Nanako Stream Stormwater Upgrade

MAP was engaged by Tauranga City Council in a multi staged ECI with the aim to effectively manage stormwater catchments, mitigate potential flooding events, improve water quality and accommodate future planned developments in the area. The completed B1/2 Separable Portion involved upgrading the existing Kennedy Road embankment to meet current dam safety design standards, reduce the risk of dam failure and provide flood mitigation. The project installed a 2.5m dia. culvert beneath the existing road and embankment and 1m low flow pipe connected by a reinforced concrete chamber with upstream and downstream buttresses constructed.

Earthworks: Over 4,000m3 of surcharge pre-load material was placed on the upstream and downstream buttress areas and the settlement monitored until the required consolidation had occurred. This was removed for the construction of the 2.5m culvert pipe and 1m low flow pipe and then replaced for the final construction of the buttresses to the required levels and design. On the downstream section this also involved the installation of a 450m3 F2 filter layer 1m thick between the natural subgrade and the buttress fill material, with the 2.5m pipe also bedded and surrounded with an F2 layer.
On the upstream side a 0.5m thick low permeable clay layer was placed including a cut off trench prior to topsoiling and grassing. The use of GPS enabled excavators and hand held GPS rover units ensured that the design levels and quantities of each element of the earthworks design could be accurately achieved.

Downstream shaping on the outlet structure was required to tie back into the existing stream and rip rap apron basin construction to specific levels for a rock ramp fish passage.

Stormwater and Concrete Structures: The first stage of the project involved the relocation and relaying of some of the existing stormwater infrastructure. This included removing existing 1050 concrete pipe sections and constructing new 1200 pipes and manholes with concrete encasement of some sections. A major element of the project was the thrusting of 39m of 2.5m steel pipe beneath the existing road and embankments. This was then connected to a further 12m of 2.5m Frank PKS pipe beneath the downstream buttress. On the upstream side a 1.0m Frank PKS low flow pipe was installed including the concrete encasement of some of this length.

Overall it was a very well delivered high quality project in difficult conditions, which the client was very happy with