The Forth Replacement Crossing in Scotland

Contracting entity FCBC (Forth Crossing Bridge Constructors)
Execution time: October 2011 to March 2014
General description of the works:
For the construction of a new bridge over the river Forth near Edinburgh, foundation works, consisting of dredging and blasting operations were required on the marine section. The actual foundations of the 3 main towers were carried out using the caisson method of construction. Colcrete - von Essen performed the dredging work within the caissons for the sinking process and supported the sinking process by means of a bentonite lubrication system at the outside surfaces of the caisson. Having reached the final depth, the caisson bottom was cleaned using the airlifting process. For the central pylon the „Beamer Rock“ was prepared by means of blasting operations. In addition, the pits for the foundations of the neighboring bridge piers were excavated using precision milling and picking techniques supported by DGPS.
Main scope of work:
  • Excavating, transporting and dumping 145,000 m³ of dredged mud/soft material
  • Creating 3,000 m² of level ground for initial set-down of caissons, (tolerance +0.10m/ -0.15m) 
  • Precision excavation by rotary cutters of 8,000 m³ of Dolerite material
  • Sinking 3 pcs caissons Ø 32 m, circa. 38,000 m³
Special features of the site:
  • dredging with cable-operated dredgers with cable grabs and DGPS-assisted monitoring systems
  • excavation depths up to -32.00 m below mean sea level
  • working in highly confined spaces with grab sizes of 2.5 – 5.5 m³
  • working under tidal influences and with ongoing shipping traffic
  • difficult planning of workflow and great coordination efforts
  • working in 24/7 operation
  • high demands for accuracy in the dredging work, in order to avoid tilting of the caissons upon set-down. Inaccurate profiling of the sea-bed may have resulted in the caissons sitting in an incorrect and irreversible position.