With a swift turnaround time of one pour per week, facilitating the next pour is crucial to the production cycle. 'In terms of programme, the divers' involvement in making this happen is right on the critical path;
and in fact governs how everything else falls into place, ' says Brookes.
The main task of the half dozen divers on site is construction of the temporary steel waling that supports the shutter 8m below the surface. This requires the use of hand tools to bolt the reusable steel sections to what is left of the existing wall. A temporary platform is also installed for the divers to work from.
Once the waling is in position the shutter can be lowered using the floating 115t capacity crane.
This is designed to fit snug onto the previous pour. The scissoreffect shutter's five 100t hydraulic jacks then extend, levering the shutter faces shut - clamping onto the foundations.
An end seal section slides down the 'free end'. Divers come in to fit plates to make the shutter concrete-tight, if not watertight.
Then 63mm diameter Dywidag bars are threaded through plastic tubes to tie the shutter shut.
When fabricating the shutter, John Martin tried to source panel connectors for these bars to run through. 'Our suppliers didn't have any as these bars are so big and rarely used. In the end we had to make them ourselves, ' says the firm's site agent Mike Brookes. 'To start with we thought we would need to use 100mm bars.'
A bentonite medium is used to expel seawater for the total 200m 3concrete pour. Three distinct mixes are used in the same pour. The first to be tremied in has 30% cement replacement by pulverised fuel ash (PFA) with an underwater additive, plasticizer and 10mm aggregate. On top of this goes a layer with the same PFA content, 20mm aggregate and a foaming agent, followed by a topping of a similar mix with steel fibres Just 15 hours later the shutter is struck and the scissors opened.
Once free, the crane lifts the shutter away and onto the barge.
'Here we jet wash the surface and wax the surface to prevent sticking next time around as the divers set about moving the waling, ' Brookes explains.