Oborn, L.E. 1970 Engineering geological comment on the effect of the proposed Belmont Gully storm-water retention scheme on the no. 2 railway tunnel, Tawa Flat deviation. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 101 6 p.
Abstract: Truebridge, Callender and Beach, Consulting Engineers, acting for the local catchment authority have proposed that the motorway embarkment crossing Paparangi Stream (Belmont Gully) about 0.8 miles northeast of Johnsonville be used as a retention dam to impond floodwater and to delay its discharge into the remainder of the drainage system. It is planned that the 11 ft culvert under the motorway have a suitable control incorporated into its structure. It is proposed that with the design flood, the imponded water reach a height about 40 ft above the present stream level, and remain at this level for about 24 hours before slowly subsiding. The No.2 railway tunnel from Ngauranga to Glenside passes about 80 ft below the gully, 6-8 chains upstream from the culvert. The request for an engineering geological appraisal of the proposal came from Messrs. Truebridge, Callender and Beach, at the suggestion of the N.Z. Railways engineers who are reluctant to give their approval for this scheme unless there is convincing geological evidence supporting the contention that the railway tunnel would not be endangered, and that the flow of traffic through the tunnel would not be impeded. Subsurface information in the vicinity of the proposed reservoir is scarce. Few records of the tunnel's design and of the problems encountered during construction are available. Outcrops in the reservoir are few, and largely defended by vigorous growths of blackberry and gorse. There appears to be no useful information available from the motorway construction. (auth)