Environmental Engineering Reference
In-Depth Information
upstream, might have
maximum
tangential stress
levels in the range 103 to 135 MPa, and
minimum
tangential stresses as low as (-) 29 to (-) 42 MPa,
easily enough to exceed the tensile strength of the
basalts.
We know that the former, whatever their real
magnitude, had been sufficient to cause stress frac-
turing in the first tunnels excavated, and loss of
100's up to 1000's of m
3
of stress-fractured rock
sile stresses) would clearly be large enough to cause
tensile fractures on ns sides (or 3 o'clock and
9 o'clock positions) around the pressure tunnels,
which were yet to be completed—if these pressure
shafts passed through sufficiently massive flows
for the above kirsch elastic isotropic solutions to
be relevant (max. tang. stress = 3a-B, min. tang.
stress = 3B-a, where a and B are the major and
minor principal stresses.).
as it happens it was also discovered during this
first involvement with the project that the most
massive h and i flows had 'mysteriously' given
the highest permeabilities. This mystery is easily
explained if the
minimum
horizontal stresses were
also of the same order of magnitude as the above
vertical stress assumption. Vertical tension cracks
along the n and s sides of these parts of the inves-
tigation boreholes could readily explain the 'inex-
plicable' high permeabilities in the most massive
rock mass. This is another illustration of the need
for separate
characterization
and
classification
for
before and after excavation, at whatever scale. such
differentiation when using the Q-system is empha-
sised in Barton, 2002.
Figure 15. consistent stress-induced fracturing of 2
to 3 m depth (also in the invert in places where scoured
by flood—flows. These large diversion tunnels measure
15 × 17 m.
to 1.38. Taking σ
c
as an average 200 MPa, the above
implies that the maximum tangential stress may
have been as high as 120 to 140 MPa. if we further
assume relevant vertical stress ranges from about
1.25 to 2.5 MPa from 50 to100 m overburden depths,
and an elastic isotropic theoretical σ
ϕ (max)
= 3σ
h
- σ
v
,
we obtain estimates of σ
h
of about 39 to 46 MPa.
The implication is therefore that the ratio of princi-
pal stresses (σ
h
/σ
v
) may be as high as approximately
20-25, which of course is exceptional.
5
cRackinG oF The PRessURe
TUnnels
The foregoing 'situation report', which can be
summarized effectively by Figure 16, was deliv-
ered in 1997, two years before the author's sec-
ond visit to the site in 1999, following completed
excavation and lining of the five pressure shafts/
tunnels (mostly a 55 degrees inclined section of
140 m length and a lower horizontal section con-
taining the final steel penstocks). Raised boring
of the 'central' core of each inclined shaft had
been followed by drill-and-blast excavation of the
complex (sometimes double-curved) 9m diameter
pressure conduits, which had been temporarily
supported with fibre-reinforced shotcrete and
rock bolts, followed by about 0.5 m of reinforced
concrete—
but with reinforcement only in the lower
half of each shaft
, where the water pressure head
would vary from 55 to 110 m. This omission of
reinforcement proved in the end to be a false
economy.
4.1
An estimation of negative minimum
tangential stresses
as the critical pressure tunnel excavations were at
a very preliminary stage, some re-evaluations of
potential tangential stress anisotropy was appro-
priate, for the partly horizontal, partly steeply
inclined shafts. For the case of excavation through
the massive h and i flows, a similar assumption
to the above, of σ
h
(max) of 35 to 45 MPa was uti-
lized, together with a vertical stress range assump-
tion of 1 to 2 MPa. This is an unusually extreme
stress anisotropy, but phenomena from around the
site appear to support it, as we shall see.
Based on the above, and application of simple
kirsch equations, the eW oriented pressure tun-
nels, like the eW oriented diversion tunnels further
