The polystyrene is not the central part of the campaign. It is not the first time it has been used in these sorts of contexts, although it does give rise to some possible worries which, as with so much else, have been neither shared nor discussed with the public.
It was placed there to shore up the sides and prevent further shearing away, particularly as a result of vibration during the making of boreholes. This prompts the question: if vibration was a problem, how did the voids to the side and under it fare during vibration? They had no packing and the boreholes actually penetrated into them. Will we ever know? There has been silence since 2001 about if there have been further collapses.
One could ask: since polystyrene is virtually impervious to water, are the sides of the shaft next to it waterlogged? And will that have weakened them? And will it be very hazardous to remove the polysytrene?
One could wonder whether open shoring might have been better, so there was ventilation, and control of the water table and a means to visually monitor.
These difficulties should also be seen in context: below the polystyrene is the collapse material. And it's weight is presumably bearing down on the unsupported voids below and to the sides.
You might think that all sounds a bit hazardous. You might think the centre will need grouting urgently before there's an attempt to move the polystyrene filling. Grouting seemed to me the right thing to do from the start. But EH rejected grouting totally at the start. They still had no intention to do it last September. This year, following our letter, their Chief Executive wrote to us and said grouting was now one of the options to be considered.
"Could be that the polystyrene fill at Silbury is the most appropriate solution in the short term."
That remains to be seen, in view of the above unanswered worries. But maybe we'll never know, as the short term has become the long term and I'm doubtful that there's any data on it's performance in such a context after this amount of time. It certainly wasn't put in there with a view to leaving it for this long.
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