Civil Engineering Reference
In-Depth Information
Figure 5.70 shows an example of the front face of a jacked box tunnel
shield for a project in Lewisham, London and the excavation techniques
used on this project. Further details of shields are provided in the jacked
box examples in section 5.10.3.
Jacking equipment
The required jacking forces are calculated based on
the frictional resistance between the box surfaces and the anti-drag systems,
the frictional resistance and/or adhesion between the exposed box surfaces
and the ground, and from the shield embedment loads. Figure 5.71 shows
all the components associated with the jacking process. The hydraulic jacks
only have a limited extension and hence spacers between the hydraulic
jacks and the cross beam are used to transfer the forces into the jacking
Steel section designed to
penetrate inducing
stability through applied
pressure and friction
between cutting edges
and ground
Very weak, loose
silt and sand
Soft squeezing
clay
Concrete section
designed to buttress the
clay, allowing it to span
between the walls
Composite steel and
reinforced concrete
shield, 1.7 m of cover to
the underside of the
tracks
a) Composite steel and reinforced concrete shield
b) Hand mining in a top compartment
through very weak, loose sand and
silt
c) Rear view of the face during excavation
Figure 5.70
Example shield as used on a jacked box tunnel as part of the
Docklands Light Railway, Lewisham, London, UK (box size 17 m
wide, 6.2 m high, 48 m long) (courtesy of BAM Nuttall Ltd and
John Ropkins Ltd)
