TUNNEL ENGINEERING

20.52 n Section Twenty
mean water level. Live load includes seabed
erosion and siltation, and variations in water level,
current, storm loads and earthquakes, each with a
return period of 5 years or less. Exceptional loads
include loss of support (subsidence) below the
tunnel or to one side, and storms and extreme
water levels with a probability of being exceeded
once during the design life. Extreme loads include
sunken ships, ship collision, water-filled tunnel,
explosion (e.g. vehicular), fire, the design seismic
event predicted for the location, and the resulting
movement of soils. Conditions to be investigated
should include normal, abnormal, extreme, and
construction.
20.21 Shafts
In tunnel work, shafts are starting points for
excavation in rock or firm material or shields. For
long tunnels, such as aqueducts, several shafts
are used to divide construction into shorter
sections that can be worked simultaneously. For
vehicular tunnels, especially subaqueous shield
tunnels, the shafts are used as bases for
ventilation buildings. In construction of shafts,
regulations of the Occupational Safety and Health
Administration should be observed (Arts. 20.6,
20.8, and 20.12 to 20.16).
Timber shafts are mined and braced in the same
manner as tunnels in similar material. Usually,
poling boards 5 to 6 ft long are driven into the
ground and braced at regular intervals by
rectangular timber frames. Then, the soil is
excavated to the ends of the polings and a new
frame installed at this level.
A relatively shallow shaft may be started
oversize with sheeting 10 to 20 ft long driven
vertically on the outside of the frame bracing.
Intermediate frames are installed as the excavation
proceeds. At the bottom of the tier of sheeting, the
sides are stepped in to make room for the next tier
of vertical sheeting.
In rock shafts, timbering is used to prevent loose
rocks from falling off the walls. Its placement usually
lags an appreciable distance behind the excavation.
Steel liner plates alone, or in combination with
horizontal ribs, may be used in soft ground where
excavation can be made in increments equal to the
width of the liner plates. H beams driven vertically
as soldier piles, with wood or steel lagging and
horizontal bracing, may be used for rectangular
TUNNEL ENGINEERING
shafts. Enclosures of vertical steel sheetpiling, for
round or rectangular shafts, are suitable for waterbearing
ground.
Where ground conditions are poor and waterbearing,
shafts may be constructed with a caisson
(hollow box), with compressed air as needed to
exclude water. Gravity pulls the caisson down as
excavation proceeds. Since its weight is relatively
small, the caisson may have to be temporarily
ballasted or jetted for sinking. The depth to which a
caisson may be sunk is limited by the high cost of
compressed-air work, which results from the short
working hours permitted under high pressure.
Open-bottom shafts with heavy walls, often
circular or subdivided into compartments, may be
built on the ground and sunk by excavating the
ground underneath. In dry soil, the excavation may
be done directly; if water is present, clamshell
buckets and high-pressure jets may be used to
loosen the soil and remove it. On reaching the
proper depth, the bottom of the shaft is closed by
tremie concrete.
As an alternative method for shaft construction,
water-bearing ground may be frozen in a circular
ring around the shaft location and the excavation
made in the dry. Closed-end pipes are driven
vertically into the ground around the periphery,
and open-end smaller pipes inserted into them. A
refrigerant, usually brine, is circulated at temperatures
as low as 230 8F from the interconnected
inner pipes into the larger ones and from them
returned to the refrigeration plant. Several months
may be required to freeze a deep ring solidly. The
ventilation shaft of the Scheldt River Tunnel in
Antwerp was built in this manner, as were a
number of mine shafts in Germany and France.
(J. O. Bickel and T. R. Kuesel, “Tunnel
Engineering Handbook,” Van Nostrand Reinhold
Company, New York.)
20.22 Seismic Analysis and
Design
Earthquake loads, or more correctly seismic loads,
are included among the loads on a structure that
are required to be considered by most current
design codes. Seismic effects can occur during the
construction phase and should therefore also be
considered during that period; an appropriate level
of risk should be agreed with the owner. The
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