We opened a batter on a Brooklyn job in dry weather and it stood at a steeper angle than the drawings called for, clean and firm, for the better part of a week. Nobody touched it during that week. Four days after a 60 millimetre rain event, with the excavation still sitting open over a public holiday, a metre-wide slab of it let go and slid into the base of the cut. The site had not moved during the rain itself. It moved after, once the water that had gone in during the storm had time to work through the clay and reach the plane where it was going to fail. That delay is the single most important thing to understand about how Wellington’s clay behaves once it gets wet.
What the clay actually is
Most of the hill suburbs sit on weathered greywacke, a hard sedimentary rock that breaks down over time into a fine, plastic clay as it weathers near the surface. That clay shrinks as it dries out over summer, opening fine cracks, and swells again as it takes on water over winter. The swelling is not just a volume change — it is also a strength change. Clay that is strong enough to stand at a steep angle when dry can lose a large share of its shear strength once its moisture content rises, because the water pressure between the clay particles reduces how tightly they can grip against each other under load.
Why the slide comes after the rain, not during it
Water does not move through clay quickly. Rain that falls during a storm has to work its way down through cracks, root channels and any looser fill above the clay before it reaches the depth where it actually changes the strength of the ground that matters structurally. That is why the failure I saw in Brooklyn showed up days after the rain stopped, not during the downpour itself, and it is why site staff sometimes get a false sense of security watching a slope come through a storm intact, only for it to fail on a clear day once the water has had time to reach depth. Anyone assessing risk on a clay slope needs to think in terms of the days following heavy rain, not just the rain event itself.
Safe angles for a temporary cut
For short-term excavations in this kind of clay, a batter of around 1:1, or 45 degrees, is a common starting point in dry, undisturbed material, but that number drops fast once the clay has been rained on or left open for more than a few days. On the job above, the specified batter was steeper than 1:1 because it was meant to be backfilled within 48 hours. Once the timeline slipped, the ground conditions the batter angle relied on had already changed. Anyone leaving a clay excavation open through a wet spell should either flatten the batter, cover the face, or accept that the original angle is no longer the one they are actually working with.
What drainage does and does not fix on clay
Surface drains that intercept water before it reaches a clay slope help, and cutting a diversion swale above an excavation or a bank is genuinely useful. What does not help much on its own is a single subsoil pipe laid at the base of a slope, because clay is slow to release water into a drain even when one is present — the water has to migrate through the clay to reach the pipe, which can take longer than the storm cycle that caused the problem. Regional council guidance on stormwater and land management reflects this by focusing on keeping water off a slope in the first place rather than relying on drainage lower down to remove it once it is already in the ground. Where subsoil drainage still has a role, it is as one part of a system, alongside surface interception, not as a stand-alone fix, in the same way a steep section’s stormwater needs a controlled path for the whole descent, not just a pipe at the bottom.
Frequently asked questions
Why did a slope fail days after the rain stopped, not during it? Water takes time to move down through clay to the depth where it reduces shear strength, so the ground often stays intact through the storm itself and fails once that delayed strength loss reaches the critical layer.
What batter angle is safe for a temporary excavation in Wellington clay? Around 1:1, or 45 degrees, is a common figure for dry, undisturbed clay, but this needs to be reduced if the cut will stay open through wet weather or has already been rained on.
Can a subsoil drain alone stop a clay slope from slipping? Not reliably on its own, since clay releases water into a drain slowly; intercepting surface water before it reaches the slope is usually more effective than trying to remove water after it has already soaked in.