TUNNEL ENGINEERING

more economical because of their strength and
ease of installation. These are made of I beams
cold-rolled into shape. For small tunnels with
circular arches, the sets may be continuous
frames. In larger tunnels or for flat arches, the
sets consist of separate posts and arches (Fig. 20.11).
Where roof supports only are necessary, the
arches may be supported on plates resting on
rock ledges. Steel sections are usually uniform for
the entire tunnel, and spacing of sets is varied
according to rock loads. Normal spacing is 4 ft.
but spacing may be reduced to 2 ft or increased to
as much as 6 ft.
The sets should be erected as soon as scaling of
loose rock has been completed. Blocking should
immediately be wedged between the steel and the
rock surface at 3- to 5-ft intervals to prevent rock
movement from starting. The steel frames should
allow space at the crown, between the lower flange
and the concrete surface, for a pipe for placing
concrete.
Timber or steel lagging should be placed
between the sets. The amount of lagging depends
on rock conditions. Lagging may be practically
solid, or there may be gaps of various widths
between the sheets, as required by circumstances.
Badly fragmented rock may require metal panning
between sets if water is present. The pans are made
TUNNEL ENGINEERING
Fig. 20.10 Drill and Blast (TARP).
Tunnel Engineering n 20.25
of interlocking channels. The space between pans
and rock should be dry-packed to allow water to
run off into the drainage system.
The concentrated loads on the sets at blocking
points produce bending moments in the frames.
Table 20.2 presents formulas for loads on supports
in rock tunnels (R. V. Proctor and T. L. White, “Rock
Tunnels and Steel Supports,” Commercial Shearing
and Stamping Co., Youngstown, Ohio).
Through badly faulted rock or pressure areas,
circular tunnel sections and ring supports are
preferable, particularly in seismic areas (Fig. 20.12).
Rock Bolts n In good rock, but also for some
rock that may be classified as poor, rock bolts may
be used to secure the excavation. They are usually
1 in in diameter and 8 ft long. They may be
coupled, however. The bolts provide anchorage in
sound rock, where they are held by wedges driven
into split ends when the bolts are inserted or by
expansion sleeves gripping the sides of the hole
when the bolts are threaded in. The bolts are tested
for pull-out and prestressed by nuts bearing
against face plates on the rock surface. Untensioned
deformed bars, bolts, or steel or glass-fiber
tubes are used as rock reinforcement and fully
grouted with cement or high-strength resin grout.
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