First Gas owns and operates the 400 Line (Maui) which supplies natural gas from Taranaki to the Waikato while the Maui pipeline supplies gas to the Hamilton and Central North Island regions. Historic and more recent instability of the Maui pipeline easement fill occurred at two sites near Awakau Road. These instability features were located on the steep slopes to the side of the pipeline. This project’s solution included installation of soil nails and steel wire mesh to stabilise the slope instability areas at both Awakau sites and an area of shotcrete on one of the sites to prevent further deterioration of a timber crib wall.
During the tender stage, GeoStabilization New Zealand Ltd (GeoStabilization NZ) identified a number of potential construction risks. Our in-house design engineers spent considerable time improving the conforming design and developing design and construction optimisations to reduce the construction risks. The optimised designs allowed for a reduction in the level of construction, reduced time on site, reduced costs and improved pipeline accessibility for future inspection and maintenance work.
The key design enhancements, particularly on Site 2, removed the need to undertake extensive steep slope earthworks. They also removed the risk of light-weight drilling equipment penetrating through tree roots. In place of the soil nails, mesh and earthworks on the slope tied back the micropile wall with a Geosynthetically Confined Soil® retaining structure and kept all treatment close to the crest of the slope. These changes resulted in significant time and cost savings.
Following a series of sub-tropical rainstorm events, Kawakawa and the wider Auckland region suffered extensive landslip issues. Many roads in the region developed both underslips and overslips, severely affecting the flow of traffic.