Environmental Engineering Reference
In-Depth Information
models of these joint configurations showed such
trends.
in Figures 9 and 10, a more simple-minded clas-
sification of columnar basalt is suggested, using
the first four Q-parameters. Jw and sRF might be
extreme.
ratios k
0
as high as 25:1 could be interpreted in the
50 to 100 m deep tunnels.
The first telltale signs of high horizontal stresses
and strong stress anisotropy developed gradually
as the project itself progressed, during the four
years of construction time. separate phenomena
in different locations in the project eventually built
a convincing picture of a highly stressed, narrow
rock ridge in which the river meander itself presum-
ably had acted like an 'over-coring' agent. (see the
assumed regional stress anisotropy was concen-
trated in the narrow, pillar-like ridge, and with each
new excavation, stress concentrations proved to be
close to the limit of stress-induced fracturing—and
sometimes exceeded the limit, despite the high
strength of the basalts.
Popping noises, some thin slab ejection, and
larger than expected deformations were recorded
during excavation of the
five diversion tunnels
, at
depths of only 50 and 100 m beneath the ridge.
since stress problems and deformations were
more notable as the tunnels reached their full
height, the previously provided rock bolting in the
arch of each top heading proved, in retrospect, to
be insufficient, as some areas of excessive scour-
ing in the arch and invert were later experienced
following river diversion. several metres thickness
3
sTRess PRoBleMs aT iTa hYDRo
BRaZil
in the case of the Brazilian 1,450 Mw ita hydro-
electric project, contrasting Q-values in adjacent
columnar and entablature flows were the focus
of stress-fracturing predictions for various tun-
nels. it was found that the least jointed flows with
high Q-values attracted extremely high stresses in
the topographic ridge defining the project location
across a meander in the river. stress fracturing and
extensive, many meters deep, 'dog-earing' occurred
in the five large 150 m
2
diversion tunnels. in the
higher-elevations of five pressure tunnel linings,
cracking occurred when contact grouting, spe-
cifically in the 3 o'clock and 9 o'clock positions,
over total lengths of hundreds of meters. There
was also extensive erosion loss of basalt in the
first flood-operation of the spillway, which could
be attributed to the stress-aligned fracturing. stress
Figure 11a. The 1450 MW ita hydroelectric project was
built across a rock ridge formed by a 12 km meander in
the Uruguay River in south east Brazil
Figure 11b. stress-related problems were noted first in the
(Tuneis de Desvio) diversion tunnels, later in the (Tuneis
Forcado) pressure tunnels and finally in the spillway.
