Figure 5 : Pour Set- Up at Caisson Figure 6 : Continuous Pour 15 days
2.2 Central Tower
For construction of the reinforced concrete foundation in dry conditions for the Central Tower a modular cofferdam was selected .
The ten modular precast cofferdam units were constructed in the port of Rosyth . The sheet pile sections were slid together horizontally with the reinforced concrete base slabs being cast as a wall and then the unit rotated into an upright position .
To form the foundation pocket blasting was undertaken to first fracture the strong basalt rock . Bulk excavation followed four production blasts to form the general pocket . Following excavation and before the units were installed , 44 number 63.5mm diameter rock anchors were installed from a jack-up barge and a small modular sister platform . These were designed to hold the cofferdam units in place during the dewatered stage .
The cofferdam units were installed into the perimeter trench from the GPS Atlas floating crane with diver support . Each unit was placed and then the annulus between the unit and the formation filled with underwater concrete . With all the cofferdam units fixed into position , the gaps between each unit were filled and sealed . Large section props were then installed to ready the cofferdam to resist hydro static loads . Finally , de-stressing wells were installed in the rock formation to reduce up-lift pressures which might heave the formation on de-watering .
Once the cofferdam was fully constructed several large 150mm pumps were engaged and the water removed to expose the formation within the centre of the cofferdam .
Figure 7 : Cofferdam during construction Figure 8 : Jack-up rig within the foundation pocket
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