TUNNEL ENGINEERING

layers may be sprayed on as needed. Total
thickness may be as much as 8 in.
The nozzle may be held directly by an operator
or attached to a boom manipulated by a worker
stationed under the protective roof of the jumbo.
Automatic application has been successful in a
machine-bored tunnel (Heitersberg Tunnel in
Switzerland). Robots, controlled by an operator
on the jumbo, can be used to apply either dry or
wet mix shotcrete.
Shotcrete is sprayed on the sidewalls after
completion of mucking. Heavy water inflow must
be intercepted and drained through inserts in the
shotcrete. Well-trained operators and careful
supervision and control are essential for good
results. Properly executed, the method can be used
successfully for fractured rock.
TUNNEL ENGINEERING
Tunnel Engineering n 20.27
Fig. 20.12 Typical section through Berkeley Hill Rock Tunnel (heavily faulted rock) for the
San Francisco Bay Area Rapid Transit.
Strength of concrete in place reaches 200 to
250 psi in 2 h, 1400 to 1500 psi in 12 h. The ultimate
compressive strength of 4000 to 5500 psi is about
15% less than that of the same concrete without
accelerator.
Waterproofing n Above the groundwater
table, waterproofing is usually applied to ceilings
in transportation tunnels to prevent dripping.
Drainage paths may be provided along the base
of the walls to handle any water that does appear.
False walls with finishes are often used to hide
walls where leakage is expected. For most tunnels
below the groundwater table, a waterproofing
membrane enveloping the tunnel is used between
the initial ground support and the final lining. If the
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