How quick is base heave?
Battersea Station Box is a top down method of construction; we are currently excavating in London clay and blinding in order to fix steel prior to pouring concrete for the propping structure. The first blinding was poured about 4 weeks ago and what at first we thought were surveying inaccuracies appears to be the concrete blinding heaving. We have excavated about 10m of soil so the total stress will have reduced by about 200kPa.
I believe there are two forms of heave, elastic rebound and swelling. The maximum rise in the blinding is currently about 10mm, and we are now trying to track the rise as it appears to moving about 2-3 mm a week. I am surprised that this is happening over such a short time frame and as we are casting primary beams prior to digging down again we risk the soffits being out of tolerance.
Currently all I can surmise is over such a short time frame it is elastic rebound rather than swelling. Swelling is due to changing effective stress due to unloading and the change in pore water pressure which I would have thought would take place over a longer period.
The GDR only quantifies heave in relation to pile design and the pressure it will apply to the base slab in the long term (drained), it mentions short term relaxation but does not quantify it.
Has anyone come across a similar situation before?
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NIce piece Henry….you’ve a TMR there at least.
In fact it is a good thesis topic.
When Neil and I want for supper with last year’s CPR reviewers I asked a general question ( since 2 were GEO and another was a senior temporary works man) regarding how they estimate heave…That it is really difficult to determine quickly how to design the base slabs in these circumstances…all I can say was that the answers were disappointing
I found it really difficult to find decent guidance on this
As you say most of what you are seeing would be categories as elastic rebound the long term swell will take place over many years as the pwp equilibrates.
You could just to a PL/AE BUT , as with all geo problems the q is what is L? So there is an FE piece in the GEO suite you used. I played with it a few month ago to model the damage expected on a structure which had cracked next to a basement of mine
As always though the other great geo issue is what is the stiffness?
In the software one is asked for Eu and E’ .. in LC the first will be about 400 cu and the second will be something like half of that.
The soil parameters bring performance sensitivity.
Id you send me the GDR stuff and the dimensions of the thing I would be interested in setting up the FE model
Henry, I appreciate that it would only be in for a short period but are you not using Heave board? As this is normally a system used under permanent slabs in the long term it has probably been dismissed because you will subsequently excavate below it and therefore take it out. That said if you install the concrete prop and have an inconveniently timed programme delay, heave could begin to pose a risk to the concrete element installed without it in place.
If the concrete slab is being used as a prop it is obviously designed to span between supports, heave board therefore might work.
Its difficult to imagine the problem without seeing a sketch but when I read about heave I naturally revert back to experience of installing heave board on placement.
Tom,
No we’re not, we are probably only looking at about a 6 week period from blinding to casting the beams.
I remember visiting your site and seeing the thickness of the heave board you were using, hence why I wondered if anyone else had come across this in such a short time frame. When we get there our permanent ground slab will not use heave board either, unlike your ground slab which I believe was designed as a propping slab, allowing the clay to swell underneath it, the slab here is ground bearing and designed to resist the potential swell of the clay.
However whether you are estimating the distance of the heave or the pressure induced from the heave I have not seen anything that is time based. I suppose ‘who knows’ what is short term and what is long term!