Quick, there’s a condition…!
A short one this week as I finish off my first draft of AER1 – wow they come around fast!
WARNING – GRATUITOUS MUD SHOTS!
The majority of my site is London Clay, unsurprising given it’s location. Damo’s site is similar. I know this as when I met him in the pub last week he brought some to show me! The clay here is so overconsolidated that it has started to form localised soft mudstone deposits. Therefore it is to water what John’s worked examples were to me: impossible to get through!
Area in general
Area in detail
The clay is overlaid by river terrace deposits, a much more permeable material. And since the whole site is one big cofferdam I’m fairly sure I’m getting flash-backs.
The clutches on the sheet piles have been welded down to the puddle flange which will eventually sit at the bottom of the slab. Below this they’re not welded and in the sandy gravel areas this has led to water flowing in under formation level.
The spec states that where there are localised areas of clay in the sandy gravel, that it should be removed and replaced with type 1 or concrete blinding. But it makes no mention of areas of coarse material within the clay. So when an area of sand was found within the clay the sub-contractor kept digging in an attempt to find the bottom. And then stuff started sinking. Most notably a bloke holding a levelling staff.
Dig for victory!
They’d made two mistakes. They hadn’t considered why areas of fine material in course need to be removed, but areas of coarse in fine don’t – settlement. And by trying to dig to find solid ground they had created a hydraulic gradient and therefore a quick condition.
The sandy hole
Once they stopped digging and started pumping the ground solidified. They filled the hole with some concrete that got rejected from a tower crane base pour (another story involving slup tests) and continued on their merry way!
Holdfast Training Services Trial Blog 
The ground makeup is identical to my site (river terrace deposits overlaying London Clay), although I hope to just avoid working in the clay. Are you concerned about heave at all from the London Clay, not sure if your excavation will be deep enough for that? Any suggestion of building loads being greater than uplift effects therefore heave not a risk?
Guz
It is so interesting getting out of the ground!
Regards
Damo,
Thought that any change to the overburden would have an effect on the clay and it becomes a question of putting back more, less or the same load as to whether you see consolodation heave or nothing at all? Surely first question is therfore what are the limits to the zone of influence – boundaries….