Civil Engineering Reference
In-Depth Information
requires considerable experience and judgment,
and should only be carried out after thorough
geological investigations have been carried out.
The test procedure consists of pumping water
at a steady rate from the well and measuring the
drop in water level in both the pumped well and
the observation wells. The duration of the test can
range from as short as eight hours to as long as
several weeks, depending on the permeability of
the rock mass. When the pumping is stopped, the
water levels in all the wells are measured until a
static water level is determined—this is known as
the recovery stage of the test. Plots of draw down
(or recovery) against time can be used to calcu-
late permeability values using methods described
by Cedergren (1989), Todd (1959), Jacob (1950)
and Theis (1935).
Because of the cost and time required for con-
ducting a pump test, they are rarely carried out
for rock slope engineering. An example of a situ-
ation where a pump test may be justified would be
to assess the feasibility of driving a drainage adit
for stabilization of a landslide. Generally, install-
ation of piezometers to measure the ground water
table and conduct variable head tests to measure
hydraulic conductivity in boreholes provide suffi-
cient information on ground water conditions for
slope design purposes.
(a)
Infiltration
Original ground surface
Original phreatic line
(b)
Infiltration
(c)
Infiltration
Higher
K
Lower
K
Arbitrary boundary
Figure 5.16
Cut slope in rock with water infiltration
on ground surface behind crest for Example 5.1:
(a) position of the ground water surface before
excavation; (b) slope with variety of surface
infiltration and rock conductivity conditions; (c) slope
with higher conductivity rock close to face, and
various climatic conditions.
5.7 Example Problem 5.1: Influence of
geology and weather conditions on
ground water levels
Statement
objective of the exercise is to sketch in the ground
water table on each slope based on the general
behavior of ground water in slopes as governed
by Darcy's Law.
Figure 5.16 shows a slope, cut in isotropic, frac-
tured rock, under a variety of operational and
climatic conditions; in all cases ground water is
infiltrating the horizontal ground surface behind
the crest of the slope (Terzaghi, 1962). In
Figure 5.16(a) the slope has been recently excav-
ated, and prior to excavation, the water table
was horizontal and at a shallow depth below
the ground surface. In Figure 5.16(b) and (c) the
slope has been open for a sufficiently long time
for ground water equilibrium conditions to be
established, but the climatic conditions vary. The
Required
A
On the cross-section in Figure 5.16(a), draw
the approximate position of the ground water
table after excavation of the slope.
B
On the cross-section in Figure 5.16(b), draw
the approximate positions of the ground
