Holdfast Training Services Trial Blog

In my last blog I talked about the use of the top down construction methodology in the construction of high rise towers. It presents a number of considerable structural challenges which have resulted in a quite unusual and, so I’m told, unique construction sequence on our project.  A senior design consultant from Sydney, Australia who is now working in the London structural engineers office confirmed that view of Friday morning when he described the 100BG basement as engineering insanity. Therefore, as it’s a Wednesday and because pictures are far more interesting than my boring text, see below some images outlining current progress.

Fig1: The concrete core, allegedly one of the largest currently under construction in Europe, is paused at a temporary hold point on floor 8.

Fig2: The full vertical load of this core is carried through steel plunge columns which are cast 5m into large load bearing concrete piles just below dredge depth. Interestingly we were able to excavate the Tower basement to 2.0m OD from 15.0m OD without use of any temporary internal propping regime.

Fig3: Sheet piles have been installed around the edge of the plunge columns to allow further excavation of the core pile cap to a depth of 0.0m OD. The Tosa pushes 6m long steel sheet piles in at a rate of approximately 24 per day. The 4m embedment seems more than sufficient to me considering they are in stiff London clay and are for the temporary condition only.

Fig4: Pile Cap excavated to depth in stiff london clay material. The water table is approximately 6m above dredge depth but ground water has not presented any issue during any stage of this excavation.

Fig5: Machinery breaking out the top of the core concrete piles in preperation for construction of the core pile cap. In the back right of the image you can see a column where the process of pouring the concrete caused the rebar cage in the column to rise in the pile shaft.

Fig6: Tower secant pile wall restraint