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PRECAST CONCRETE DRAINAGE CHANNELS PREFERRED FOR ABALONE FARM UPGRADE
November 15,
2022 451 451 
South Africa’s largest abalone producer, Abagold, has installed drainage channels constructed with precast concrete sections manufactured by CMA member, Concrete Units. The installation forms part of an upgrade of Abagold’s Bersig Farm, established in Hermanus in 2002 as Abagold’s second abalone farm.
Abalone is one of the planet’s most desirable sea foods and Abagold cultivates the world-renowned Haliotis Midae species which it exports live, canned and dried globally. Bersig is one of three Hermanus based Abagold abalone farms, the others being Sea View and Sulamanzi. Abagold is also invested in a number of subsidiary aquaculture ventures locally such as port Nolloth Sea Farms (an abalone ranching facility) and Specialised Aquatic Feeds. Internationally Abagold has invested in and built Oman Aquaculture in Mirbat Oman, the Middle East’s first abalone aquaculture facility.
Concrete Units precast concrete channel sections being delivered to site.
Abalone farming involves maintaining a continuous flow of fresh sea water through cultivation tanks in which the abalone is grown.
The new system was designed by consulting engineer, Jean Dippenaar, who conducted extensive analyses of flow rates on the old system before he determined the optimal flow rates for its replacement. Once the new design was finalised the original plant was gutted; the old tanks, pipe work and the PVC drainage pipes were removed and were replaced with a new tank system and the precast concrete drainage channels.
Biological growth inside the old system’s underground PVC pipes made them almost impossible to clean and this impeded water flow to the detriment of the farm’s yield curve,” said Nico Hough site manager of main contractors, Exeo Khokela.
Freshly laid precast concrete channels.
“By contrast, using precast concrete drainage channels has not only improved flow rates, but has simplified operation and maintenance. What’s more, we anticipate that far less maintenance will be required in the years ahead. For instance, after three months in operation, the channels required no maintenance at all,” said Nico Hough.
“Originally designed as an in-situ installation, we switched to precast concrete channelling because it gives the smoothest possible concrete finish and it saved a month’s installation time. Smooth finishes inhibit biological growth, unlike porous surfaces which encourage it.”
Freshly laid precast concrete channels.
Implemented over two phases, Phase 1 involved the construction of 13 channels and took six months to implement, and with nine channels Phase 2 took just under four months to complete.
In order to minimise the number of channels, Dippenaar’s design has two rows of tanks draining into one channel. Two 110mm diameter PVC outlet feeder pipes were cast into the side walls of each channel section at an angle of 45º to yield a tank flushing flow capacity of approximately 180m³/hour.
Concrete Units supplied 327 channel sections for both phases. Weighing 2.3 tonnes apiece, the channel sections were also cast with interlocking tongue and groove ends to ensure watertight joints and were laid end-to-end using Jointex and an expansion filler.
“One hundred and sixty four channel sections were used for the project’s first phase and 163 units were deployed for the second,” said Alwyn Carstens, production manager of Concrete Units.
A precast concrete drainage channel flanked by two rows of abalone tanks.
“With external dimension of 700mm (width) x 600mm (height) X 4.04m (length), we cast the channel sections inverted to provide the smooth off-shutter finish. We used just over 23kg of rebar on each channel section. Inner concrete cover was 75mm (typical for marine work) and outer wall cover was 60mm.
“We used a 40MPa durability mix with a GGCS slagment extender. And a special water resistant and crystalline waterproofing admixture, SIKA WT 200P, was used to reduce the permeability of the concrete. The admixture will also improve the durability and design life by healing all post cracks that might form over the design period,” said Carstens.
Sea water drains from the abalone tanks into a channel from two outlet feeder pipes.
In addition to installing the new plant and the channels, Exeo Khokela were also responsible for the earthworks.
“We created the channel platforms so that the drainage channels could be installed at a constant slope of 1%. We used imported fill for the subgrade and we raised the platforms by 350-500mm to achieve the slope. After excavating the channel trenches we cast a 15MPa blinding onto which the channel sections were installed,” concluded Hough.
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