c. Loads. The support pressures in the underground openings are
governed by the unit weight of the soil, groundwater table, soil properties,
deformations during excavation, interaction between soil and the supports,
shape of the opening, and the length of time that has elapsed since the
installation of lining. Other factors such as the presence of another
opening adjacent to it, excavation of a large deep basement near an existing
opening, load from neighboring structures, and change in groundwater
conditions, will also affect the design pressures on the tunnel supports. A
schematic representation of the load action on underground openings is shown
in Figure 23 (Reference 23).
Estimate of load for temporary supports in earth tunnels may be
obtained from Table 2 (Reference 23). For further guidance see Reference 23
and Reference 24, Tunneling in Soft Ground, Geotechnical Considerations, by
Peck.
5.
PRESSURE ON VERTICAL SHAFTS.
a. Shaft in Sand. In the excavation of a vertical cylindrical shaft
granular soils, pressures surrounding the shaft approach active values. If
outward directed forces from a buried silo move the silo walls into the
surrounding soil, pressures approach passive values as an upper limit.
(1) Pressure Coefficients. See Figure 24 for active and passive
pressure coefficients for a cylindrical shaft of unlimited depth in granular
soils.
(2)
Modification of Active Pressures. For relatively shallow shafts
(depth less
than twice the diameter), rigid bracing at the top may prevent
development
of active conditions. In this case, horizontal pressures may be
as large as
at-rest pressures on a long wall with plane strain in the
surrounding
soil. (See DM-7.2, Chapter 3.)
(3) If groundwater is encountered, use submerged unit weight of sand
and add hydrostatic pressure.
b.
Shaft in Clay.
(1) Pressure on Walls of Shafts in Soft Clay. For a cylindrical
shaft, no support is needed from the ground surface to a depth of
2C
z+o, = ))))))). To determine the approximate value of ultimate horizontal
[Upsilon]
earth pressure on a shaft lining at any depth z, use
ph = [Upsilon] [multiplied by] z-c
where
[Upsilon] =
effective unit weight of clay
z=
depth
c=
cohesion
This pressure is likely to occur after several months.
7.1-198
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