3.6 Foundations
The Firth of Forth is a fjord , formed by the Forth glacier in the last glacial period . The maximum water depth on the line of the bridge is 45m with rock being as low as about -85m OD ( Ordnance Datum ). The alignment of the bridge is dictated by the desire to position a tower on Beamer Rock . It is formed by a steep-sided dolerite outcrop that reaches an elevation of about + 3m OD and forms a ridge trending north-west to south-east , which is almost perpendicular to the bridge alignment .
The area of rock exposed varies with the tide , reaching about 45m x 95m at low water springs . The rock is strong and , except within 2 – 3m of the surface , joints are usually tight , sometimes with calcite infill .
The central tower foundation is a 25m x 35m gravity footing founded at -5 m OD .
The envisaged construction methodology * was that a platform would be formed using marine plant working under water . A precast cellular foundation would then be floated and ballasted into position on pre-installed landing pads and the gap beneath the unit infilled by underwater grouting to form the contact with the excavated rock surface . The cells would then be filled with in situ concrete .
The flanking towers are supported on 29 m x 41 m pile caps with a group of 16 No . 3.4 m diameter cast-in-place piles . The top elevation of the pile caps is at 25 m OD so that the pile caps are not visible and are sufficiently deep at low tide so as not to pose a hazard to yachts or other leisure users of the Forth estuary . Foundation conditions vary , with the more challenging south tower being
located in 22m of water with rockhead at -40m OD . The sedimentary rock is overlain by glacial deposits and alluvium . The envisaged construction methodology was again based on float-out of cellular precast caissons which make up the pile cap and tower base .
A similar sequence was considered for the north tower , although some dredging was required as the pile cap is partially below the existing seabed . Four piles were to be constructed using a template placed on the seabed . The caisson was floated in at high tide and sunk over the piles at slack water when the tidal current drops below 0.5 knots for at least two hours . The structure was ballasted on the falling tide so that it remains seated on the guides at subsequent tides .
The first four pile sleeves were grouted and temporary buoyancy elements removed . Further piling operations then took place , using the pile cap as the guide for driving the pile casing . Various dockyard sites close to the bridge site or further afield were considered for the precast units . Construction on a submersible barge moored alongside a quay or quayside construction followed by load-out onto a transport barge were also viable construction option .
* It should be noted that the construction methodology described here was that envisaged in the Specimen Design . In the final design , FCBC chose to adopt gravity foundations using caissons and cofferdams , rather than a piled solution . The final design solution is described in Part III ..
Figure 3 : Crossing stay cable system : cable tensions due to out-of-balance loading
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