An innovative underwater shuttering system is allowing a quay wall to be built in double-quick time, reports Alan Sparks.
Reconstructing a 120m length of decaying Victorian concrete quay wall sounds a straightforward task. But by turning the construction technique on its head, engineers at the Port of Tilbury will deliver the £1M project in half the time and at a fraction of the cost.
Ground heave had caused the front face of the mass concrete wall to crack and crumble. Divers confirmed the problem up to 8m below water level, so a replacement was needed to accompany a new warehouse facility at the key Essex port Precast concrete solutions dominated the proposals made at tender stage. Building large concrete blocks in the dry and dropping them into place was one suggestion, or installing precast panels and filling insitu behind - both commonly accepted methods.
But temporary works master John Martin felt the variable existing foundation levels would pose problems for the sealing of precast panels. Its designers trusted in their specialist abilities in the world of the limpet dam to solve the problem - much of the team having worked together on Deal pier refurbishment last year (NCE 18 September 2003).
The solution was a sealable 12m long shutter cell that clamps into position allowing concrete to be tremied from above. It is effectively a pair of shutter faces that are held 3.25m apart and connected from a hinge above.
'A precast panel is effectively a shutter anyway, precast boxes still need a shutter on the joints, so we thought that using a full shutter would not be that much extra work anyway, ' explains John Martin site agent Mike Brookes.
The £200,000 shutter was built specially for this job and may never be used again, although John Martin hopes the methodology behind the concept will see similar shutters on future port jobs for the Norfolk-based firm.
'We had to design this from scratch and we have been learning to work with the shutter as we have progressed.
But all said, the shutter has worked very well and provides far more versatility and flexibility than precasting, ' says John Martin works manager Adrian MacDonald.
'Teething problems have meant striking of the shutters has been tricky as the tie bars have snagged on the shutter. This has been easily resolved and has not affected the programme at all. But with these lessons learnt, we know that version two, if ever we get to build it, will be perfect.'
Even though the shutter had to be designed and built ahead of the project, this was much quicker than the expected lead-in time had precasting been selected. The 30t unit took three weeks to design and fabricate in the yard and three weeks to assemble on site.