Top Retaining Walls Christchurch builds the drainage system that decides whether your retaining wall is still straight in twenty years. Nearly every failed wall we get called to in Christchurch failed for the same reason. Water sat behind it, the soil got heavy, and the wall slowly gave way.
BRANZ recommends a minimum of 300mm of clean metal or gravel immediately behind the wall. We lay the first 150mm along the base, sit the perforated coil on top, then cover the coil with another 250mm of metal. Geotextile cloth wraps the top and back so fine soil cannot migrate in.
The coil then has to go somewhere. To daylight at the low end where the site allows, or into stormwater where it does not. A coil that runs into a dead end is worse than no coil at all, because it collects water and holds it against the wall.
| Metal | Minimum 300mm of clean free draining metal behind the wall |
|---|---|
| Coil | 110mm perforated drain coil at the base, full length |
| Cover | 150mm under the coil, 250mm over it |
| Geotextile | Wrapped over the top and back of the drainage zone |
| Discharge | To daylight or into stormwater, never a dead end |
| Maintenance | Rod or jet the line if flow drops after heavy rain |
| Retrofit | Possible on walls that lean but have not failed |
Dry soil is heavy. Wet soil is far heavier, and saturated soil adds water pressure on top of that. A wall designed for dry conditions can be carrying twice what it was meant to after a wet Canterbury winter.
Drainage is not an upgrade. It is the thing that keeps the wall inside the load it was designed for, which is why we quote it as part of the wall rather than as a line item you can delete.
Out east through New Brighton, Bexley, Aranui and Kaiapoi, the water table often sits within a metre or so of the surface. Finding fall for a drain coil on a flat section with a high water table takes planning, and sometimes we have to run the line much further than you would expect.
On the Port Hills the opposite problem shows up. There is plenty of fall, but loess soil loses strength when it gets wet, so uncontrolled water in the ground is a slope stability issue, not just a wall issue. We are careful about where the discharge point goes.
If your wall is tilting but not failing, we can sometimes excavate behind it, install a proper drainage layer and coil, and give the wall another twenty years. It costs a fraction of a rebuild.
If the posts have rotted at ground line or the wall has moved a long way, drainage will not save it. We tell you which situation you are in at the site visit, and we will say so even when the answer is the cheaper one.
Every wall we build gets drainage. At least 300mm of clean free draining metal, a perforated coil at the base running to a real outlet, and geotextile over the top so silt cannot block it. It is not an optional line on our quotes.
Around 15 to 25 percent of the materials on a typical job. Rebuilding a failed wall costs the whole job again, so it is the cheapest insurance on the quote.
Often yes, by excavating behind and installing the metal, coil and geotextile properly. Whether it is worth doing depends on the condition of the wall itself.
Check it after heavy rain and clear the outlet. If flow drops away over the years, the line can be rodded or jetted. Most people never touch it, and most blocked coils were never wrapped in geotextile in the first place.
Our old wall was rotten right through and leaning into the neighbour's place. They pulled the lot out, put proper drainage behind the new one and had it finished in three days. Swept the site clean every afternoon before they left too. Tidiest crew we have ever had here by a long way.
They sorted the consent, which was the bit I was dreading. Dealt with the council so I never had to, and kept me in the loop the whole way through. The driveway sits right above it and has not shifted a millimetre.
Steep section, shocking access, and they still made it look easy. They suggested two smaller walls instead of one big one, which saved us thousands in engineering fees. The terrace in between is now my favourite part of the whole garden.
Call [PHONE] or answer three quick questions. The site visit and the written quote cost you nothing.
