Anchorage Grove, Tauranga – MagnumStone™ Gravity Wall

At the bottom of Anchorage Grove, a small recreational reserve lines the edge of the Waimapu Estuary, connecting the local Maungatapu Peninsula community with the Tauranga Harbour. Over time, tidal forces have caused the existing riprap and concrete sea wall to disintegrate to a point where it was beginning to collapse into the sea. While the existing sea wall incorporated stairs and a small access ramp used for kayaks and other lightweight marine equipment, the reserve also contains an underground Council sewage pump station, which Tauranga City Council desired to protect.

To withstand the tidal pressure and give greater protection to the pump station, Tauranga City Council required a new sea wall solution that could be built efficiently during low tide within a tight site without the need for heavy machinery.

DESIGN

A tiered MagnumStone gravity wall was designed to replace the old wall, extending along the width of the reserve between two existing timber pole walls. As there is no vehicle access to the boat ramp, no extra loadings were required. Designed to be embedded into the marine mud and placed on a solid aggregate footing, the 1.5m high bottom tier will deflect approximately 300mm of normal high-tide surges while remaining exposed at low tide. The second wall will be staggered back 1.5m and retained to a height of 1.2m, retaining the general slope of the reserve. Both walls will incorporate the stair design and access point for kayakers and other recreational users. The design will also follow the natural curve of the reserve, creating non 90° corners around the stairs and a curved wall around the lightweight boat ramp.

INSTALLATION

With the construction programme set around low tide, specific sections of the existing wall were first removed with all debris cleared away, depending on which part of the wall was to be placed. Once a stable footing had been embedded into the marine mud, Higgins began construction of the stairs. This was done to ensure the correct placement and alignment of the stairs before extending the blocks either side. With the compacted aggregate footing and cut surface wrapped in geotextile, aggregate was then placed and wrapped before the removal and construction of each adjacent section could begin.

On the staircase construction where the corners were greater that 90°, corner blocks were secured onto the adjacent MagnumStone blocks to maintain the correct fascia alignment. A concrete filler was then poured within the extended angle to tie each corner block in place.

MagnumStone provided a versatile construction method that could quickly be placed at low tide, maintaining durability and aesthetic appeal at both high tide and when exposed. The underground sewage pump is now further protected, with greater support given to those wishing to enjoy the reserve’s green spaces and utilise the newly built access ramp into Tauranga Harbour.

The MagnumStone system provided the following benefits to the project:

• Quick to construct, make it a great retaining solution for sea walls
• Able to withstand being submerged beneath tidal surges
• Can be constructed using relatively small machinery, ideal for tight sites