"MetroGnome" wrote in message
news:u1omc.10$a7.7@newsfe6-win...
Not true. The entire length of the station has the same geology - which
at
platform level means that it is wholly within the Woolwich and Reading
Beds
("mushy stuff"!), as are the running tunnels on either side for a
considerable distance.

Further to this, the following is taken from "Jubilee Line Extension - From
Concept to Completion", by Robert Mitchell:-

"Further consideration was given to how settlement and disruption during
construction could be minimized. The geology of the area consists of made
ground overlying terrace gravels, London Clay, and Woolwich and Reading Beds
followed hy the Thanet Sands. Given the substantial presence of silty and
cohesive ground at the station tunnel axis, it was originally planned to use
compressed air with its attendant health and safety issues. It was decided
to raise the track level through the station by about 5m, thereby reducing
ground pressures and enabling the escalators, ventilation shaft and escape
stairs to be housed in a deep cut-and-cover box. Ironically, while this
reduced the amount of mining required, it had the potential to increase the
settlement problem owing to the proximity to the surface. The track
alignment was therefore shifted horizontally towards Jamaica Road to
minimize the risk of settlement to properties."
[...]
"Construction of the station was by Aoki:Soletanche as part of contract
105, awarded in November 1993 for £69.6 million. The diaphragm wall station
box is approximately 50m square with panels around 30m deep. The main
problem encountered was the thickness and density of the Thanet Sands and a
dewatering system was installed to lower the groundwater below the
foundation level. The contract allowed for compensation grouting at the
discretion of the contractor and limits of distortion and settlement were
specified for particular structures, extensive instrumentation being
introduced to monitor movement."


Hope this is of interest.