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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.
City Pollution – Old Habits Die Hard
In the sustainability study group this morning we discussed the Construction industry and current lack of regulations regarding control of plant vehicle pollution. The following link takes you to an article that discusses the problems with construction plant pollutants in London and the challenge of regulating them in an industry where enforcing higher standards would generate absurd replacement costs.
It turns out that a lot of plant equipment currently used in London is Red diesel powered and outrageously inefficient. One of the Geo boffins told me they had recently received some environmental data from a site showing that it currently takes one piling rig 1000 litres of diesel to bore every 60m deep pile. There are 74 of them on the project!
The construction industry might grind to a halt if the Government begins to pay as much attention to plant equipment engines as they do VW diesel cars.
Holdfast Training Services Trial Blog 