2.1 North and South Towers
The North and South ( flanking ) towers were built within caisson foundations and were designed to be founded on the bedrock . The steel caissons are typically 30m in diameter up to 41m in height and weighing in excess of 1200 tonnes .
The caisson consists of two parts . The bottom part of the caisson ( permanent caisson ) is a double walled 30m diameter and 30m high steel ring . On top is a temporary caisson , a single skinned sheet pile wall that was removed at a later stage .
The temporary caisson is 11m high and creates a dry working environment for the foundation and early stages of the tower works . Both parts of the caisson were fabricated in the CRIST ship yard in Gdynia , Poland and trial-assembled to guarantee a tight sealing .
Throughout fabrication weld and pipework testing was undertaken , and after completion as-built dimensional surveys and trial builds with the permanent and temporary caissons were undertaken .
The caissons were transported by a semi-submersible barge from Poland to Rosyth . They were off-loaded in the middle of the Forth from the semi-submerged transportation barge by a shear-leg crane and moored . The barge had to be partly submerged to reduce the lifting weight of the caissons from up to 1200 ton to 650 ton by using the uplift effect of the hollow double skin wall .
The initial placing of the caisson on the seabed had to be within a design tolerance of +/ - 250mm in 20m water depth . Placing was measured using GPS at the shear-leg crane mounted on the caisson which enabled the monitoring of the exact position of the caisson including its tilt during the whole initial placing process .
The caissons were lowered by removing material from the inside creating ground brakes underneath the caisson wall and therefore overcoming friction and sinking the caisson using gravity and its own weight . Two barge mounted clamshell excavators were used in parallel to control the tilt during sinking process .
A “ sealing-ring ” made of a double-interlocking row of jet grout columns at -40m between the caisson and the rockbed was required to allow final excavation and cleaning of the rock bed .
Figure 1 : Typical Caisson cross-section
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