Issue
Drainage Control undertook an inspection of this property on a moderately steep hillside site. The problem reported by the homeowner was that the ground underfoot at the rear of the house was very wet. This was near the back entrance to the house which was the main access in use. The washing line was in this part of the property as well. The flat area to the rear (About 3 metres in depth) was terminated by a steep bank, partially retained, which flattened out to a still steep plot above that, which rose consistently to the property boundary. This wet area was the main outdoor useable space at the property, and thus affected the amenity enjoyment of the owner.
Solution
The steep area above was dry. This was not the source of the problem. Wellington is as complex underground as you would expect in an earthquake prone precinct. Full of distressed rock and various sorts of clay in the hills around. It often surprises people that spring water can be found on steep hillsides, or suddenly begins where before there was nothing. Such was the case here. A minor problem had become much worse. Why? Really hard to say – we are not geologists, but water will follow the path of least resistance – had an earthquake realigned these paths? Maybe. Clearly fresh water supply was not an issue here, as topology would lead it away from this particular property. If in doubt, a simple test for fluoride will settle the issue.
This is not an unusual occurrence in our experience, and the solution was to intercept this groundwater at the boundary of the flat yard at the base of the retained wall and lead it to the existing stormwater drains on the property which we determined were serviceable into the future. A second line of defense would be installed closer to the
house beside a concrete path that ran alongside the house and past it to the washing line to one side.
We emplaced a novacoil perforated drain under the retained wall wrapped with geocloth and filled with drainage chip with a layer of good friable topsoil above which would allow easy access by groundwater to the drain.
We also fitted the drain with rodding holes for maintenance and cleaning. In the process, we uncovered an existing unmaintained novacoil system closer to the house, which was simple to renovate and return to service – the more the merrier!
Alongside the path, we fitted a channel and grate drain running the full length of the path to remove any water that got past the Novacoil drain, and to prevent the path from acting as a barrier to escaping groundwater.
This was the hard bit, as the run was long and flat, which meant the channel and grate system needed to be emplaced with precision so that there was a gravity run to the new silt sump we installed before connecting to the existing stormwater system.
Finally, all waste material was removed to an approved disposal facility and the lawn re-sown.
Observations
As things go, this was not a particularly difficult job, and we fully expected a good result. Indeed, improvement was immediate, and the yard had completely dried out well before we were finished to the delight of the homeowners, who were already planning a new lease of life for their backyard as a lawn for lounging/entertaining.
Outcome
Before we were finished, the back yard of this property was dry again, and the homeowners were interested in the project and how it worked, and more importantly, how they could maintain it. Due to this, we are reassured that they will be able to enjoy their backyard for the foreseeable future.