The entrance to Chatham Commercial Dock has two locks with four caissons. Hydraulically operated geared chains draw them into chamber recesses within the island that sits between the two locks. The locks were constructed by the Royal Navy in the 1870s, and the caisson gates had last been replaced in the 1960s.  

In 2010, Beckett Rankine was commissioned by Peel Ports to inspect and structurally assess two of the lock gates, which had become difficult to move. Our detailed structural survey found that the gates needed repairs, while casts taken during dive surveys showed that the cills and gate skids that the caissons slide along were heavily worn. 

We specified the repair works and developed all of the marine civil aspects of the project, which included planning and overseeing the controlled lifting.  

With no available drydock in the vicinity, the challenge was to develop a suitable methodology for repairs. Following an options study, it was agreed to lift the gates onto the quayside one at a time, which had to be strengthened with a piled support pad behind the quay to handle the 800t weight. A floating sheerlegs with 800t capacity couldn’t fit through the lock, so a pair of 400t capacity sheerlegs were used for a tandem lift. 

The corroded and damaged elements were replaced, and the gates repainted to extend their operational life. The inner caisson had been damaged by ship impacts, and so some of its structural members were cut out and replaced. The timber decks were also replaced and strengthened.  

To overcome the worn cills, we widened the gate skid beams so that they bore upon unworn granite, resulting in smoother movements when the gates are operated. 

The dockyard remained operational throughout the refurbishment, and the works extended the caissons’ life by approximately 20 years.