Kennedys Bush Road Widening – MSE + Anchored MagnumStone Walls

A developer needed to transform a steep winding farm track into a compliant access road to create 50 new high-end sections in Christchurch’s Halswell suburb. The project involved cutting into a steep slope and infilling a gully to achieve the design road levels. The Cirtex MagnumStone retaining wall system was chosen to retain the cut and fill earthworks due to its pleasing aesthetic, and its design flexibility.

DESIGN

Four MagnumStone retaining walls were designed to help strengthen and widen the road, ranging from 66m to 113m long. MagnumStone MSE walls using StrataGrid were used to support fill up to 3.6m in height on the downslope side of the road. The MSE wall was designed and installed using Site-won fill material leading to massive construction savings.

To minimise excavation the MagnumStone walls retaining cuts were designed as a reinforced concrete wall, supported by a single row of Platipus B6 anchors.

INSTALLATION

The wall over the gully was the first to be built. Working outwards from the lowest point around the culvert, the first row of blocks was placed on a compacted gravel level footing. Several rows of blocks with StrataGrid attached were laid, with the road slowly being built up as each compacted layer of engineered fill was placed. As the road can be used with heavy machinery once compacted and tested, TruLine were able to build a stable platform to access further into the tight site and begin work on the upper retaining walls. Careful construction monitoring was undertaken to ensure the placement of the Site-won fill material met the design specifications.

For the walls supporting cuts, the MagnumStone blocks were used as form work with steel reinforcing and concrete place within the hollow core of the blocks. A single row of B6 Platipus anchors was installed at 2.4m spacings approximately 0.9m in height from the top of the wall. Anchor load testing was undertaken to confirm the 130 kN ultimate capacity required was achieved with the B6 anchors.

The anchors were installed through the dovetail in the MagnumStone blocks with a load plate placed against the inside back of the block. Once the anchor was positioned, the block could simply be removed while the anchor was being driven in, then put back into place, tied to the anchor with the load plate, then filled with concrete to tie the whole wall together.

Thanks to the versatility of the MagnumStone retaining system, all 4 retaining walls were constructed with a consistent face finish, despite the variation in design and construction methodologies used for each wall.

The MagnumStone system provided the following benefits to the project:

• Various design methods can be used to create a uniform finish
• Easy to handle with small machinery for tight site conditions
• Relatively quick to construct compared to other retaining solutions