Projects

Browns Bay

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This project was identified under North Shore City Council’s wastewater strategic plan to reduce the number of overflows from the wastewater system in the Deep Creek, Browns Bay and Mairangi Bay catchments – all key public recreational areas.

The proposed works were completed as one of many projects under Project Care, the Council’s 20 year $210m programme underway to improve stream and beach water quality.

The overall project objective was to upgrade the existing wastewater system by providing 1.5 kilometres of new trunk sewer which would also be utilised as storage (approximately 4,000m3) for peak wet weather flows.

The new pipelines were designed to be located within the carriageway of Anzac Road, through the beachfront reserve and within the low-lying area of Browns Bay. The contract requirements stipulated that the main trunk sewer lines were to be installed using microtunnelling methods to minimise disruption to traffic, pedestrians, residents and businesses, and to ensure no damage to surrounding buildings, services or roads through groundwater drawdown or any other construction activity.

The upgrade of TS7 and TS7D was a FIDIC design and build contract based on an alternative tender submitted by the joint venture parties, Harker Underground Construction and Works Infrastructure Limited.

For this particular project, given that the ground conditions to be encountered included saturated running sands, siltstone and clay, Harker Underground Construction built and manufactured their own equipment - the TT2100H slurry TBM – which included a separation plant, guidance systems and software development.

The two main drives for installing the 2100mm diameter pipe, with a PVC ‘t’ lock liner, commenced from the Main Catchment Chamber, a 9 metre diameter, 7 metre deep precast concrete panel structure constructed using caisson techniques, at the intersection of Anzac Road and Beach Front Lane. This was used as a bi-directional jacking shaft during construction and retained as a manhole chamber on completion of the project.

The first section was two drives of 200 metres length each, which passed under the Browns Bay foreshore area and terminated in a new chamber constructed in Bayview Road.

Ground conditions encountered included alluvial sands and silts, buried timber and the water table approximately one metre below the ground surface.

The second section commenced with the retrieval of the TT2100H TBM from Bayview Road and re-installed in the Main Catchment Chamber for a single drive of 600 metres under the carriageway of Anzac Road, terminating in Glencoe Road.

Ground conditions were similar to the first section with the underlying silt / sandstones encountered towards the end of the drive.

 

The final section to be installed by microtunnelling methods for the overall project was the interconnecting 1067mm diameter polycrete pipe under the existing tidal Taiaotea Stream, approximately 20 metres from the beach. Design survey levels had determined an average of 3 metres minimum cover was available for the new pipeline. A further survey of competent ground strength and depth along the proposed alignment indicated that the competent ground would not provide adequate long-term cover to the pipeline. Hence options to provide permanent protection to the pipeline were investigated. The option selected - considered to be the least disruptive to the coastal marine environment – was for the installation of a 400mm thick concrete protective cap with footings, across the stream.

This project is one of Harker Underground Construction’s most significant slurry TBM contracts undertaken to date.

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