Issue
Drainage Control undertook an inspection of a property in Brooklyn, Wellington, at the request of the property owner. The issue was with the basement level of this property which houses 3 bedrooms, bathroom, and toilet as well as an internal storeroom and stair access to the upper level. The area behind the rooms had issues with water ingress during periods of high rainfall. Water pools at the back of the internal storeroom and bathroom area.
The basement level has been constructed up against a clay bank without any proper drainage for getting this water away. It had become an issue for the tenants. There was some drainage installed in the past to the accessible part of the basement level behind the back wall and in front of the clay bank, which had been diverted into a small sump and drained by a submersible pump which was connected to the exterior stormwater drain by a makeshift pipe. This only covered about a third of the wall behind the basement level enabling stormwater and ground water to pond and access the inhabited part of the basement level. Further access to this area was denied as the clay bank at this point abutted the back wall of the basement level.
Solution
Drainage Control had to access the full length of the basement level to install new drainage to deny access by any liquid to the basement level. This meant that the clay bank had to be excavated so that a trench well below the ground level of the structure could be formed in which to lay comprehensive drainage, and this was done. This clay was very hard, so it was probably not done in the past due to the difficult access and the quantity of material that needed removal. This was accomplished with the use of power tools and hand carrying the spoil to a staging area in the back yard for later removal to an approved landfill.
Once we had this trench formed with a gravity fall to a collection point, a subsoil drain was installed consisting of 100mm perforated novacoil pipe wrapped in geocloth. A larger silt sump was installed at the collection point and the Novacoil drains from each end of the basement level connected to it. The trench was then backfilled with drainage chip to the new ground level established by our excavations.
A new and more robust submersible pump was then emplaced and connected to the power, and the pipework from the pump to the house stormwater drain was replaced to meet drainage and plumbing requirements. The subsoil drain has covered ground level access points fitted at both ends for future cleaning purposes.
Observations
It is understandable, but certainly not desirable, that due to the difficult access and clay so hard, a kanga hammer was deployed to help break it out, this was not a comprehensive drain as originally installed. Something of the order of 5 cubic metres of clay was extracted by hand from the basement area, and replaced with 2 cubic metres of drainage chip.
Outcome
The property now has comprehensive basement drainage with sufficient capacity to deal with all foreseeable future requirements. The owners of this property can now make improvements as required to the habitable areas of the basement level confident that their efforts will not be sabotaged by water damage.
Please get in touch with me if you require further clarification.
