Environmental Engineering Reference
In-Depth Information
magnitude for se Brazil appears to have been seri-
ously concentrated by maximum-stress-aligned
river erosion. This occurred in the narrow 150 m
high ridge separating a 12 km river meander, which
was chosen for the site of this hydroelectric plant.
Further concentation of horizontal stress in this
ridge was caused by general stripping and excava-
tion of large surface structures such as the auxil-
iary spillway. Within the ridge of basalt are two
particularly massive, high Q-value, high modulus
flows, which probably concentrated the horizontal
stress even more. Tunnelling in these flows pro-
duced many surprises, even when tunnel depths
were only 50 to 100 m.
excavation of the five diversion tunnels in an
e-W direction beneath this highly stressed ridge
apparently increased the already high stresses
within the massive flows, to tangential stress levels
as high as 120 to 140 MPa, even at 50 m depth. For
the large 15 by 17 m temporary diversion tunnels
with minimum rock bolting and shotcreting, the
resulting 2 to 3 m deep stress-fracturing allowed
major erosion, amounting in places to loss of sev-
eral meters of hard basaltic rock in the invert as a
result of river-flood diversion through the tunnels,
and of course a similar loss of 2 to 3 m of stress-
fractured rock in the arch.
For the five pressure shafts excavated in the
same e-W direction through the ridge, the most
serious consequence of the extreme stress anisot-
ropy was the highly negative minimum tangential
stress, which caused tensile cracking of the rock
and later of the concrete lining ( in the 3 o'clock
and 9 o'clock positions), when even low pressure
contact grouting was performed. The location of
the cracking followed simple rock mechanics the-
ory, and appears to have been repeated earlier when
drilling vertical investigation boreholes, which
showed
greatest permeability in the most massive
flows
, probably due to n- and s-side tension cracks
down the massive-rock parts of the boreholes.
lessons to be learned include the need for stress
measurements in general, when lightly reinforced
lined (or unlined) pressure tunnels are contem-
plated, and topographic reasons suggest insufficient
minimum rock stress. The extreme stress anisotropy
at ita heP would have far exceeded the limits for
hydraulic-fracturing based stress measurement—
due to drilling-induced tensile cracks that would
not have given a break-down pressure nor a shut-in
pressure, except at a larger, unknown radius. The
maximum principal stress could not then have been
estimated in the normal manner.
extreme, rock stress-induced, systematic tensile
fracturing around prospective pressure tunnels,
prior to their operation, is rather unusual, and
presents a dilemma. high pressure grouting of
the rock could probably have helped to eliminate
the two regions of negative tangential stress along
each pressure shaft, prior to reinforced concrete
lining.
Partial 'homogenization' of the tangential
stresses and general rock mass improvements
through systematic grouting would also have
reduced the need for heavy reinforcement of the
concrete liner, but would need to be proved by
post-treatment permeability and stress measure-
ment, and local cross-hole seismic and more gen-
eral tunnel wall refraction seismic.
ReFeRences
andrioli, F.R., Maffei, c.e.M. & Ruiz, M.D. 1998.
improving the lining of the headrace tunnel in charcani
V Power Plant, Rio chili, Peru. Proc. 5th s. american
conf. on Rock Mech. and 2nd Brazilian conf on Rock
Mech., saRocks 98, santos, Brazil.
Barton, n. 1986. Deformation phenomena in jointed
rock. 8th laurits Bjerrum Memorial lecture, oslo.
Publ. in Geotechnique, Vol. 36: 2: 147-167.
Barton, n., By, T.l., chryssanthakis, P., Tunbridge, l.,
kristiansen, J., løset, F., Bhasin, R.k., Westerdahl, h. &
Vik, G. 1994. Predicted and Measured Performance of
the 62 m span norwegian olympic ice hockey cavern
at Gjøvik. int. J. Rock Mech, Min. sci. & Geomech.
abstr. 31:6: 617-641. Pergamon.
Barton, n. & Grimstad, e. 1994. The Q-system following
twenty years of application in nMT support selec-
tion. 43rd Geomechanic colloquy, salzburg. Felsbau,
6/94. pp. 428-436.
Barton, n. 1995. The influence of Joint Properties in
Modelling Jointed Rock Masses. keynote lecture,
8th isRM congress, Tokyo, 3; 1023-1032, Balkema,
Rotterdam.
Barton, n. 2002. some new Q-value correlations to assist
in site characterization and tunnel design. int. J. Rock
Mech. & Min. sci. 39/2: 185-216.
infanti Jr, n., Tassi, P.a., Mazzutti, R., Piller, M. & Mafra,
J.M.Q. 1999. Tensões residuais nas obras subterrâneas
da Uhe itá. XXXiii seminário nacional de Grandes
Barragens, comitê Brasileiro de Barragens. Belo hori-
zonte, Brazil.
king, M.s., Myer, l.R. & Rezowalli, J.J. 1986. experimen-
tal studies of elastic-wave propagation in a columnar-
jointed rock mass.
Geophys. Prospect.
,
34
: 1185-1199.
Martin, D.c., christiansson; R. & soderhall, J. 2002.
Rock stability considerations for siting and con-
structing a kBs-3 repository, based on experience
from Äspö hRl, aecl's hRl, tunnelling and min-
ing. skB (swedish nuclear Fuel co.) stockholm,
TR-01-38.
Zimmerman, R.W. & king, M.s. 1985. Propagation of
acoustic waves through cracked rock.
26th US Symp.
On Rock Mechanics, Rapid City SD.
ashworth (ed.).
739-745. Rotterdam: Balkema.
