This photo was taken the morning near Burnham Bucks Station and shows one of the mountings for the OHLE gantries. The process appears to be a 3 stage one drill hole, insert mounting part way, pile drive mounting down which can take a couple of weekends to complete. This process does not appear to be being carried out in any logical sequence however my guess this posts are only in certain places and are seated well int the ground in areas which a vulnerable to subsidence.
XF