Environmental Engineering Reference
In-Depth Information
existing block with two vertical and two horizon-
tal fractures becomes in the process 9 blocks. With
respect to erosion resistance this is a catastrophic
increase—and was readily observed in the floor
of the spillway, sometimes with greater frequency
than this, in one of the directions of fracturing.
several 1 m long cored and instrumented slots
had been drilled in the floor of the spillway, fol-
lowing the scouring event, and these showed slow
closure when oriented roughly eW (up to 0.5 mm
surface-measured closure), while an almost ns
aligned slot showed opening, by up to 0.15 mm.
such is broadly consistent with a strong stress ani-
sotropy, but would need to be modelled in three
dimensions for interpretation to be meaningful.
The measurements could also be influenced by
sub-horizontal fracturing.
interestingly, occasional 'radial' blast-gas induced
fracturing seen at the base of some remnant, vertical
blast holes, was actually not radial but elliptical—
with the long axis inevitably oriented ns, with some
particularly extended (gas-and-stress-induced) frac-
tures in this direction. at another location in the
spillway floor, a newly stress-fractured slab had
lifted (buckled) making a gap of several centimetres,
beneath its bridge-like structure. high stress anisot-
ropy was evident in many forms at Uhe ita, and
was a valuable learning experience for all parties
involved, including the author who was engaged by
the consortium of contractors.
7
conclUsions
an unexpected linear stress-deformation behav-
iour was measured in the block test that was per-
formed at the BWiP project in hanford, Usa. This
apparently was due to the contribution of both
shear and normal components of joint deforma-
tion. Joints that are closing normally exhibit con-
cave load-deformation curves, while joints that are
loaded in shear exhibit convex load-deformation
curves. These contrasting trends appear to 'cancel'
and a linear curve results.
Basalt flows encountered near the surface may
exhibit low horizontal stress, if there are no topo-
graphic or tectonic reasons for stress concentra-
tion. This would seem to be related to the tensile
nature of joint formation, and is clearest in the case
of columnar basalt. at the BWiP site, the thermal
and flat-jack applied stress in the heated block test
was able to close the vertical columnar, part entab-
lature joints, giving much stiffer behaviour.
cross-hole seismic performed between four
boreholes drilled into the wall of an experimen-
tal tunnel, also at shallow depth, exhibited strong
contrasts in velocity between horizontal (lowest
velocity) and vertical ( highest velocity ) ray paths.
Figure 19. Views of the site during construction and
after reservoir impoundment. The narrowness of the
ridge of rock made for a very compact site (compared
to 16.6 km of mountain tunnels for a much larger river
meander at Jinping ii).
Diagonal ray-paths showed intermediate veloci-
ties. in all cases there was a strong reduction of
velocity of about 2 km/s in the outer 2 to 3 m of
the holes, due to an eDZ caused by blast damage,
stress reduction, and somewhat reduced moisture
content.
at the candidate waste site, the cohasset flow
was encountered at about 900 m depth, and due to
strong stress anisotropy and high stresses (60, 40,
30 MPa) core discing was experienced in the more
massive columnar-jointed rock. stress-induced
fracturing was predicted for the tunnels in this
massiveflow, but joint deformation maybe would
have protected tunnels from rock bursting in the
more jointed entablature.
at the ita 1,450 MW heP in Brazil, an aniso-
tropic horizontal stress distribution of 'normal'
