Civil Engineering Reference
In-Depth Information
little increase in the water table. In con-
trast, a strong rock with widely spaced joints
will have low porosity so ground water flow
will rapidly fill the joints and increase the
water pressure within the slope. It is com-
monly found that rock falls on steep rock
faces occur soon after heavy rainfalls, par-
ticularly if the water freezes and expands
behind the face.
(b)
The completion zone of the piezometer
should be positioned where the rock mass
contains discontinuities. For example, if
drill core is available, it should be stud-
ied to locate zones of fractured or sheared
rock where ground water flow is likely to
be concentrated. Positioning the completion
zone in massive rock with few discontinuities
may provide limited information on ground
water pressures. The length of the com-
pletion zone in rock is usually longer than
that in soil because of the need to intersect
discontinuities.
(c)
Faults comprising clay and weathered rock
may have low conductivity and act as ground
water barriers behind which high water
pressures could develop. In contrast, faults
comprising crushed and broken rock may
have high conductivity and act as a drain
(Figure 5.11(c)). Measurement of water pres-
sures on either side of the fault will indicate
the hydraulic properties of these features.
(c)
Other geological features that may be con-
sidered in piezometer installations are fault
zones. These may act as conduits for ground
water if they contain crushed rock, or they
may be barriers to ground water flow if
they contain clay gouge. In the case of
high hydraulic conductivity faults, the com-
pletion zone may be located in the fault,
and for low hydraulic conductivity faults,
the completion zones may be located either
side of the fault to determine any pressure
differential.
5.5 Measurement of water pressure
The importance of water pressure to the stabi-
lity of slopes has been emphasized in previous
chapters. If a reliable estimate of stability is to
be obtained or if the stability of a slope is to be
controlled by drainage, it is essential that water
pressures within the slope be measured. Such
measurements are most conveniently carried out
by piezometers. Piezometers are devices sealed
within the ground, generally in boreholes, which
respond only to ground water pressure in the
immediate vicinity, and not to ground water pres-
sures at other locations. Piezometers can also be
used to measure the
in situ
hydraulic conduct-
ivity of rock masses using variable head tests as
described in Section 5.6.
The following are a number of factors that
may be considered when planning a piezometer
installation to measure water pressures in a rock
slope:
(d)
The number of piezometers, or the number
of completion zones in a single piezometer,
may be determined by the geology. For
example, in a sedimentary deposit contain-
ing low hydraulic conductivity shale and
relatively high hydraulic conductivity sand-
stone, it may be necessary to install comple-
tion zones in each rock unit.
(e)
The hydrodynamic time lag is the volume of
water required to register a head fluctuation
in a piezometer standpipe. The time lag is
dependent primarily on the type and dimen-
sions of the piezometer and can be significant
in rock with low hydraulic conductivity.
Standpipe piezometers have a greater hydro-
dynamic time lag than diaphragm piezo-
meters because a greater movement of pore
or joint water is required to register. The
term slow response time is used to describe
a long hydrodynamic time lag.
(a)
The drill hole should be oriented such that
it will intersect the discontinuities in which
the ground water is likely to be flowing. For
example, in a sedimentary rock containing
persistent beds but low persistence joints, the
hole should intersect the beds.
(f)
In rock slopes where the piezometer is being
used to measure joint water pressure in
