foreshore sewer was built in the early 60’s to service North Shore
City’s East Coast Bays suburbs and extends from Murrays Bay in the
north to Takapuna in the south.
Easterly swells that buffet the coast from time to time have taken their toll and several sections of the sewer pipe and their support require maintenance. To protect the sewer pipe a protective concrete casement is being constructed.
Repairing the foreshore sewer is reducing the risk of the sewer leaking and having a detrimental effect on beach water quality and the marine environment. The work is improving the visual appearance of the structure, and the finished shape of the supporting structure blends in with the cliff and shore rock formations.
The new structure also slows wave action and allows sand to build up on the beach. Already new marine flora and fauna are making their home on the first section repaired during 2003.
The project involves constructing a new concrete casement over the existing sewer pipe and lowering two manholes to improve the visual appearance of the coast.
Michael Khrapko of Stevenson Concrete worked with the North Shore City Council, main contractor Gideon Construction and Shotcrete operator Natural Pools and Rock to provide the concrete mix solution for the project.
Constructing the new concrete casement over the sewer is being done in five stages; cleaning the pipe, cutting a sandstone trench, application of bulk fill concrete, application of steel fibre concrete, application of a decorative concrete layer.
Because of concern for the natural environment and the effect of the tide, work has only been able to be done over six hours during low tides, when the seawater is well clear of the work site.
The first stage is to water blast the pipe to remove seaweed and loose rock. A trench is then cut into the sandstone - this is about 700mm deep and 300mm wide. The trenches provide a ‘key’ for the structural concrete. Polythene is then laid over the pipe and the bulk fill concrete is poured over the top and down both sides. The polythene bond breaker is designed to prevent cracking in the existing encasement being transmitted through the new encasement.
This first layer of concrete contains Microsilica 600.
The next work day the surface is water blasted again before the next layer of concrete, which contains steel fibre is applied. This layer is at least 200mm thick and the steel fibre provides the additional strength required to withstand the loads applied to the structure for a period of 50 years.
“We have used a special type of steel fibre for Shotcreting that was recommended by manufacturers Novocon,” explained Michael Khrapko. “This steel fibre provides the structural integrity required without the need for traditionally placed steel reinforcing bars.”
“The tricky final decorative concrete layer contains Microsilica, Super Plasticiser to give the 50 Mpa mix workability, and oxide colouring so that it more closely resembles the natural rock formations. This layer is also sculpted so that the protective concrete casement more closely resembles the rock formations.”
Already the rock concrete sections that were completed during the urgent work two years ago have been colonised by a range of shellfish species.