My main concern, however, is what they did next. The hole was filled with polysyrene http://www.megalithic.co.uk/modules.php?op=modload&name=My_eGallery&file=index&do=showpic&pid=1334
and this was capped and weighed down with chalk. Once again, in the absence of any explanation of the thinking behind this or any technical explanation I have some layman's concerns:
First, the delay has meant that "temporary" has now become semi-permanent. Does this matter?
Second, why polystyrene? Is it supposed to strengthen the sides? It lacks the cohesion to act like that, surely?
Third, is it a known and tested procedure? Not that I can discover. This is merely an unstable tunnel, the same as many thousands that are treated worldwide, yet nowhere else can I find polystyrene having been used. Surely something like Silbury wouldn't have been subjected to an untried and untested procedure?
Fourthly, polystyrene has almost zero water absorbency. It merely traps water including, presumably, in the area between itself and the tunnel sides. Has it kept the rock dry or has it resulted in the rock becoming saturated?
Fifthly, is the polystyrene contained in a plastic sleeve (it seems it may be, from the photo). If the sleeve is permeable, then don't the above concerns about the rock being soaked still apply? And if it isn't, don't they apply even more?
Sixthly, if you fill the void of the tunnel, doesn't this reduce the chance for the rock to "breath" through evaporation, so it's bound to be more waterlogged than it would have been? Wouldn't it have been safer to leave the void empty, so the surface moisture could have been monitored and controlled? Or alternatively, shouldn't a permeable fill material have been used, so that there is even water distribution, as seems to be done everywhere else?
Seventh, if you fill the void won't the seasonal water table raise water much higher up the sides of the shaft than would have been the case?
Eighth, does water matter? Well clearly, EH think it does a lot, hence their original cover. And clearly it does since it's ingress caused a collapse. But more compelling is this: there is a perfect precedent - Silbury's sister at Marden had a vertical shaft. Water was allowed to accumulate, the chalk became waterlogged and weak, and the whole thing collapsed. I'm a layman, but to me this would impel me to make keeping the shaft dry, or at least at a controlled level of humidity, the one absolute priority.
Ninth, do the surveys, which have centred on identifying voids, also provide data on the degree of saturation, and the position of the water table, and any possible reduction in the strength of the chalk? Is EH confident that no harm will occur the moment the polystyrene is removed?
I cheerfully admit to knowing next to nothing about mine stabilization and sincerely hope that EH can show my concerns are naive and groundless, but so long as this ridiculous lack of public disclosure is maintained then worries about their actions and competence are not going to go away.