A Closer Look at Prevailing Civil Engineering Practice - European ...

A Closer Look at Prevailing Civil Engineering Practice, What, Why and How
Vincent T. H. CHU
(ii) Open mode
It refers free air hand-dug tunnel or compressed air handing tunneling
with manual access to face of excavation.
(iii) Mixed mode
It is similar to close mode except that it allows access to the face of
TBM for manual removal of obstructions.
8. Why does settlement occur in microtunneling
Settlements occur in microtunneling, or other tunnel construction methods
in two forms: large settlements and systematic settlements. The cause of
large settlements is the over-excavation by microtunneling machine
leading to the loss of stability at the tunnel face and the formation of empty
space above the tunnel. The occurrence of large settlements is attributed
to the improper operation of the tunneling machine or rapid unexpected
changes in ground conditions.
Systematic settlements are mainly caused by the collapse of the radial
overcut between the jacking pipe and the excavation. The annular space
between the jacking pipe and the excavation is essential in microtunneling
and pipe jacking for the following purposes:
(i) Reduction of jacking forces
(ii) Injection of the lubrication
(iii)Steering of the microtunneling boring machine
During tunneling, the soils may collapse onto the pipe, resulting in
subsidence at the ground surface. Systematic settlements can be
controlled by limiting the radial overcut and by filling the annulus with
bentonite lubricant during tunneling, and with cement grout after tunneling
is completed.
9. How could ground freezing stop ingress of groundwater in
excavation
The concept of ground freezing involves the lowering of temperature of
ground near the excavation area. Drillholes with designed spacing are
installed so that a chilled brine or liquid nitrogen is introduced into the holes.
Brine requires continuous circulation while liquid nitrogen is for rapid
freezing and it is unrecoverable. With the addition of a chilled brine or liquid
nitrogen, the groundwater is frozen into ice. Upon frozen, soils exhibit
higher shear strength and the frozen zone acts as an impermeable barrier
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