Construction World December 2018

An excavator drives interlocking steel sheet piles to create a coffer dam before the mass concrete footing can be poured.

The concrete footing is cast down to bedrock between the steel sheet piling.

its load before it could turn and dump into the bucket of a wheel loader, which also used the ramp to get close enough. Some 230 cubic metres of material was removed to create the void for the mass concrete footing. A higher water level was maintained within the cofferdam to equalise the pressure within the cofferdam and to prevent migration of sand into the cofferdam during excavation. Special-purpose underwater concrete As mentioned, however, the coffer dam waiting to receive the concrete was full of water, so special consideration was given to both the concrete mix and the pouring technique. For this underwater application, AfriSam’s special-purpose underwater concrete, Aqua Beton Admixture was used, and the concrete was placed in the coffer dam using the ‘tremie’ method, supplied by a static concrete pump. Tremie concrete requires the placement of a vertical pipe extending from above the surface to the underwater floor. As concrete flows from the bottom of the pipe, more is pushed through to ensure that the tremie pipe is continuously charged with fresh mix. This underwater feed method ensures that neither air nor water is entrapped or mixed in the concrete mass. As the mass expands with the concrete being pushed out of the pipe, only the outer layer is in contact with and affected by the salt water, leaving the rest of the concrete intact and in optimal condition.

“The readymix was supplied from our Peninsula Quarry about 17 km away, and the logistics had to be finetuned to ensure that delivery times exactly matched the low tides when pouring could take place,” says Melanie Ross, AfriSam’s territory manager for this contract. “This was not always easy, as the tide times could also fall when traffic was heavy so this required special attention to detail and timing.” Placing of concrete for the footing had to be conducted in two pours of about 70 cubic metres each, due to the short time window provided by the low tide cycle. Dowel bars were then drilled into the foundation and shuttering prepared for the above surface section of the sea wall, which was completed in six 5,4 metre panel sections cast in situ. With a base of 450 mm wide tapering to 350 mm, the visible wall is stylishly finished with a curved in situ coping along its top. Each of the six panels was completed in a single pour with 40 MPA readymix concrete from AfriSam. The formwork for the coping was specially designed and manufactured to complement the surrounding design; but the outward-facing curve also has a practical function – rolling back the storm waves that reach the wall’s height. This sea wall solution has provided the client with a stylish barrier that matches the architectural impact of the home, while ensuring a solid and economical design that promises longevity for many decades to come. 

A panel of the in-situ cast sea wall is completed while formwork goes up in preparation for the next panels.

The new sea wall is complete, while workers apply finishing touches to the cut-off tops of steel piles.

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