I find this difficult to believe and even if true I don't think it a wholly adequate explanation. The geology of the area is very well known. It is well surveyed and the large amount of mining in the area produced a lot of information. Even if not well known in detail the geology of the area is known to be of successive layers of different rock and the unpredictable nature of each individual base could have been foreseen. This being the case it would have been possible to dig a few bases as test holes, perhaps only a small % of the total. Whilst this would not have speeded up the process it could have given much more data to produce a better estimate of how long and what resources were needed. Not coping with the geology isn't an excuse, it is an admission. None of the conditions encountered have been, ahem, ground breaking.