d already probed both sides of that line with a hand auger, and the fill side gave way another 40 mm before hitting anything firm. That’s not shrinkage. That’s eleven years of a cut-and-fill pad settling unevenly, and the slab had been quietly telling everyone about it the whole time.
Most flat sections in the hill suburbs and river-terrace areas around Christchurch weren’t flat to begin with. Builders cut into the higher ground and pushed that material sideways to fill the low end, then poured the slab across both. The cut side sits on original, compacted ground. The fill side sits on material that was never going to compact to the same density unless it was placed and rolled in proper layers with testing at each stage — and on a lot of older pads, it wasn’t. Over years, the fill consolidates under its own weight and the load of the house, while the cut side barely moves. The result is a slab that’s effectively hinging along the old cut line. You see it first as a crack, then as a door that won’t latch, then as a gap opening between the slab edge and the cladding.
A straight crack that tracks the boundary between obviously different ground conditions is a different animal from the fine, random shrinkage cracking you get across an entire pour. I look at three things before I write anything in the report: whether the crack direction matches a plausible cut-fill boundary, whether there’s a step you can feel with a straightedge across the crack (anything over about 3 mm of vertical offset is worth flagging), and whether nearby doors or windows show racking that agrees with the same line. On post-quake Christchurch sections this gets an extra layer — TC1, TC2 and TC3 land classifications tell you something about liquefaction risk generally, but they don’t tell you what a specific pad’s fill was doing before the earthquakes even happened. A section can be TC1 and still have a badly placed fill pad from 1998.
Between the liquefaction history, the EQC-era repairs some of these properties went through, and the number of sections that were re-levelled or partially rebuilt after 2011, Christchurch has more cut-and-fill pads with an undocumented history than most cities its size. A LIM report will sometimes show consented earthworks or a geotechnical completion certificate, and sometimes it shows nothing at all because the work predates the file or was never consented. When the LIM is silent and the slab is talking, I trust the slab. Repair costs for a genuine cut-fill settlement issue vary enormously depending on how much of the pad is affected, but re-levelling a garage-sized section of slab with underpinning or engineered fill replacement typically lands somewhere between NZD 8,000 and NZD 25,000 once you include the concrete work and reinstating services.
My job stops at describing what’s happening and why, with enough detail that a structural engineer can pick it up without redoing my measurements. But once the settlement is confirmed as a cut-fill issue rather than surface shrinkage, the next step usually isn’t the engineer alone — it’s someone who can actually re-profile the ground, place compacted fill correctly, and get a compaction test signed off before anything gets poured over the top again. That’s specialist earthworks, not general concreting, and getting it wrong twice on the same section is expensive. Buyers who go ahead on a property like this are generally better served getting a quote from earthworks contractors in Christchurch before they finalise a negotiated price, because the earthworks scope changes the whole repair budget, not just the slab line item.
Is a diagonal slab crack always a cut-fill problem? No. Plumbing trench backfill, tree root activity and simple pour shrinkage can all produce a similar-looking crack. The giveaway is whether the line matches a plausible old ground contour and whether there’s measurable vertical offset across it.
Will a LIM report show the original cut-and-fill work? Only if it was consented and the file has survived. Many older pads, especially pre-1990s builds, have no earthworks record at all, which is why a physical inspection matters more than the paper trail on these sections.
How much does this typically knock off a negotiated price? It depends on severity, but buyers commonly factor in the full remediation estimate plus a contingency of 10 to 20 percent, since cut-fill issues rarely stay confined to exactly the area you can see.