TUNNEL ENGINEERING

conditions. Stone or brick masonry has been used to
a great extent in the past, but currently concrete is
preferred. The thickness of the permanent concrete
lining is determined by the size of the tunnel,
loading conditions, and the minimum required to
embed the steel ribs of any primary lining.
The lining is placed in sections 20 to 30 ft long.
Segmental steel forms are universally used and
must be properly braced to support the weight of
the fresh concrete. The walls are usually concreted
first, up to the spring line. Next come the arch
pours. It is important that the space between the
forms and the rock or soil surface be completely
filled. Grout pipes should be inserted in the arch
concrete to permit filling any voids with sand-andcement
grout.
Concrete is placed through ports in the steel
lining or pumped through a pipe introduced in the
crown, a so-called slick line. Placement starts at the
back of the pour, and the pipe is withdrawn slowly.
A combination of both methods may be used.
Concrete is either pumped or injected by slugs of
compressed air. Admixtures are added to get an
easily placed mix with low water content and to
reduce concrete shrinkage. If there is leakage
of water, it usually occurs at shrinkage cracks,
which may be sealed with a plastic compound. Or
the water may be carried off by copper drainage
channels installed in chases cut in the concrete
(Art. 20.9).
Footings for side walls in rock tunnels are cut
into the rock below grade. They give adequate
stability unless squeezing ground is encountered,
in which case a concrete invert lining is placed. In
soft ground, a concrete slab is placed, to serve as
pavement in highway tunnels. If heavy side
pressure exists, this slab may have to be made
heavier to prevent buckling.
Unreinforced Concrete Lining n A concrete
lining is placed to protect the rock and
provide a smooth interior surface. Where the
concrete lining is exposed to compression stresses
only, it may be unreinforced. Most shafts not
subject to internal pressure are lined with
unreinforced concrete. Shrinkage and temperature
cracks are probable and may cause leakage. Where
there is a risk of non-uniform loading, unreinforced
liners are not used, such as in squeezing ground
and through soil overburden.
TUNNEL ENGINEERING
Tunnel Engineering n 20.37
(Recommendations in Respect of the Use of
Plain Concrete in Tunnels, AFTES c/o SNCF,17
Rue d’Amsterdam, F75008 Paris, France.)
Reinforced Concrete Lining n In most
cases, reinforcing steel will be required to withstand
tension and bending stresses. Reinforcement
is usually required at least on the inside face to
resist temperature stresses and shrinkage, although
reinforcement elsewhere may be needed to resist
moments.
Linings for Shield Tunnels n Linings for
shield tunnels may be one-pass or two-pass. A onepass
lining system is when the final lining is also
the initial lining, usually for tunnels in soil. With a
two-pass lining system, an initial lining is installed
behind the shield just sufficient to allow the shield
to advance while a waterproofing membrane is
installed and the final cast-in-place reinforced
concrete lining is prepared. The advance rate is
thus usually faster and costs fall. The initial lining
may be segmental rings with minimal bolting for
ease of erection (Fig. 20.18), or steel ribs with
lagging. Precast concrete segments are now widely
used and the use of cast iron and fabricated steel
are rare due to their high cost. Although the initial
lining may be designed as part of the final lining,
any leakage through the seals would result in the
full hydrostatic pressure acting on the inside final
lining for which it should be designed.
Pipe in Tunnel n Water and sewer tunnels up
to 14 ft diameter are often provided with an
internal pipe that forms the inner lining. After the
pipe is secured against movement, the space
between the initial ground support and the pipe
is filled with cellular or mass concrete. Sewer pipes
may require a further interior lining to protect
against corrosive liquids and gases. Water tunnels
with a high internal pressure exceeding the
expected external pressures are usually provided
with a steel lining if a reinforced concrete lining is
insufficiently strong. Since the pipe may be
dewatered, it must also be designed for the external
pressure, which, if the pipe has leaked, may equal
the internal pressure.
(U.S. Army Corps of Engineers Manual, 1997,
Design of Tunnels and Shafts in Rock, EM 1110-
2-2901.)
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