Vista Grove Sewer Reinstatement
Background to Resource Consent Application and Description of Proposal to Reinstate the Slip and Sewer Mains.
Background
On 7 August 2006 after heavy rainfall throughout July 2006 a large slip occurred in the rear of properties at No 6, No 8 and No 10 Vista Grove and No 7 and No 9 Levin Grove. As a result of this slip a 40m section of 225mm public sewer main running through No 6 Vista Grove and No 7 Levin Grove was removed together with 10m of a 150mm public sewer main that intersected with the 225mm sewer main within No 7 Levin Grove. In addition a large portion of the house at No 6 Vista Grove at the top of the slip had to be demolished. The rumpus room at the rear of the property of No 6 Vista Grove has also since been removed.
A comparison with old contour plans pre-development of the subdivision determined that the gully had been filled and that the current 11m high slip face is composed of fill material. There is a large area of fill remaining to the north and there is also an area of fill remaining to the west adjacent to No 7 and No 9 Levin Grove. Investigations determined that the fill material is quite saturated and that there is subsurface groundwater flowing through it. Uncontrolled stormwater from the natural catchment above the slip naturally drains down the gully and there is evidence of surface drainage problems on a number of the properties.The sewer mains have been temporarily reinstated and investigations carried out to assess the reinstatement of the slip and the sewer main.
Letter to residents of Vista and Levin Groves, Kelson, from HCC Chief Executive, Tony Stallinger, 19 October 2007
(PDF 11kb)
Proposed Reinstatement of the Slip and Sewer Mains
The proposal to reinstate the slip and the sewer mains is to construct a new reinforced fill in the place of the previous fill and relay the sewer mains within the fill as previously, this proposal is shown on the attached plans, numbered 51-24457-C051 (PDF 266Kb) and 51-24457-C052 (PDF 113Kb).
The main points describing this proposal are :
- The toe slope of the fill would be unreinforced reusing existing selected onsite material at a slope of 2.5 horizontal : 1 vertical, with the main body of the fill being selected imported material and geogrid reinforced at a slope of 1.25 horizontal : 1 vertical.
Surplus existing onsite material and unsuitable material would be removed from the site. - After removal of unsuitable material from the site, the foundation surface of the proposed filling would be benched into the existing surface material.
- Subsoil drainage is to be constructed beneath, within and around the filling to enable any subsurface groundwater to be collected and removed from the fill. A drainage layer is to be placed between the main face of the slip and the imported fill, connected to the subsoil drainage to capture water flowing through the drainage layer.
- A stormwater cut-off drain is to be constructed above the fill along the boundary of 6 and 4 Vista Grove. This will have the dual purpose of providing for cut-off and collection of subsurface groundwater at this point and providing additional surface stormwater drainage control for the properties adjacent to the cut-off drain.
- Separate stormwater collector drains would be constructed up each side of the filling to collect direct stormwater discharges from the adjacent properties, this is to prevent these discharges entering the new fill.
- The final surface of the unreinforced toe slope would where possible be planted with shrubs/ trees etc. The geogrid reinforced slope would be hydroseeded.
Drawing 2 (PDF file 113 Kb)