North Otago Irrigation Company

North Otago Irrigation Company


Client: Sub-Contracted to McConnell Dowell Constructors Limited for pipeline installation
Contract value: $1.4M
Contract Period: Jan – May 2016

Trench, lay, and backfill 3635m of 1200mm diameter GRP (glass fibre reinforced plastic) irrigation main, including 11 horizontal and 26 vertical bends, 5 air valves and a drain valve.

Technical Difficulty:
The pipeline traverses across rolling farmland from the Headpond on Ngapara Georgetown Road to the Paradise Gully main, adjacent to Weston Ngapara Road.

As it closely follows the alignment of the Stage 1 main, care must be taken to maintain cover to this main to maintain its integrity.

Work is confined to a 30m wide corridor which, combined with a trench up to 5m deep and 15m wide at the top, doesn’t leave much space for stockpiling topsoil and spoil, and for a platform to transport and lay pipe.

Where the grade exceeds 1 in 5 cut-off blocks are constructed to both anchor the pipeline and to help contain the bedding material, which is also wrapped in geotextile to stop the groundwater flow from displacing this material.

Trenching is generally through loess and clay based material that poses few problems. However there are outcrops of soft rock requiring ripping, and through the valley floors groundwater and saturated clays have been encountered. This has necessitated constructing raft foundations, consisting of Geo-tensar grid and crushed rock, to support the pipeline. The raft foundation and the pipe are wrapped in geotextile, again to contain the bedding material.