Ground Water and Retaining Wall
This is one for those of you with a keen eye for geotechnics I think.
I am currently looking at getting all the ducks in order and the sequence of the bulk dig on One Nine Elms. Prior to the excavation, a ground water pumping test will check that the basement box is water tight (more likely reduce the risk it leaks like a sieve). This has raised a slight contractural issue which has been missed in the scope of works and allowances the sub contractor has agreed.
The design of the D Wall assumes the ground water level at the level of the soil on the passive side throughout the excavation. However the pumping test contractor has not allowed for recharging the box after the test and the substructure contractor have specifically stated their rates for excavation assume the ground water level is below formation.
During a planning meeting I was quizzed fairly robustly, below I have summed up my response. Are my comments sensible or have I been an idiot?
1. Why would the sub contractor specify the GW level in their rates? If the GW level is at the surface, the excavated soil will be wet. Dry material will need to be imported and mixed with the wet before it can be transported and accepted at the tip, therefore resulting in cost. I also suspect bulking factor is dependant material and water content.
2. Would a lower GW effect the working surface? Yes it would improve it, effective stress goes up which is directly proportional the the shear strength of the soil.
3. Why would the wall be design like that, it’s not how it is constructed? It’s the worst case, the designer may have been unsure of the methodology so will have taken worst case GW and therefore higher pore pressure.
4. What would happen to the D Wall if the GW level was below formation during the dig? Errmmm….. it would effect the wall stability. See scribbles below, in short passive pressure would increase, therefore stability would be better.
In terms of the strength, the SF & BM capacity of the wall won’t change as the strength of the steel and concrete is unchanged but the loads exerted on the wall will. I suspect the position of the max BM will change but be lower in value. Running it through software would have quickly told me this but there was none to hand and I didn’t fancy getting my pencil out, plus I would need to check at the various excavation stages. So my answer was weak in this regard.
The wall carries a vertical load, this can essentially be modelled at a pile with some of the shaft resistance gone i.e. where it is excavated. The wall will experience the max vertical load in the long term therefore it would be safe to say the construction methodology will not effect the vertical capacity.
Multiplex being a management contractor the would never rely on my advice, even if I put numbers behind it. In the end the design contractor will be paid to alter the GW level in their software and give the thumbs up. A 10 min job charge at a day or two fees.
Holdfast Training Services Trial Blog 
Brad,
Looking in the New Rules for Measurement Vol 2 (NRM2) might shed some light on the costs for removal of wet/dry soil and other associated costs. I haven’t been able to get hold of a copy to check myself, but would be pretty sure it is in there.
NRM2 would also confirm the allowed bulking factor – I remember this seemed ridiculous to me, but I can’t remember why. I think it was massive and it didn’t account for the type of soil, it was just a blanket figure.
Your QS should have a copy of this document, it is worth a look just for general information on how the sub-contracts are broken down.
Chris
Cheers Chris, I’ll look into it.
I’m not convinced its accurate to say dry material would be bought to site and mixed with wet spoil before being sent to the tip. This approach would significantly increase the number of vehicle journeys required, which is already a serious issue in Central London, and would hammer their programme. I could be wrong but that sounds mental.
How deep is the excavation and do you break into the London Clay? Having now seen de-watering operations on site they are not insignificant. By the time you take into account numerous pumps, storage of water, management of the process and personnel the cost can quickly become notable. Furthermore if the pumps break down, as they often do, who carries the risk for the inevitable delays to other programmes? Its a fair assumption that a concrete contractor can’t install a basement slab or pile cap if they are wading through a knee deep puddle. Delays to these aspects can result in very serious compensation events as they are critical path activities.
I only say about the mixing as that is what is currently happening with piling spoil at a ratio of 1 truck of dry to 3 wet. There are two levels of basement so approximately 12-15 deep in places, generally the excavation will break into the LC but there is a scour feature in the north of the site so it will be in gravel. The D Wall provides cut off, so I am under the impression the dewatering is more of an integrity test. Once complete the equipment will be removed, I could be wrong though.
I suspect the stipulation that cost have been based on GW at formation might also come with a ‘subject to re-measure’ clause. This indicates that the contractors have considered GW, not assumed a fully drained box and made suitable allowance. The point being that on comparison with other tenderers they demonstrate competence and flush out those that are quoting on the assumption of beneficial conditions who will sting you later if GW is at formation i.e. this doesn’t mean there is a need to deliver GW at formation merely that this was the basis of the provisional sums. The rest of your points are absolutely fine but too much thinking for a contractor! I suspect we’re talking method statement, deliver and re-measure. Although this would not earn management fees for a PM so if it can go to a consultant and they invoice a tidy sum then Multiplex can add their percentage and hey ho the world continues to turn. No incentive to do this for less than it can be – it’s extras outside the pain gain…
Almost as cynical as me there Rich. I am not to hot on the commercial side yet, this is something I aim to get a better understand of during the attachment. The re-measure sounds sensible, current the sub contractor is contracted to Wanda a Chinese company who are not the most forthcoming with payment and agreeing a fair price for works. As a result the contractors are very reluctant to do any work that vary from the contract without a formal instruction.
OK seems fairly sound
Didn’t understand the dewatering/recharge thing. Why mention recharge?
ON 1,2 The benches as the contractor goes down can be steeper if dry ( in theory 45+phi’/2 if there is no surcharge)…but if there is pwp then bets are off
In the bulking
Material,Bulking Factor, Shrinkage Factor, Diggability
Clay (Low PI) 1.30 – M
Clay (High PI) 1.40 0.90 M-H
Clay and Gravel 1.35 – M-H
Sand 1.05 0.89 E
Sand & Gravel 1.15 – E
Gravel 1.05 0.97 E
No idea what the mixing is about — sure not importing for working platform? But I’d like to know , if they do import and mix, why they do.
3 the long term design will be worse than that ( sort of) The pwp will recover to pre construction head So there will be a higher pwp that gw to dredge .But there will be a ground slab acting as a prop- so the BM V regime will probably be less than those in the short term
4 As you have it – all good nothing bad
Brad. Nice handwriting. As the PM is this your responsibility to explain or should this come from those that have designed it?
Chris,
We don’t send engineers out as project mangers; we expect more of them than that. Brad’s notes are a sensible explanation of the engineering that is presenting itself, which is the minimum we expect of the engineers we send to site. Too many site engineers forget to think about what is going on and can’t or don’t do the basics before asking questions of others. Our expectation is that a PET officer demonstrates a reading of a problem rather than acting like a project manger and deferring the technical to others. Only through doing the basics do you add value on site i.e. to allow this to be raised with the designer from a position of knowledge. So in short, not only should he explain his take on it, if he hadn’t done so he would have been asked to! Note also that doesn’t matter if it’s right or wrong as long as it represents an honest attempt to address the fundamentals of a situation. We can all then consider what we make of it. That’s what collective learning is about. And now you know.
Chris, thanks I’ve been working on it. By ‘your’ I’m taking it you mean Multiplex. It should, however the client has not been very savvy with their contact and therefore changes in methodology generally mean a design fee. This results in delays, as in the case of One Nine Elms approvals have to go back to Beijing, even for small amounts which is mental. It is therefore beneficial to have an idea if something is possible, scale of the design and implications before Multiplex embarrass themselves with unfeasible and expensive requests to the client.