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
Chapter 8. Tunneling
1. How can Earth Pressure Balance TBM maintain stability of tunnel
face
Earth Pressure Balance (EPB) TBMs are used in excavating and
advancing tunnels through any type of soft ground or soil condition,
particularly below the water table. The EPB method consists of a cutting
chamber located behind the cutterhead. This chamber is used to mix the
soil with water foam. It is maintained under pressure by the mucking
system. The ground at the cutting face is supported by earth pressure by
balancing the advancement of the tunnel with the discharge rate of the
excavated soil.
The underlying principle of the EPB method is that the excavated soil itself
is used to provide continuous support to the tunnel face by balancing earth
pressure against the forward pressure of the machine. The thrust forces
generated from rear section of TBM is transferred to the earth in the
cutterhead chamber so as to prevent uncontrolled intrusion of excavated
materials into the chamber. When the shield advances at the face of
excavation, the excavated soil is then mixed together with a special foam
material which changes its viscosity or thickness and transforms it into a
flowing material. This muck is then stored and is used to provide support
and to balance pressure at the tunnel face during the excavation process.
2. Why do Slurry Shield TBMs have difficulty when tunneling in clay
In Slurry Shield TBMs, the slurry forms a filter cake on the face of
excavation which has the following purposes:
(i) It provides the surface for slurry pressure to act on.
(ii) It acts as a seal against the y of groundwater into the tunnel.
(iii) In case the TBM breaks down, this filter cake serves as a sealing
membrane at the tunnel face which allows man-entry into the
excavation and working chamber upon provision go compressed air.
Slurry Shield TBMs are widely used for non-cohesive soils ranging from
fine-grained sand to coarse-grained gravel. It is less suitable when
operating in clay because:
(i) Most slurry separation plant could not separate clay from slurry. As
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