Te Ara Ihutai Coastal Pathway, Christchurch – MagnumStone™ Gravity Wall

As part of Christchurch Council’s Cycleways Network, the 6.5km long Te Ara Ihutai Coastal Pathway will provide a protected shared pathway, giving coastline access between Ferrymead and Scarborough Beach. Already attracting a growing number of cyclists and pedestrians, the last section stretching across Moncks Bay will incorporate the Christchurch Yacht Club as well as a jetty along Main Road.

To support the shared pathway around the jetty and protect it from tidal surges, Fulton Hogan required a system that could be quickly installed during low tide within tight working conditions and timeframes, without having to cut too far back into the existing road. As the original design caused both cost and construction challenges, MagnumStone was adopted as an alternative that was fast and cost-effective to construct.

DESIGN

A MagnumStone gravity wall with extenders, approximately 115m in length and 2.4m in height, was designed to support this section of the pathway and the road. The wall extends either side of the jetty and cuts into the tight space between the jetty and the main road. As water will rise above the lowest section of the completed wall, tidal influence and the maritime environment were carefully considered, with seismic Collapse Avoidance Limit State and traffic overload case checks included within the analysis for the final design.

A higher grade concrete was specified for the MagnumStone blocks and extenders, which included a black oxide to help blend the sea wall into the surrounding environment. The design layout of overlapping extenders was considered for the sharp corners around the jetty. A drainage pipe was also required within the wall. Engineered granular fill would allow water to move in and out of the lower section once it became submerged. The top two layers would then be reinforced with concrete, with a handrail to be set at the edge of the wall. Because the MagnumStone wall and aggregate fill have some flexibility, the reinforced concrete fill required strategic construction joints to make sure it aligned with any movement of the wall.

INSTALLATION

Due to the tight site space, it was important to deliver specific blocks to site when required. Working within a 4-5hr programme around low tide, a concrete foundation was laid, using pumps to extract any water while it set and the first layer of blocks were placed. Using an excavator, each block and extender was lowered, placed, then the cut area was back filled. MagnumStone’s fast installation methods allowed construction to progress during the tight programme.

The gravity wall design also reduced the excavation area required between the jetty and the main road. Easy handling of each block meant the space needed for pipe penetration could either be cut or an entire block could simply be lifted out, with concrete poured around the pipe before the next layer of blocks was placed on top. At the top of the wall, the concrete cantilevered capping beam was tied back into the concrete fill using steel reinforcement within the top two layers of blocks.

Once complete, the Te Ara Ihutai Coastal Pathway will not only connect communities from Scarborough to Ferrymead, but will offer visitors to the region a safe, long-lasting shared pathway for cyclists and pedestrians to enjoy.

The MagnumStone system provided the following benefits to the project:

• Can be coloured to blend in with natural surroundings
• Quick to install within tight site conditions
• Can be engineered to different strengths to suit varying conditions