This project upgraded an existing path put in during WWI and WWII to service the submarine booms and searchlight batteries of the Harbour Protection Defence Screen.

The original path was in places highly dangerous; but also very popular with the public as a recreation walk. Falls of up to 8 metres from a 600mm wide rough stone path hacked out of a cliff face posed an unacceptable health and safety risk to DoC(Department of Conservation). The decision was made to construct a new path that cantilevers over the cliff face, protected with an aesthetic handrail of precast exposed pebble posts, stainless steel top rail and 10 strand intermediate wire barrier. The project was a high risk one from a health and safety point of view and OSH were involved at the outset. It was also extremely difficult from an access point of view with materials, including aggregate and concrete being transported to a point approximately 300m away from the construction site down a chute specially envisaged and designed by Fort Projects.

 

The first step in the project was to re-lay a historic concrete path that had been badly cracked and displaced by roots of Pohutukawa trees growing along the foreshore. The path was cut into sections, lifted away and the roots of the trees wrapped in polystyrene. The area was backfilled with SAP6/10 and the slabs relaid to a higher and level line. During this process holes for new posts were excavated, often around existing root systems that had to be all protected prior to positioning the posts and concreting them into place. After this step was completed access was then gained to the area, approximately 170m long, of new 1.4m wide cantilevered path. Fort Projects selected a special concrete mix for the path using 30MPA 19mm standard, super plasticized and also retarded with an addition of 12kg/m3 of Jet Black oxide. The resultant mix has stood the very exposed conditions well, with the resultant colour matching the cliff face as desired by DoC in their specification.

Access to the site for pouring concrete was very difficult. Pump access to the top of the cliff face was not easy and the resultant fall from the pump locations prevented this from being a reliable way of delivering concrete. Several alternatives were considered, including using helicopters. In the end it was decided that manpower and wheel barrows were the best solution and 63m³ was delivered via this means. With the wheel barrows at the base of the cliff and the only access to this point being through a steep, narrow and unlit tunnel, the trucks had to discharge their contents some 18m above the path.

A chute arrangement was devised and operated very successfully after some trials sorted out teething problems. The distance to the end of the path from the loading point was 380 m and often up some pretty steep slopes so the site staff had their work cut out to deliver the concrete. An average concrete pour saw site staff each delivering up to 12 loads for one path section, each travelling a total distance of 8km for one truckload of concrete. Not bad when considering a full barrow load of concrete was pushed for half this distance!

Safety first

Chris Smith was the project manager for this project and the site foreman was Ryan Jenner. Their team started by constructing the cantilever walkway to a high quality while minimizing environmental impact. Safety harnesses were worn attached to a wire rope lead around the cliff face. It wasn’t practical to wear harnesses during the pouring operation so this work was only carried out once the permanent handrails had been grouted into place.