Environmental Engineering Reference
In-Depth Information
tertiary holes etc. However unless proper attention is paid to the equipment used and
selection of the area to be grouted, the results can be misleading.
18.3.11
Durability of cement grout curtains
From the authors' combined direct knowledge, which goes back to the early 1950s,
cement grout curtains of dams that have operated for 50 years or so seem to have done
their job admirably. Nevertheless there is evidence to suggest that seepage water will,
albeit at very slow rates, gradually leach some of the compounds from the hardened
grout. At Thomson (Victoria) and Corin (ACT) dams, there are calcium ions (along with
magnesium and sulphate ions) in the seepage water at the dams' toes, but only traces of
calcium ions in the storage water. For many dams this very slow deterioration probably
would have little impact. The grouting will have done its main job of sealing up the large
defects and the slow loss of the grout will not result in much of a change in the overall
permeability of the rock mass.
Loss of compounds from the hardened grout probably begins with the slow loss of
excess lime (CaO 2 ) that dissolves in the seepage water as calcium hydroxide (Ca(OH) 2 ).
On contact with air (either direct contact or from dissolved air in, say, rain water), cal-
cium carbonate (CaCO 3 ) forms. Depending on the flow conditions, the CaCO 3 may come
out of solution. Another mechanism would begin with the solution in water of carbon
dioxide (CO 2 ) from bacteria in the ground. The resulting weak acid (H 2 CO 3 ), then com-
bines with the excess lime to give CaCO 3 .
One would assume that this solution process is not unlike the processes described in
Chapter 3 for the gradual loss of carbonates from carbonate rocks. If a partly-grouted
joint in the rock mass has some unfilled zones and water seeps along those open parts of
the joints, grout exposed to the flow will begin to suffer some loss. The loss rates would
be expected to be very slow, particularly if only effectively narrow joints were available
for the seepage water. Even if the loss of material did allow the cement grout to disinte-
grate to some degree, some of the inert particles in the cement grout may be able to join
up the open parts of these joints to help maintain a reasonably low permeability for the
rock mass.
A process like the one just described seems to have occurred at Blowering dam, a
112 m high rockfill dam in the Snowy Mountains Scheme (personal communications
with C. Houlsby in the early 1980s). At Blowering much of the grouting was probably
done with 5:1 and 6:1 W/C ratio (by volume). The weak grout in the joints of the rock
“bleeds” significantly, leaving patches of weak, hardened cement grout and water.
Ultimately on storage filling seepage water was able to “attack” the weak grout patches
and reduce the effectiveness of the grout curtain. Under the spillway CaCO 3 has deposited
in the underflow drains and lowered the capacity of the drainage system. The actual
change in total seepage at Blowering is unknown, as seepage under the dam is not meas-
ured, but the left abutment ridge grouted by a single line curtain has significant seepage
flows through it.
Talbingo dam (Snowy Mountains Scheme - completed 1971) and Thomson dam
(Victoria - completed 1984) are two well-monitored dams that have long seepage records.
Both are 160 m high rockfill dams. The storage at Talbingo is held close to full all the
time, while Thomson, a water supply storage, does vary slowly with time and weather
conditions.
At Talbingo, some 14,000 40 kg bags of cement were used on the curtain and
12,000 bags on the blanket (over the full core contact area). Generally, a 3 to 1 by
volume water/cement was used as a starting mix, but the mixes were sometimes thinned
to 5 to 1 if the ground proved to be tight. The seepage has been steadily dropping since
records were started in late 1982, nearly 11 years after the lake's first filling was
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