Completion of bored piling and start of first stage excavation
12th August 2008 saw the completion of the bored piles for the Tower.
Cementation Skanska has completed the installation of a secant pile wall around the site. This comprises 250 bored piles, 900 mm in diameter installed such that the pile diameters intersect. At alternate positions along the length of the wall “female” piles were installed first. Before the concrete in these had set too much, the rig returned and installed the intermediate, male, pile, cutting into the females. As the bore is concreted it forms a close bond with the piles on either side creating a wall of vertically reinforced piles. These secant pile walls form the perimeter support to the surrounding streets and carry the vertical loads around the perimeter of the Tower.
Our next operations are to complete a reinforced capping beam on the top of the secant pile walls and excavate to a partial depth across the site. The capping beam links individual piles , such that they do not act separately but as a continuous wall. This is 50% complete. This lower capping beam will later be capped by heavy steel beams to transfer individual column loads along the wall.
Completion of a proportion of the capping beam releases our next operation, first stage excavation of the site down to some 7 m below street level, with a perimeter bank of material left some 3 m higher around the inside of the secant pile walls to control lateral movement of the perimeter. Our target is to complete the first stage excavation by the end of September.
This first stage excavation will include the removal of the foundations of the previous buildings. Machine mounted hydraulic beakers will return to break these reinforced concrete bases. Inevitably this will create a certain amount of noise and dust, but there is no effective method for breaking up large masses of reinforced concrete. We will be following the City of London guidelines by avoiding the use of the breakers during the hours of 10.00 to 12.00 and 14.00 to 16.00. The machines will be heaviest duty that can be deployed. These produce less intrusive noise than lighter machines and producing higher outputs will hasten the end of this stage.
Dust suppression will be in the form of a fog spray. The fine particles in such a spray can look like a column of smoke, but use much less water, and because they coat the debris and airborne dust with a thin film are much more effective than hoses and conventional sprays.
Broken concrete and steel reinforcement from the former foundations will be separated and sent to waste handling centres for recycling. The soil beneath and around the foundations has been tested. It is free of contamination and is available for land reclamation.
A large amount of material will leave the site in this stage. We will have traffic marshals at the site entrance to avoid pedestrian and traffic congestion, road sweepers cleaning the roads and labour ready to clean any mud carried out onto the footways and carriageways at the site entrances.