Environmental Engineering Reference
In-Depth Information
Subsidence from
consolidation
Filter blanket
30 cm thick
or greater
Surcharge
Ver y
soft
soil
Strong soils
Sand drains
30 cm in diameter
or greater/at 3 m
or larger spacing
(Wick drains are
alternate)
FIGURE 8.55
Vertical sand drains to accelerate drainage
and consolidation of weak soils beneath a
surcharge.
enter from the surface and a layer of free-draining material is provided. Its thickness
depends on its permeability. The roadway is crowned and sloped to allow percolation in
the drainage layer to exit beyond the pavement and enter drainage ditches.
Blanket drains
are optional considerations to control high hydrostatic pressures beneath
the roadway and are barriers to capillary moisture. They are connected to trench drains as
shown in
Figure 8.57a.
Longitudinal trench drains
are installed along the roadway, even in level or near-level ter-
rain, as well as in side-hill cuts (Figure 8.57b), to intercept groundwater and permit base-
course drainage. Perforated, jointed, slotted or porous pipe is placed near the trench
bottom and surrounded with pea gravel. Trench backfill is of carefully selected pervious
filter material designed to prevent the piping and infiltration of adjacent soil. The surface
is sealed with clay or some other impervious material to prevent rainwater infiltration.
Rainwater runoff is controlled by surface ditches.
Transverse interceptor drains
or a
drainage blanket
, as shown in Figure 8.57c, are necessary
to control flows beneath side-hill fills and provide stability.
All drains
must flow freely under gravity and discharge at locations protected against
erosion.
Tunnels
Predrainage
of high-pressure water trapped by a geologic structure in rock masses such as a
fault zone may be accomplished by pilot “feeler” holes drilled in front of the tunnel head-
ing. At the very least, the pilot holes help to disclose severe groundwater conditions. The
holes are spaced to explore at angles from the heading, as well as directly in front, since in
rock masses a water-bearing zone can be present at any orientation with the tunnel heading.
Grouting
is used in soil or rock. Very difficult conditions are caused by saturated crushed
rock zones associated with faulting and folding. They have been combatted in some cases
by sealing the tunnel heading with a concrete bulkhead and injecting grout to stabilize the
mass. Lancaster-Jones (1969) describes a case where four phases of treatment were carried
out to stabilize a wet, crushed quartzite sand before tunneling could proceed. A prelimi-
nary injection was made with sodium silicate to assist penetration of the subsequent
cement grout, which was pumped under a pressure of up to 1400 psi (100 tsf) to fissure the
ground and compress the sand. This formed a strong network of cement in the fractured
ground and sealed the main water passage. A third treatment was made with chemical
