Holdfast Training Services Trial Blog

Prior to Christmas the Groundworks contractor took the level down to 150mm above formation and handed over to concrete contractor (PC Harrington of Southbank slipform fame). Concrete contractor took down to formation level, but had no intention to blind for an additional 3 days, despite direction to cover within 4 hours. Shear vane tests had been directed by the designer (Arup) which could not be conducted immediately, thus the contractor’s reasoning for the delay. Due to concern for the quality of the surface material at formation level – it’s London clay and tends to suffer from surface cracking if left to dry out and turns into an ice rink when wet – the contractor was instructed to blind immediately but leave holes for the test to be conducted. Seems strange that a specific test was specified other than “test for this property”, but it’s my first day and what do I know? I haven’t got a computer or a log on yet, let alone a clue what’s going on!

A real digger!

Day 2 and I witnessed a group of hairy Irishmen with their asses hanging out of their trousers (one for John) attempt to put a very large, prefabricated RC cage into hole. It didn’t fit. For 2 reasons: the hole was square, the cage wasn’t; and when the piles were cut down all the rebar was bent all over the place and wouldn’t fit through the spaces between the rebar. They got it in eventually but if Harry had seen how he’d have probably had kittens! It’s led to some cover issues. The rebar is touching the blinding round the hole in places, this probably isn’t an issue since it’s a tower crane base, therefore temporary, therefore the durability is unlikely to be a problem. But PCH are the problem children on site so we’re not given them an inch at this early stage. So the contractor will have to do some localised breaking out of the blinding to ensure correct cover, mostly since they have no idea what strength of concrete was used when they poured the blinding! They’ve learnt from it and the steel fixers will make the other 4 in the hole – sensible!

Tower crane footing

Today (Friday) I was sent onto site to hold the hand of a Geo Engineer from Arup (they must have heard of my fondness for quiche). He came to observe some shear vane tests on the foundation level clay prior to the blinding being poured. The minimum required value was 40kPa. The very lowest of our results was 120kPa. Does this mean the basement slab, sheet piling, pile shaft resistance and anything else that might have been based on cu is massively overdesigned? I tried to gently probe the area by asking questions, but didn’t get very far, the bloke looked about 12 and a bit scared that anyone had asked him a question with the word “overconsolidation” in it, all I got was that c’ was taken as about 25 (comment John?). Afterwards the Chartered Engineer on site asked me how I’d remembered things like Atterburg limits, I told him what we did last week and he looked horrified. I think I’ll be babysitting the Geo bloke every time from now on. Ahh well, at least it’s not making the brews!

Shear vane test

By the end of the week I finally have a computer, a log on and a McAlpine fleece, but still no clear idea of what I’ll be doing. Very TBC but it looks like I may be responsible for QA on Tower cranes, basement raft slab and, most excitingly, slipforming! Looks like I’d better dig back through Rich Hall’s blogs!