Environmental Engineering Reference
In-Depth Information
Figure 16.23.
Effect of block sliding and rotation on joint apertures uplift pressure. (a) Initial condition,
parallel sided joint, (b) Block sliding, joint c width reduced, joint a width increased; (c)
Block rotation, joint b narrows at downstream end (Adapted from EPRI, 1992).
discontinuities in the rock foundation can be narrower at the downstream than upstream
because of the stresses and deformations induced by the dam and the reservoir load on the
dam, and by geological features which limit the flow of water.
Figure 16.23 shows the effect of block sliding and rotation in the foundation;
Figure
16.24
the effect of varying joint apertures and
Figure 16.25
the effect of a low permeabil-
ity shear zone.
This emphasises the need for careful consideration of the foundation geology in design-
ing new dams, or assessing existing dams which do not have piezometers in the founda-
tion. However there is no substitute for actual measurement of the pressures, with a
carefully designed and well-monitored system of piezometers.
16.6.2.2
Analysis of EPRI (1992) uplift data
An analysis by Khabbaz and Fell (1998) of uplift data recorded in EPRI (1992), gave the
results shown in
Figure 16.26
. In this figure the uplift coefficient
is the ratio of the meas-
ured pressure above tailwater level to the difference between reservoir level and tailwater
level. It is equivalent to
K
in Equations 16.21 to 16.24. Data from 25, mostly USA, dams
was available and represent the maximum pressures recorded across each dam. X/L is the
ratio of the distance from the line of drains to the measuring point compared to the distance
from the line of drains to the downstream toe of the dam.
Table 16.5
summarises the data.
This information shows that the “standard method” assumption of
0.33 at the
drains is usually conservative. The highest value recorded was 0.47 for piezometers in one
block of a dam where the drains were somewhat blocked, and the next highest, 0.33 for
a dam with drains spaced at about 11 m centres rather than the normal 2 metres.
However analysis of some Australian dams showed some had areas with maximum
uplifts greater than the “standard method”.
Analysis of the data to assess the effect of depth of drains, height of dam and other vari-
ables showed few clear trends. There did seem to be a tendency for lower uplift pressures
for high dams, which may reflect greater care in the construction of the drains or grout
