Environmental Engineering Reference
In-Depth Information
6
aUXiliaRY sPillWaY oPeRaTion
The fourth, major stress related phenomenon at
Uhe ita was experienced in the flat bottomed
spillway excavations, in other words right to the
surface in excavations intersecting the two massive
flows h and i. Partly unlined, or mostly unlined
auxiliary spillways have operated with success in
Brazilian dam projects built in basalts in the past,
and experienced consultants accepted a similar
design for Uhe ita. one of the specific reasons
that the basaltic rocks strongly resist erosion dur-
ing the infrequent, but sometimes extreme water
flows, is that they have an interlocked, 'jigsaw-like'
pattern of vertical and sub-vertical jointing, fre-
quently with curved interlocking roughness (e.g.
Figures 9 and 10). alternatively the joints may have
a smaller-scale 'open saw-tooth' roughness, caused
by the brittle-ductile cooling front that allows cool-
ing cracks to develop downwards from the surface,
but only as an intermittent process, a few centi-
metres at a time. There are often minor changes
of direction with each cooling-joint propagation,
which helps to add to the deformation resistance
of blocks that are jointed in this way.
The suspicion of a fourth stress-related phenom-
enon was occasioned by an unprecedented loss of
18,000 m
3
of rock from the floor of the 20,000 m
3
/s
auxiliary spillway, but during only 2½ hours (!) of
spillway operation with flows of only 800 m
3
/s for
1 hour, and 1,600 m
3
/s for 1½ hours in the 2000/2001
rainy season. The author's third visit to the site in
the dry season in mid 2001 coincided with the pos-
sibility to inspect, in dry conditions, if there was
any evidence to suggest stress-enhanced erosion.
Besides a previously hidden 'junta falha' or
joint-fault beneath part of the spillway, the most
important phenomenon proved to be the exist-
ence of numerous, well-oriented tension fractures,
which crossed or ran sub-parallel with some exist-
ing ne trending joints. The stress-induced fractures
had—inevitably—the familiar ns (ridge-parallel)
orientation. There was also an equally pervasive
development of sub-horizontal and equally fresh
(unweathered) tension fractures, which also satisfy a
ns maximum principal stress orientation. Thus the
basalt had lost its prime property of non-systematic
jointing (if we ignore the familiar columnar jointing,
which resists erosion with reasonable efficiency).
The now systematic fracturing of the basalt
effectively divided existing, irregular-shaped blocks
into smaller, more easily eroded units, and as the
upper or front parts were removed by traction and/
or pore pressure during spillway operation, the
next sub-blocks were exposed for a similar treat-
ment. an existing basalt block, if divided by just
one vertical and one horizontal fracture, becomes
in the process 4 blocks. a finer division of each
Figure 18. longitudinal impression of the grouting
induced cracking along considerable lengths of each of
the inclined pressure shafts, which unfortunately did not
have steel reinforcement in their upper 50 to 60 m.
shafts into the rock mass above the deep power-
house excavation. an extension of the drainage
fans from an extension of the existing drainage
gallery was recommended, in order to be able to
intersect the above cracks (between each pressure
shaft) at more acute angles, to improve drainage
efficiency. More fans of drain holes were added.
Piezometers were already installed and more were
added.
although some additional drainage capacity
was installed, there was preference for the more
extensive crack (and potential crack) repair using
'epoxy taping'. leakage under first filling was
well controlled, though was higher than desirable.
a scientifically interesting phenomenon was also
discovered. There was a very minor rotation of the
powerhouse inclinometers, when a pressure tunnel
was taken out of operation for inspection, suggest-
ing a coupled (effective stress controlled) deforma-
tion of the rock mass surrounding the pressure
shafts, in the hillside above the power house. need-
less to say, extensive improvement of the grouting
in this area was recommended, in order to reduce
sensitivity to potential effective stress changes, and
more drain holes were drilled from the drainage
gallery.
