ENCOUNTERING PINGOES

摘要

Lessons learned from the Lee Tunnel experience can be divided into two categories. In terms of our understanding of DFHs we have learned that their geology is far more complex than previously thought, large DFHs adversely affect the ground at greater depth than previously thought and large DFHs reaching the chalk may be in hydrological communication over considerable distances. In terms of the management of the tunnelling process through DFHs we have learned that even well-controlled encounters with a DFH at depth can lead to settlement at ground level, and good communication and planning are imperative for a satisfactory outcome. Looking to the future, the designers are satisfied that there are no long-term implications for the tunnel given the disturbed geology at depth, and thankfully the Northern Outfall Sewer is not at risk.