With the original plans established when Ashburton was first laid out in 1864, the lake within the Ashburton Domain was the result of two dams being created when bringing water in for the town. As Ashburton grew, the Domain matured, catering for the public’s modernised needs. The lake is now nestled amongst recreational gardens and sports pavilions, but repair and strengthening work to the failing tilt slab concrete wall surrounding the lake is now required as part of a modernisation programme set out for the entire Domain.
With the current wall and the sides of the pond eroding away, Ashburton District Council required a cost-effective, easy-to install solution that would last another hundred years and could be built without disturbing existing mature trees. There was an initial plan to reinstate the wall with half ton rocks, but this was set aside once Cirtex and the contractors showed the Council what long-term benefits a MagnumStone gravity wall could offer instead.
DESIGN
With no tidal surges or active loading from pathways to cater for, two 100m long MagnumStone gravity walls were designed to extend either side of the newly planted Southern Corner Plaza. Two layers of blocks would be placed, retaining the wall to a height of 1.2m, with grass to be installed right to the edge. With large tree roots nearby, Tarbotton Land and Civil decided to add small extenders to the bottom row of blocks, future-proofing any possible increased loadings put upon the wall over its proposed design life.
The embedded design would allow the wall to curve around the mature trees, creating an aesthetic beauty to the lake, with the grassed top sitting approximately 200mm above the waterline. This would give ducks and other local wildlife easy access to the lake and shoreline. The MagnumStone blocks would also remain stable if ever the lake needed to be drained.
INSTALLATION
The lake was initially pumped out until the water table was 200mm below the surface. To reach the retained levels required, the gravel footing was embedded 400mm into the ground. All soft material was removed, with the footing filled with GAP65 and MDM tested to the required specifications. With the platform laid, blocks were stored locally offsite and brought in as required. MagnumStone can be handled and placed with small excavators, which made installation ideal when working on this tight site beneath branches and around the mature trees.
MagnumStone’s design allows for easy placement of inside and outside curves, with the extenders simply slotting in behind. The area was then backfilled with a compacted layer of drainage aggregate, then wrapped with geotextile. MagnumStone’s standard blocks were used on the top layer, which allowed grass to be laid right to the edge of the wall.
Tarbotton Land and Civil found MagnumStone to be strong and easy to install, with Ashburton District Council now having an aesthetically pleasing, cost-effective solution to help further develop one of their town’s most iconic landmarks.
The MagnumStone system provided the following benefits to the project:
- Can be used as retaining in water application.
- Easy to install using small excavators and machinery.
- Aesthetically pleasing finish with simple curved designs.
