TUNNEL ENGINEERING

effective. Bolts are tightened with hydraulic or
pneumatic wrenches where possible otherwise
with hand wrenches.
Welded steel segments, similar in shape to castiron
segments, have been used for economic
reasons in some subaqueous tunnels. They were
welded in jigs to tolerances as close as practicable,
but flanges were not machined and no calking
grooves were provided. Difficulties were experienced
in making them watertight with gaskets.
An improved design includes calking grooves
and fabrication tolerances similar to those for
cast iron.
Precast Concrete n Precast segments are
essential to increasing the speed of machine
tunneling. A compromise must be reached between
the segment size and the number of segments to be
installed, directly affecting the weight of the
segments, the size of the equipment needed to
handle the segments, and the number of operations
to be carried out. The width of the segments is
governed by the stroke of the jacks pushing the
head of the shield, usually in the range of three to
five feet. Tapered rings, narrower on one side, are
used on bends. At least three segments per ring are
required, with five to eight being more common.
The closing segment in a ring is usually smaller and
wedge shaped to facilitate insertion. Joints in
adjacent rings are usually staggered so that all
joints are discontinuous, helping to stiffen the
rings.
Connection details to adjacent segments vary
widely and can be flanged (Fig. 20.19). Straight
bolts with nuts, washers and grommets are the
most common, but the use of curved recessed bolts
result in smaller pockets. Gaining popularity are
straight bolts placed at an angle to minimize
recesses; the bolts couple into sockets cast into the
adjacent section. Dowels may also be used between
adjacent rings. The bolts ensure that the rubber or
neoprene seals between segments are compressed.
The addition of a hydrophilic seal near the outside
face may reduce leakage even further. Due to the
very close tolerances needed to ensure seals remain
watertight and that the diameter remains constant,
a high degree of mechanization with steel forms is
used. The segments must be installed within the
shield tail and the space behind them (the tail void)
grouted at a pressure at least equal to the external
pressure, making lateral alignment modifications
TUNNEL ENGINEERING
Tunnel Engineering n 20.39
very difficult. It is not uncommon for most bolts to
be retrieved once the grout is set. Secondary linings
are not essential.
Heavy, interlocking concrete blocks have been
used successfully in relatively dry or impervious
soil. They present difficulties when exposed to
water pressure due to leakage.
Except where steel rings and lagging or concrete
blocks are used as primary lining, no secondary
concrete lining is used, unless required for
appearance and interior finish of highway tunnels.
In this case, a concrete lining of the minimum
thickness practicable is placed. When the tunnel is
to be faced with tile, provision should be made for
attaching it. (To facilitate maintenance and improve
lighting, walls and ceilings of highway tunnels are
usually finished with ceramic tiles.) To provide
good adherence of the scratch coat, scoring wires
may be welded longitudinally on the steel forms
for the lining to provide a rough concrete surface.
Coating of smooth concrete surfaces with epoxy
compound may result in satisfactory finishes at
less cost.
See also Art. 20.18.
20.18 Design of Tunnel
Linings
Article 20.17 discusses the types of linings usually
used for tunnels. The following paragraphs
describe design of a liner ring.
A liner ring is statically indeterminate. A onepass
lining is designed for transport and erection
loads, loads during grouting, and ground loads
including seismic. In lieu of computer analyses,
which might be as simple as a two-dimensional
analysis of a grid framework supported on springs,
or as complex as finite element or finite difference
three-dimensional analyses using soil-structure
interaction for each step of the construction
process, stresses in the liner ring may be computed
after the ring is made statically determinate by a
cut at the top and one end is fixed (Fig. 20.20).
For a circular ring of constant cross section
symmetrically loaded the thrust at the crown C is
Tc ¼ 2
pR
ð p
0
M cos f df (20:2)
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