Environmental Engineering Reference
In-Depth Information
in most codes. Unless the “leak” is wide spread, in most dams the localised effect would
be countered by the crack not propagating. The authors believe that a conservative atti-
tude to pore pressures within the dam wall would cope with any such localised pressure
development.
CaCO
3
formation can also occur in dams within the foundations, below the water sur-
face in the drains. The reaction here involves CO
2
being released by microbes in the
ground and H
2
CO
3
being formed. This very mild acid then reacts with the excess lime
from, say, a grout curtain and CaCO
3
results. This phenomenon has been seen at the
pipeline inlet structure, a 34.7 m high gravity dam, at the head of the pipelines to Tumut
3 power station in the Snowy Mountains Scheme.
The possible formula of CaCO
3
, limonite and other products in drains clearly points to
the need to clean these drains regularly. At best, this cleaning would include high pressure
water jet cleaning; at worst, the holes may have to be reamed. If the drains in the dam wall
do play a key role in the dam's stability, then this cleaning operation should extend to
them as well as to the foundation drains. In new dams, therefore, care must be taken to
detail all drains so that cleaning can be done without the expenditure of a lot of money.
Without a regular cleaning program in place, full reliance on the drains must be viewed
with some doubt.
16.6.2.4
Hydro-dynamic forces
Spillway sections of concrete dams are subjected to additional hydro-dynamic forces due
to the passage of water over the crest.
At the design discharge capacity, an ogee crest has zero pressure at the crest due to the
water. However at discharges higher than the design discharge, there is a negative pressure
which adds to the overturning moment and must be accounted for. These effects are
shown in
Figure 16.1
. The negative pressures are most significant in small height structure
such as spillway crests leading to a chute spillway.
In a flip bucket or any location where flow is changing direction in a concave upwards
manner, the water exerts positive pressures. These may also contribute to overturning if
they are downstream of the centre of the base of the dam.
Methods for calculating these pressures are given in FERC (2000), Brand (1999), and
USBR (1990).
As pointed out by FERC (2000), and shown in
Figure 16.27
, if the outlet for a drain is
in the flip bucket of a spillway, it may be subject to very high pressures. Such pressures can
also be transmitted to the foundation through open joints in the spillway, or poorly posi-
tioned drain holes in spillway aprons. These situations should be avoided or, if present in
a dam, the drainage system should be amended.
As discussed by FERC (2000), the uplift pressures in the dam foundation will respond
rapidly to reservoir levels because in a saturated rigid rock foundation, extremely small
volume changes are required to transmit the pressures. Such behaviour has been clearly
recorded for many years at Tumut Pond dam (an arch dam). In the absence of measured
data to prove otherwise, the uplift should be assumed to vary linearly with changes in
reservoir and tailwater levels. However there are examples (EPRI, 1992) of non-linear
response giving higher pressures.
16.6.2.5
Aprons
Upstream and downstream aprons have the effect of increasing the seepage path under the
dam. For an upstream apron properly sealed to prevent leakage, the effect is to reduce the
uplift under the dam. The effectiveness of upstream aprons in reducing uplift is compro-
mised if cracks and joints in the apron permit leakage.
Downstream aprons, such as stilling basins or spillway chutes have the effect of increas-
ing uplift under the dam as shown in
Figure 16.28
.
