Installing new lock gates at London's Royal Docks involved a precise cutting operation on the granite quoins that housed the gate hinge posts. By using diamond tooling, the contractor saved time and money, and ensured an exact fit.
Trimming the Locks with Diamonds
A purpose-built diamond router head has completed one of the most precise milling operations ever undertaken at the refurbishment works on the King George V lock, main entrance from the Thames into the Royal Docks.
The work involved routing out a 780mm diameter semicircular vertical groove in each of 6 granite quoins which form housings for the hinge posts for the 3 new pairs of lock gates which have recently been installed. An accuracy of 1mm on diameter was required over the full 16m depth of cut to ensure a sufficiently tight fit between the hinge post and housing, to prevent excessive water seepage when the lock is emptied. In fact the diamond routing method, proposed by the Special Projects Division of Marcrist International Ltd of Doncaster, who also supplied all the equipment, completed the job to accuracies of around 0.25mm.
London Dockland Development C corporation is spending £6 million on refurbishing the lock, which has hardly been used since the docks closed in 1981. Now however, with the massive redevelopment work going on in the docklands, the lock is needed so that building materials can be delivered by ship. The refurbishment work involves the replacement of all three existing gates and hydraulic machinery, together with the repair of all cracked and badly spalled concrete. Two temporary steel coffer dams had to be built at each end of the lock before it could be emptied for the work to be carried outIncreased diameter.
The huge King George V lock, which was opened in 1920, is 247m long, 30.5m wide and 15.2m deep, and is constructed with mass-concrete walls up to 6m thick.. The existing gates were hinged on granite quoins built into the concrete walls. Over the years, these quoins, and indeed the lock walls themselves, have moved slightly and are no longer vertically true. It was decided therefore, to cut out a new groove that would increase the existing 760mm diameter by 20mm. Also, the Centre of the new hinge post was to be offset by 10mm from the original position, so that the inner edge of the post remained tangential to the raised sill built into the lock floor, enabling the gate to shut tight against this sill. The depth of groove cut, therefore, varied from zero at the inner edge to 30mm at the outer edge.
Main Contractor for the works, Taylor Woodrow Construction, was happy when the Special Projects Division of Marcrist International came up with the diamond routing solution since, quite frankly, no other alternative seemed feasible apart from a labour-intensive manual operation. An idea of the complexity of the job can be gained by looking at how the quoins were originally built. Blocks were cut from Cornish granite and the groove roughed out using a template. These blocks were then delivered to the site and laid as accurately as possible, one on top of the other. The final task was carried out in-situ by a mason who laboriously dressed an accurate groove into the granite quoin working slowly from top to bottom. There is no record of the time taken for this but it must have been a long process particularly as it was carried out using hand tools.
Buoyancy chambers
The six new lock gates, manufactured by Ravelistein BVof Deest in Holland, weigh 250 tonnes each and contained two buoyancy chambers. Initially these contained air to allow the gates to be towed by barge across the North Sea to London.
On arrival, the lock was refilled with water and the gates lifted over the outer coffer dam using a 400 tonne capacity Taklift 3 floating crane supplied by Smittak of' Rotterdam.
Water was then pumped into the lower buoyancy chamber of each gate to bring it upright and a mobile crane used to maneuver it into final position over the new hinge post.
Once the gate was in place, water was pumped out of the lower buoyancy chamber and scupper pipes located in the base of the upper chamber, were opened.
These allow water to enter the upper chamber and fill it to the same level as that in the lock itself, thereby controlling the buoyancy of the gate as the water level around it rises or falls.
With this simple method all six gates dropped perfectly into place and are now in full working order- a testimony to good planning, good workmanship and the accuracy of diamond tooling.