Environmental Engineering Reference
In-Depth Information
n.R. Barton
Nick Barton & Associates, Oslo, Norway
aBsTRacT: contrasting geophysical, rock mechanics and rock engineering experience in basalts,
caused by either exceedingly low or extremely high stress are described, from projects in the Usa and
Brazil. The first involves a nuclear waste characterization project in hanford basalts in the Usa, and the
second describes, in much more detail, stress-fracturing problems in numerous large tunnels at the 1450
MW ita hydroelectric project in se Brazil's basalts. Particular phenomena that were noted, include linear
stress-strain loading curves when columnar basalt is loaded horizontally, and a k
0
value reaching about
20-25 at ita heP.
1
inTRoDUcTion
The beauty of columnar basalt, and the huge areal
extent of basalt flows across large tracts of many
countries, are perhaps the features that character-
ize basalt most profoundly. The colombia River
basalts in Usa, and the Parana Basin basalts of
s.e. Brazil, are just two of these major accumu-
lations of 10's of thousands of km
2
of basalt. in
this paper, some sophisticated characterization in
the first location mentioned, in the hope of find-
ing a nuclear waste disposal candidate, and some
major rock engineering problems due to extreme
horizontal stress in the second location, will form
the core of this paper.
2
sTRess-DeFoRMaTion chaRacTeR
one of the Usa's nuclear waste disposal candidates
of the mid-eighties was the 900 m deep cohasset
f low of the extensive colombia River basalts. This
was found some distance away at a more conven-
ient
shallow
depth for preliminary but extensive
characterization studies, at the so-called hanford
BWiP (basalt waste isolation project).
some interesting joint deformation effects were
caused by the low horizontal stress levels at this (too)
shallow location, as revealed in an
in situ
block test,
and at larger scale in some cross-hole seismic meas-
urements in a tunnel wall, showing strong eDZ
effects. at each scale, behavior was affected in special
ways by the anisotropic joint properties and by ani-
sotropic stress levels, particularly the low horizontal
stress. The latter could be controlled in the block test,
and thermal loading logically caused joint closure:
the original state. an unexpected linear stress-
deformation behaviour was measured in the block
Figure 1.
Basalt forms blocks of many shapes and
forms.
test, apparently due to the contribution of both shear
and normal components of joint deformation.
some site characterization was performed by the
author, along exposures of the candidate cohasset
along the distant colombia River. Both joint prop-
erties and rock mass properties were described, in
an attempt to evaluate their potential effect on dis-
posal tunnels planned for 900 m depth at the candi-
date site, and possible tunnel support quantities.
