Geology Reference
In-Depth Information
Problem
Mitigation
Ground too strong (intact strength and/or lack of
discontinuities)
May need to pull TBM back and advance with drill and blast
Ground too weak and collapsing (should have been an earth
balance or slurry machine perhaps)
Ground improvement might be necessary in advance of tunnel drive
-
grouting or
freezing
Major faults
Collapse of ground and TBM gets stuck. May need to sink a shaft in front of machine
and construct a tunnel back to and around the TBM to free it up. Ground treatment and
possible hand construction through fault zone may be required to get the TBM going
again
Weak ground and high
in situ
stresses leading to squeezing
action on TBM
Can cause huge delays. Ground improvement to strengthen the ground and resist the
squeezing pressures
Too much clay for slurry treatment
Can cause delay and necessitate installation of additional treatment plant
-
extra
hydrocyclones, etc
Ground abrasive because of high silica content causing too
much wear on teeth, leading to cost and delay
Cost may be prohibitive, necessitating a change of excavation method
Too much water and TBM electrics not protected
Drilling and grouting in advance of machine or possibly ground freezing or compressed
air working. Possible change of method to drill and blast or employ different machine
with suitable spec
Excess tunnel slurry pressure causes blowout at ground
surface
Lower pressure
Pressure too low causes face collapse
Reverse of the above
