An historic lock has been restored after a detailed underwater investigation revealed serious damage to the Victorian piles that supported Castle Mills Lock on the river Foss close to York.
Commercial Diving & Marine Services (CDMS) was contracted to conduct a detailed underwater condition survey of Castle Mills Lock Island as part of the York City engineers department’s routine inspection programme.
A traditional method of construction on structures of this era was by a combination of vertically positioned timber piles with horizontal supporting beams across the face of the structure. Flat stone back slabs were then positioned behind the beams to screen off the apertures in between the beams, behind which rocks stone and general infill material was deposited. On the Foss Lock Island a quantity of additional lateral beams were located extending through the structure to support the ground base between the basin and the lock itself.
A significant surface ground depression prompted the inspection diver to investigate further and he reported that the timber elements of the structure were in a very poor state of degradation with some beams being totally rotted away. Piling around the structure was not deemed an option as vibration could cause a collapse, which subsequently excluded a number of tenderers.
One other element of difficulty was that a 3t weight restriction over the access bridge onto the Lock Island site was to be strictly adhered to.
The winning design went to Keller Geotechnique, which proposed vibration free diamond core drilling in 230 strategic positions over the entire surface area at ground level and also 85 raking piles to totally stabilise the structure. Reinforced concrete filled steel jacket piles were used which were bored over 9m into the substrata.
Stainless steel dowels were required in other key areas to stitch structural cracking, which were drilled and resinned in by CDMS diving teams. The proposal was then to pressure gout the entire island, however large voids and cavities in the basin face would have made grout containment almost impossible.
CDMS director of technical services Steve Fila designed a combination of concrete formwork and bespoke brackets securing fish plates and tie bars to the face of the structure.