Holdfast Training Services Trial Blog

Slipforming of the core at the South Bank Tower began back in May. The 12^th of May to be precise. 12 floors to slipform, from +106m AOD to +154m AOD. That is 48m total.

Slipforming a reinforced concrete core is supposed to be fast, efficient and financially economical. PC Harringtons (our concrete contractor) sold slipforming the core at a pace of 1.5m a day, 5 days a week. So by more complicated maths this makes:

        Duration = 48m / 1.5m per day = 32 working days, or roughly 6 weeks

Therefore,

       Date for completion = 12^th May 14 + 6 weeks = 20^th Jun 14

Sadly, this date for the completion of the core was not met. We topped out today! 5th Aug 14!

Thus,

       Actual duration of slipping = 5 Aug 14 – 12 May 14 = 67 working days

So,

     Actual Speed of slipform rise = 48m / 67 days = 0.71m per working day

At 10 hours work per day, this makes:

     Actual Speed of slipform = 0.71m / 10 hours = 0.072m per hour  

                                                or 72mm per hour

                                                or 1.2mm per min

                                                or 0.020 mm per second

                                                or 20 micro m per second, on average.

A common snail has a speed of 0.001m/s, or 1mm/s, or 1000micro m/s. Therefore 50 times faster than our slipform has climbed!

As I said, the planned speed (or rate) of slipping was 1.5m per day.  That makes the achieved speed of the slip 48% of the planned rate! Is this adequate? It has to be asked whether this was the most economical method of building the tower.

At the South Bank Tower we have a rare set of site conditions. The build ability of tower is generally poor, the complexity of reinforcement is high, the burden of temporary works is also high, access is limited, competition for hook time on the tower crane is high and weather conditions 150m above ground mean that it can be less than conducive to work.

I’ve decided to explore this issue in my latest TMR. What alternative systems could have been used to build the core? Should the tower have been purely steel? Additionally, should Mace have known that achieving a rate of rise of 1.5m a day was over-ambitious? And how was this reflected in the contract to deliver the slipform?

I also want to explore whether the project was influenced by the fact it wasn’t fixed price at the start, and had it been fixed would we still have taken the same decision to slip it.

Having spent the last 6 weeks inside the slip on a daily basis I can confirm it can be fraught, manic and a desperate place. There was never quiet second. PCH performed as best they could in my view. The slow pace was not due to laziness, insufficient resources or commercial will. We as Mace have driven PCH to get this core finished but the sheer complexity has denied us the rate that is normal for a slipform. Other slips on the project (10 floors high) have flown up without issue.

There is very little literature available on the rate of slipforming. Is anyone aware of any? I plan to work out the financial cost the slow slip has caused, and quantify the impact this has had to the project.