TUNNEL ENGINEERING

unless the rock is highly abrasive, causing
excessive wear of drills.
Too much information can never be provided
for the engineer, to produce a realistic design, and
for the contractors, to prepare sound bids. Even at
best, unforeseen difficulties must be expected.
In addition to geological surveys and borings
(Art. 20.5), engineers may use electric-resistivity
measurements and gamma-ray absorption for
information on depth and characteristics of rock
formations. Information also may be obtained from
the U.S. Geological Survey, which has extended its
scope and geophysical studies beyond the mining
field. Where geological conditions are particularly
hard to evaluate or are especially severe, exploratory
pilot tunnels, about 10 10 ft, may be driven
part way from each end or for the entire length of a
tunnel, prior to final design and advertising of
construction. TBMs may be used for pilot tunnels
Table 20.2 Load Hp in Feet on Rock on Support in Tunnel*
either used as a seperate service tunnel, or
abandoned, or enlarged to form the final tunnel.
In these pilot tunnels, internal rock stresses can be
measured by pressure cells and strain gages
inserted in transverse drill holes, and the nature
of the rock, foliation, blockiness, and pressure of
faults and water can be inspected.
Tunnel Boring Machines (TBM) n The high
initial cost of hard-rock TBMs tends to restrict their
use to longer tunnels. Although ideally suited to
circular tunnels due to the rotary motion of the
cutting heads, variations of the excavated shape
may be technically feasible, as have been done with
a few soft-ground TBMs. Large grippers are jacked
outwards, either to each side or top and bottom,
and hold the main part of the machine and transfer
the applied thrust to the adjacent firm rock. Disk
Rock condition Hp, ft Remarks
1. Hard and intact Zero Light lining on rock bolts only if spalling or
popping occurs
2. Hard, stratified, or schistose 0 to 0.5B Light supports. Load may change erratically
from point to point
3. Massive, moderately jointed 0 to 0.25B
4. Moderately blocky and seamy †
0.35(B þ Ht)to 1.10(B þ Ht)
No side pressure
5. Very blocky and seamy 0.35(B þ H t)
to 1.10(B þ Ht)
6. Completely crushed, chemically
intact †
TUNNEL ENGINEERING
7. Squeezing rock, moderate depth 1.10(B þ Ht) to
2.10(B þ Ht)
8. Squeezing rock, great depth 2.10(B þ Ht) to
4.50(B þ Ht)
9. Swelling rock Up to 250ft,
regardless of
value (B þ Ht)
Little or no side pressure
1.10(B þ H t) Considerable side pressure. Requires
continuous support of lower ends of ribs
or circular ribs
Heavy side pressure; invert struts required.
Circular ribs recommended.
Same as for Type 7
Tunnel Engineering n 20.23
Circular ribs. In extreme cases, use yielding
supports
* If depth of rock over tunnel is more than 1.5(B þ Ht), where B is width and Ht is height of tunnel. From R. V. Proctor and T. L. White,
“Rock Tunnels and Steel Supports,” Commercial Shearing & Stamping Co., Youngstown, Ohio.
† If roof of tunnel is permanently above the water table, values for Types 4 and 6 can be reduced by 50%.
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