Holdfast Training Services Trial Blog

The tower extension is starting to finally take shape. After a few somewhat stressful weeks getting our sub-contractors temporary works together the slipform finally launched from level 30 of the old tower.

The slipform is somewhat like oil tanker destined to have a near miss with some rocks and then be broken up! Once it is going there is no stopping it easily! And at the end of it journey it becomes redundant and is ripped to bits and cast aside!

The walls of the new tower core are only 200mm thick. This has been causing issues with fitting everything into the wall. Cover, small embed plates, climbing tubes, horizontal and vertical rebar makes for a congested beam. This has meant that whenever the slipform reached a floor level the rig was stopped for two days in order to allow all the reinforcement to be fitted.  This pause also allowed us to fit bracing to the wing walls that act as sails in the wind.

The narrow walls also makes it very difficult to stop embed plates from dragging up inside the slip as it is jacked up.  The embedment plates allow the exterior steel beams to be connected, with a welded fin plate, back to the core.  If anyone finds themselves at a consultancy designing slipform makes the walls thickets and use 16mm vertical rebar.

Now I mentioned the near collision with the rocks and this ship. Well we, Mace, never really thought PC Harrington would deliver this tower core without fault. So surprise surprise on Friday when they informed us that two of the six corner MacAlloy embed plate pockets were in the wrong. This week we have been brainstorming to rectify this problem since there is no longer any free space to introduce the 1.5m x 2.5m embed plate into the core. See corner below.

The cost to rectify this sits firmly with PC Harrington. It could, worse case, require significant temporary works to remove 1m of concrete above void such that we can get vertical rebar continuity across the embed. Best case, we can locally break out the wall to get the embed in, but the the vertical rebar won’t tie, which I’m guessing the structural engineers won’t be happy with.

The slipform is now at the 35^th SSL level which is where the four outrigger (wing) walls terminate. Therefore the rig has to be temporarily dismantled in order to allow it to continue to level 42.

Here is a picture of the top deck being lifted off. Tomorrow we will be lifting off the working deck and hanging deck together. It is a pretty precious operation since most of the lateral restraint of the slipform has to be cut to lift it down to ground.

Elsewhere in the project our North Basement excavation is progressing (very slowly). The second waler is now being installed. This retaining wall is along a party wall and deflections must be limited to 10mm. Therefore the propping is very heavy. It has been on my list if ‘interesting TMR’ subjects to study sometime, but time seems to overtake!!

The focus of the project as a whole has developed too in the last two weeks. Whereas we were on a reimbursement style Construction Management Contract with the client, since there was so much outstanding design information at the start of the project, Mace have now signed a fixed price for the rest of the project. It is somewhere in the region of £200 million. However in order to make any money Mace need to accelerate the programme, and this means any major temporary works constraints made are now trying to be changed. It also means all the risk is with us (ground and all). Any sums not considered in the fixed price conversion are our of our profit. Therefore the incentive to find cost savings have significantly increased and the management of risk better managed.  I’m currently working in the design of a fifth tie for the the main tower crane.  This was never considered so finding a cost effective solution could be interesting.