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The temporary works for this project comprised the following:


  • Sheet piling to create a cofferdam to allow the culvert installation

  • Establishment of a fish passage

  • Supply and Installation of a temporary bridge

  • Dewatering of the cofferdam


The culvert installation required a reinforced earth foundation pad to be formed under the precast units. This required excavation well below the previous culvert invert level. To enable the excavation, Hunter Civil used 8m long FSP 3A sheet piles to form the cofferdam. Driving was very easy and rapid. Our team also installed sheet piles to the south to retain the approach which would house the excavator and the delivery trucks including Hiab and associated outrigger loads.


Once the sheet piles had been completed, excavation could commence. This required the surplus water within the cofferdam to be pumped out and the water table to be lowered. To ensure that the works could be completed in the dry and that reinforced earth raft could be constructed without risk of invert seepage water, Hunter Civil installed a deep casing to the north-eastern corner of the cofferdam. This casing was perforated and approximately 900mm diameter. Rail ballast was installed within the cleaned-out casing and a single closed 8” spear was also installed within the casing. This provided for both shallow dirty waters to be pumped off and the deep water to be drawn from the spear that was connected to a 6” pump. This proved very successful. Concrete aprons were required to be cast within the dewatered cofferdam following the installation of the precast units. This concrete work was also completed in the dry.

Following the successful backfilling of the culvert units and removal of temporary works a handrail/guardrail was installed. The road surface was also reshaped to flow over the new culvert.


Oct 2017 - Nov 2017 


Christchurch City Council 





The works were undertaken as part of the Groynes Reserve entrance upgrade following changes to the entrance access routes brought about by the $90M Western Belfast Bypass, being completed by Fulton Hogan. The works required excavation in a pristine and very ecologically sensitive environment, so environmental management was the key focus when considering a suitable construction methodology.

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