QUEENSFERRY CROSSING III . Final Design & Construction Process
1 . INTRODUCTION
The Queensferry Crossing is a cable stay bridge with three towers over 200m high and the cable fans arranged centrally between the two carriageways . The two main cable stayed spans are 650m , the two back spans are 223m and there are approach viaducts at each end of variable spans up to 104m .
Transport Scotland undertook a year-long tender dialogue process during 2010 for a design and build contract to construct the Queensferry Crossing and connecting roads . Forth Crossing Bridge Constructors was the successful tenderer . FCBC ’ s contract commenced in April 2011 and will be completed in summer 2017 .
2 . FOUNDATIONS
The location of the crossing exploits a narrow crossing point within the Firth of Forth estuary , but the depth of water is comparatively deep at up to 45m . However , the estuary is punctuated by a small rock island called Beamer Rock which divides it into two channels . This shallow tidal rock island was defined as the central tower location . The remaining two towers sit towards the edge of the deep channels where the water depth reduces to circa 15m .
The foundation solution had to address a range of marine conditions , from shallow tidal waters to deep water conditions . The competent founding strata for the Flanking Towers was the sedimentary rock , below alluvial , fluvio-glacial and glacial soils , at a depth of up to 20m below bed level .
While the specimen design adopted piled foundations for the piers and flanking towers , FCBC chose to use gravity foundations throughout . All three towers have reinforced concrete foundations , however the Central Tower has differences in construction compared to the Flanking Towers ( North Tower and South Tower ) due to the differing sub-structure foundation and bridge articulation .
The South Tower structural foundation ( 30m ( dia )* 9.0m high ) is also larger than the North Tower structural foundation ( 24m ( dia )* 9.0m high ), primarily due to the design for potential ship impact forces . The Central Tower foundation ( 35.0m * 25.0m * 3.50-6.0m ) sloping surface was formed using sacrificial stainless steel “ Hy-rib ” panels , however as with all other parts of the foundations this is not visible at low tides due to rock armour positioned on top of the foundation and blended into the surrounding “ Beamer Rock ”.
1 / 2017