Before my crew puts a digger on a Karori section, I walk it first, on foot, usually twice — once straight after we are asked to quote, and again the morning we are due to start, because a wet week in between can change what the ground is telling you. The section that sticks in my mind was a “gentle back garden” the owner described on the phone. On the ground it was closer to 28 degrees over the back third, with a line of small steps in the turf running across it that had nothing to do with any path anyone had ever built. Reading that correctly before a single scoop of soil moves is most of what keeps a job safe and on budget.
Measuring the slope properly, not by eye
A slope can be described three ways and they are easy to mix up: as a ratio such as 1:3, as a percentage, or as an angle in degrees. A 1:3 slope rises 1 metre for every 3 metres of horizontal run, which works out to about 33 percent and roughly 18 degrees. A 1:1 slope, 45 degrees, is far steeper than most people picture when they hear “one in one.” I carry a clinometer or an inclinometer app on my phone and take readings at several points across a section rather than trusting a single glance, because slope angle on a Wellington property is rarely constant — a bench cut for the house can sit at 5 degrees while the cut behind it runs at 30, and both numbers matter for what can be built where.
What the ground itself tells you about past movement
Small steps or terracettes in the turf, running roughly parallel across a slope, are one of the clearest signs that soil has crept downhill before, even slowly and even if nobody living there noticed it happening. Tree trunks that curve near the base before straightening up are doing the same thing in slow motion, correcting for ground that shifted under them years ago. A patch that stays visibly wetter than the rest of the slope long after rain has stopped elsewhere often marks a spot where subsurface water is being forced to the surface, frequently along the same plane a future slip would use. Fresh tension cracks running across the top of a slope, rather than along it, are the one sign that gets a geotechnical engineer called before anything else happens on site.
Telling fill from ground that has never been disturbed
A huge number of Wellington sections are built on old fill rather than natural ground, and treating the two the same is a common and expensive mistake. Undisturbed ground usually shows consistent colour and density as you dig into it, with roots and soil structure that make sense for the depth. Fill often gives itself away with mixed material at odd angles, buried debris such as old brick or timber, a colour change that does not follow a natural soil profile, or simply the fact that a flat bench sits where the original contour lines on an old survey plan show a continuous slope. None of that makes fill unusable, but it changes what it can safely carry and how it needs to be treated before anything goes on top of it. On sites where the ground is clay-based, which covers most of the hill suburbs, how that clay behaves once it takes on water is often the deciding factor in whether a slope that looks stable in a dry January stays that way through a wet winter.
When to stop and bring in a geotechnical engineer
My own rule of thumb is simple: any slope over about 20 degrees that is going to carry a structure, any site showing terracettes, tension cracks or an unusually wet patch, and any site where the fill history is unclear all get a geotechnical assessment before design work starts, not after. Independent building research guidance is consistent on this point — slope stability assessment belongs at the start of a project on sites like these, because a design based on assumed ground conditions has to be redone, at real cost, the moment a test pit or borehole shows something different underneath.
Frequently asked questions
How steep does a section need to be before it needs an engineer? As a general guide, anything approaching or exceeding 20 degrees that will carry a building, driveway or significant retaining should have a geotechnical assessment, though clear signs of past movement can trigger one at a lower angle too.
What is the easiest sign of a slope that has moved before? Small parallel steps in the turf, called terracettes, and tree trunks that curve near the base are both reliable visual clues that soil has crept downhill in the past.
How can I tell if my section is built on fill? Compare an old survey or contour plan against the current shape of the site, and look for mixed material, buried debris or an abrupt flat bench where the original ground would have sloped continuously.