Civil Engineering Reference
In-Depth Information
rock. For example, the Downie Slide in British
Columbia has an area of about 7 km
2
and a thick-
ness of about 250 m. Stability of the slope was of
concern when the toe was flooded by the con-
struction of a dam. A series of drainage tunnels
with a total length of 2.5 km were driven at an
elevation just above the high water level of the
reservoir. From these tunnels, a total of 13,500 m
of drain holes was drilled to reduce the ground
water pressures within the slope. These drain-
age measures have been effective in reducing the
water level in the slide by as much as 120 m, and
reducing the rate of movement from 10 mm/year
to about 2 mm/year (Forster, 1986). In a mining
application, ground water control measures for
the Chuquicamata pit in Chile include a 1200 m
long drainage adit in the south wall, and a num-
ber of pumped wells (Flores and Karzulovic,
2000).
Methods of estimating the influence of
a drainage tunnel on ground water in a
slope include empirical procedures (Heuer,
1995), theoretical models of ground water
flow in homogeneous rock (Goodman
et al
.,
1965), and three-dimensional numerical mod-
eling (McDonald and Harbaugh, 1988). In all
cases, the flow and drawdown values will be
estimates because of the complex and uncer-
tain relationship between ground water flow and
structural geology, and the difficulty of obtaining
representative permeability values.
Empirical procedures for calculating inflow
quantities are based on actual flow rates measured
in tunnels. Based on these data, a relationship has
been developed between the normalized steady-
state inflow intensity (l/min/m tunnel length/m
head) and the rock mass conductivity determined
from packer tests (Heuer, 1995). The flow quant-
ities can be calculated for both vertical recharge
where the tunnel passes under an aquifer, and
radial flow for a tunnel in an infinite rock mass.
This empirical relationship has been developed
because it has been found the actual flows can
be one-eighth of the calculated theoretical values
based on measured conductivities.
Approximate inflow quantities can also be
estimated by modeling the drainage adit as an
infinitely long tunnel in a homogeneous, isotropic
porous medium, with the pressure head on the
surface of the tunnel assumed to be atmospheric.
If flow occurs under steady-state conditions such
that there is no drainage of the slope and the head
above the tunnel
H
0
is constant with time, the
approximate rate of ground water flow
Q
0
per
unit length of tunnel is given
2
πKH
0
2.3 log
(
2
H
0
/r)
Q
0
=
(12.11)
where
r
is the radius of the tunnel driven in homo-
geneous material with hydraulic conductivity
K
.
For rock formations with low porosity and low
specific storage it is likely that transient condi-
tions will develop where the head diminishes with
time as the slope drains.
An important aspect of slope drainage is to
install piezometers to monitor the effect of drain-
age measures on the water pressure in the slope.
For example, one drain hole with a high flow
may only be draining a small, permeable zone in
the slope and monitoring may show that more
holes would be required to lower the water
table throughout the slope. Conversely, in low
permeability rock, monitoring may show that
a small seepage quantity that evaporates as it
reaches the surface is sufficient to reduce the
water pressure and significantly improve stability
conditions.
12.4.7 “Shot-in-place” buttress
On landslides where the slide surface is a well-
defined geological feature such as a continuous
bedding surface, stabilization may be achieved by
blasting this surface to produce a “shot-in-place”
buttress (Aycock, 1981; Moore, 1986). The effect
of the blasting is to disturb the rock surface and
effectively increase its roughness, which increases
the total friction angle. If the total friction angle
is greater than the dip of the slide surface, then
sliding may be halted. Fracturing and dilation of
the rock may also help reduce water pressures on
the slide surface.
