TUNNEL ENGINEERING

Steel liner plates are available in various shapes
and sizes. They may be used to support the ground
if a limited excavated area of the roof or arch will
stand long enough for insertion of the liner plates,
starting at the top of the arch and working down.
The flange of each plate is bolted to the previously
erected liner.
In small tunnels, ribbed or corrugated liner
plates may give adequate support. In large tunnels
or under heavier loads, the plates are backed up by
steel ribs, against which they are blocked. Liner
plates without flanges may also be used as lagging
or poling. See also Art. 20.17.
To prevent settlement or unbalanced load, all
voids behind the liner plates should be filled by
injection of pea gravel or cement grout.
Small tunnels may consist of a single heading.
For large tunnels, various combinations of headings
are used. Some of these are known by the
country of their origin, as American, Austrian,
Belgian, English, German, or Italian methods, but
are used in many variations. Originally, the
methods required wood supports, but now steel
supports are favored, where economical.
Sequential Excavation Method (SEM) n
Also known as the New Austrian Tunneling
Method (NATM), SEM was developed in Austria
but is now used worldwide. It is a tunneling
method adapted to the excavation of variable and
non-circular cross-section reaches of tunnel, such
as highway ramps and subway stations. This
underground method of excavation divides the
space (cross-section) to be excavated into segments,
then mines the segments sequentially, one portion
at a time. Excavation sequencing by the American
method, Austrian system and Belgian method are
outlined below.
The excavation can be carried out with common
mining methods and equipment (often a backhoe),
chosen according to the soil conditions; tunnelboring
machines are not used. Ground conditions
are assessed at the face of the tunnel or from the
side of a small tunnel, which helps to decide how to
proceed in the best way and determines the choice
of equipment and lining. It should be noted that the
combination of ground treatment and SEM for the
excavation of uniform cross-section tunnels would
generally be more expensive than the use of
pressurized face TBM construction under American
underground construction labor and economic
TUNNEL ENGINEERING
Tunnel Engineering n 20.29
conditions. Thus the application of SEM would
be limited economically to variable geometry
structures. However for shallow tunnels, such
structures could probably be more economically
constructed using cut-and-cover techniques.
SEM requires extremely dry conditions; dewatering
is often necessary before the excavation
can proceed. SEM involves careful sequencing of
the excavation as well as installation of supports.
Shotcrete (a kind of concrete sprayed from highpowered
hoses) may be used to line the tunnel or
support the face, and grouting (the injection of a
cementing or chemical agent into the soil) may be
used to increase the soil’s strength and reduce its
permeability. Because of the requirements of this
method, the rate of excavation is slow. Use of this
method in saturated, non-cohesive granular soils
would require the use of groundwater control and
ground improvement techniques. One real concern
with the use of SEM in granular soils is sudden
uncontrollable ground loss, often resulting in
surface sinkholes. This can happen when the
selected ground improvement method is unsuccessful
because of localized variation in ground
conditions.
One method often used to control groundwater
is compressed air. However, the high air pressures
often required might make the use of compressed
air tunneling uneconomical in comparison to other
possible methods. It is for similar reasons that shield
tunneling using compressed air has been replaced
by tunneling with pressurized face tunnel boring
machines. Another commonly used method of
controlling groundwater is dewatering. However,
unrestricted dewatering can have a significant
effect on adjacent foundations. An approach that
has been used with variable success overseas has
been to install groundwater cut-off walls (slurry
walls, etc.) along both sides of the right-of-way and
then dewatering inside the cut-off. When dewatering
sands, running or fast-raveling ground, conditions
may result so that some form of ground
improvement, such as closely spaced groutable
spiles or horizontal jet grouting above the crown of
the excavation could be required. Two other ground
improvement methods that could be used are jet
grouting and chemical grouting. Each method
would be used to create a block of stabilized ground
through which the tunnels could be excavated.
American Method n As shown in Fig. 20.15a,
excavation starts with (1) a top heading at the
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