That’ll do!
After a week off last week the learning curve had ramped up again this week.
Two tower cranes up, three to go. We’ve cast the ducts for the power cables to the tower cranes into channels under the blinding. This keeps them out the way and protected. The ducts continue across the site and spit out of the edge of where the building ends.
![WP_20150313_08_58_31_Pro[1]](https://pewpetblog.com/wp-content/uploads/2015/03/wp_20150313_08_58_31_pro1.jpg)
![WP_20150313_08_58_41_Pro[1]](https://pewpetblog.com/wp-content/uploads/2015/03/wp_20150313_08_58_41_pro1.jpg)
Ducting emerges… …from under the (still wet) concrete
Adjacent to this is the berm holding back the sheet piles (in that area they’re just there for water control really, less so for soil retaining. In order to install the cables it was asked if the berm could be locally removed in the corner to allow the cables to come straight down the pile wall and into the ducts.
![WP_20150313_08_59_02_Pro[1]](https://pewpetblog.com/wp-content/uploads/2015/03/wp_20150313_08_59_02_pro1.jpg)
The berm in question
So I broke out the cofferdam notes and off I went. I discovered that although the piles would still stand, it seemed likely the top would see a deflection of up to 40mm.
5 meters behind the wall is a public road complete with services under it. I then estimated the services would probably see a localised deflection of up to 20mm. Well beyond the acceptable 10mm. So it needs bracing.
So I broke out the Cofferdam spread sheet and off I went. I estimated the wailing load, and thus the prop load. I selected a propriety system that could accommodate and made a recommendation. My CEng took one look at it, halved the section size said “that’ll do”.
It’s due to get installed next week.
I really hope it holds!
Holdfast Training Services Trial Blog 
I kind of hope that a) you have a record of the decision to half the calculated requirment and then that it moves significant;y but that’s pure mischief…
Guz, I am amazed there is no temporary works coordinator check on this as it falls within that bracket. You must have impressed! We have a fairly rigid process which can see a site engineer design something but it then must be checked by the temporary works team depending on complexity – an excavation retaining wall falling into that category. Does your chartered engineer issue a design acceptance certificate?
Our CEng is our temporary works coordinator, and previously worked in the McAlpine temporary works design team. So he does know his stuff. He signed the design form and it has gone through the McAlpine design team who gave it a once over, but bear in mind these are people that used 2 x pi x r to calculate the area of a circle, so I don’t put too much faith in their work!
If is doesn’t hold then you can say – I told you so!!
Regards
I’m sure it will be fine. I must have over calculated something, I’m just not sure what yet…
Hi Guz – that makes sense regarding the checking procedure and Chartered Engineer.
Interesting you calculated 40mm deflection – what are the sheet piles? I have to do a section of sheet piling (where you have seen that building in the corner that is still standing). There is talk of pretty minimal bracing, or at least the same bracing of what we are doing elsewhere (7.2m between props at 3m exposed/retained height). As you have – there is an adjacent road with services in. I suspect it will all be fine but I think the models tend to put the traffic loading really close to the edge of the excavation, and in reality the traffic is a good 4m back behind a wide pavement – hence greater lateral load modelled than experienced in reality.
Damian – The reality is that all will probably be loaded less than designed for, the risk is that it won’t and the event to avoid is the laden muck away lorry that pulls up onto the pavement to let the bus through due to congestion in the road and ends up back on site via a less than conventional route (ever?!) .It’s not the ‘normal traffic that is the issue its the worst reasonable case.