 The rock is heavily fractured and has mean fracture centres varying from .The southern end of the tunnel however passes through the "less competent" Upper Hythe A and B layers which are described as "medium-dense thinly bedded and thinly laminated, clean-to-silty and clayey fine and medium sand with subordinate weak-to-strong sandstone, cherty sandstone and chert".Most of the tunnel is above the predicted water table.