Getting down to the nitty gritty
Keeping sediment out of the sea was a vital aspect of a double award-winning marina project. HUGH DE LACY reports.
A freshwater lake on one side and the sea on the other presented unique challenges to Hawkins Infrastructure’s construction of a new boat harbour on Auckland’s Whangaparaoa Peninsula.
The Hobbs Wharf sea wall at the corner of Pinecrest Drive and The Anchorage on Gulf Harbour covers 3.8 hectares, of which 2.7 hectares was to be a marina with a tidal average depth of seven metres.
Much of the marina had been excavated in the 1990s and cut off from the sea by two bunds, but groundwater and stormwater run-off had since created a small lake on the landward side.
Hawkins’ task was to empty the lake, excavate the marina to the required depth by removing 34,000 cubic metres of spoil, then let the sea in by demolishing the two bunds.
This had to be achieved within the constraints of a stringent environmental management plan that precluded sediment escaping into the neighbouring Gulf Harbour marina, or even dust settling on the pricy pleasure boats moored there.
Also the pohutukawa trees on the site had to be moved and, in the case of one old tree too big to be shifted, the land around it had to be contoured to protect its root system .
Hawkins’ successful tender had been for $9 million but the client, Gulf Corporation, later extended the project and the eventual total cost was $10.5 million.
Work started in October 2007 and was project-managed by Hawkins’ Brendan Lehrke.
It was completed in December last year and has since won Hawkins two Auckland Regional Council/New Zealand Contractors’ Federation Environmental Awards, one each for different aspects of the project, in 2008 and 2009.
Early on in the project it was realised that seawater was entering the site through rock fractures in the bunds, creating problems with the dewatering of the lake.
The combination of seawater, stormwater and groundwater meant that pumps had to operate round the clock, seven days a week, extracting nearly a thousand cubic metres of water at every tidal flush.
All water was discharged by way of retention/decanting ponds, purpose-designed because the flow rate was too great to allow the use of flocculents, and the final discharge water was cleaner than that coming into the site.
The first part of the project was to sink a sheet pile coffer-dam into the seaward bund to at least keep most of the seawater at bay.
This bund was 50 metres long by eight metres wide and seven metres deep, made of sandstone with a thin layer of compacted clay in the middle.
A total of 105 sheet piles were driven 10 metres into the bund, penetrating 1.5 metres into the sandstone base.
This allowed the bund on the landward side to then be removed and excavation of the harbour basin to take place.
The catchpits on the access and adjacent roads were covered with silt cloth to minimise the amount of sediment washed into the stormwater system, and the wheels of all the trucks leaving the site were washed.
When the excavation was completed, the three-metre tidal variation and the low-tide depth of four metres meant there was a seven-metre height differential between the existing marina and the new one, and this became the main factor in determining how the seaward bund and coffer-dam were to be removed and the sea let in.
Thorburn Consultants helped Hawkins to develop a multi-stage plan for the bund removal, a crucial part of the project which, if handled wrongly, could have seen the tidal flows carry sediment uncontrollably into the harbour.
The first step was the removal of the access road on the basin side of the bund, followed by trimming it back to a safe grade.
The excavated basin was then flooded up to the low-tide level to reduce the head differential, initially by the simple process of suspending the dewatering operation, but when this proved too slow it was speeded up by seawater being pumped over the bund.
Even so, flooding the basin took a week and a half.
The sheet piles were then removed and have since been used on another Hawkins project.
This was followed by the installation of a floating silt curtain fence, 50 metres long by 6.5 metres deep, designed and supplied by Silverdale firm G.J. Reid, on the seaward side of the bund.
The curtain was weighted to the seafloor by a chain base and locked in place by four anchors.
In practice it was found that the top of the curtain had to be kept 300 millimetres below the surface of the water to allow the top layer of clean water to flow out with the tide while the silt in the water below was trapped by the curtain.
Next came the removal of the bund itself and its 8500 cubic metres of sandstone and clay.
Any work above the high tide mark could of course proceed at any time but, to prevent sediment being deposited in the existing marina, work below that mark could be carried out only on the incoming tide.
This meant that the original hope of being able to remove the bund wall at the rate of 600-800 cubic metres a day proved unrealistic, and the actual removal rate was only about half that.
A one stage a storm threatened the integrity of the silt curtain, but Lehrke, contacted after hours by alarmed neighbours, was able to put remedial measures in place that prevented any sediment escaping.
The job was done to such a standard that it earned perfect scores in all the Auckland Regional Council monitoring inspections.
Meanwhile work progressed on keystone walls, a rip-rap wave control system and a precast retaining wall round the new basin that ensured the perimeter’s stability once it was subjected to tidal flows.
The net effect is another handsome facility to bolster Auckland’s reputation as one of the world’s great leisure boating centres.
Contractor Vol.33 No.7 August 2009
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