Dry Access for Improved Reservoir Operation

Bosley Reservoir, opened in 1831 to feed the Macclesfield Canal, covers an area of 35 hectares and holds 1,350 million litres of water. EDS was asked by the client to install an engineering solution that would enable dry access to both the upstream and downstream sides of the scour pipe. This was achieved by constructing a caisson. The works allowed the installation of a new structural liner through an existing deteriorating pipe, the fitting of a new hydraulic valve housed in the caisson (operable from the dam crest), and provided the opportunity for general maintenance works.

Key Challenges:

  • Constructing a caisson in a predefined location within the reservoir that, when pumped out, would provide dry access for the client
  • Extending the existing pipe into the new caisson to create access via concrete rings for the new liner installation
  • Fitting a new hydraulic valve and feeding leads to the dam crest
  • High silt content in the reservoir reduced visibility for the dive team during foundation slab installation
  • Placement of large elements needed to be carried out by a single diver

Solutions:

  • Due to the upstream location of the pipe, EDS deployed a NATO pontoon system fitted with a lifting gantry, dive spread and tool station
  • The pontoon system also housed a piling rig, allowing the team to take core samples of the reservoir bed and then use an excavator to clear the area around the pipe for caisson construction
  • To position the caisson at the correct level in poor visibility, a scaffold system was used as screed rails so the dive team could level the stone fill. The slab was then placed adjacent to the existing pipe, with GPS equipment used to verify installation levels and location
  • To connect the new and existing pipework to the hydraulic valve, a coring rig was used to core holes in the manhole rings prior to them being laid to form the caisson. This required accuracy and precision to ensure the pipe travelled through the caisson in the correct location
  • A grout wall was poured between the manhole rings to create a seal, after which the chamber was drained to allow re-lining of the existing pipework
  • A new hydraulic valve was installed, with hydraulic leads fed up to the dam crest so the valve could be operated with ease
  • To protect against blockages, a galvanised trash screen was fitted

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