Holdfast Training Services Trial Blog

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?