Low stress and high stress phenomena in basalt flows - Volcanic Rock Mechanics - page 2

cohasset fl flow exposed along the colombia Figure 2.

River.

Figure 4. comparative scales of boreholes in 'massive'
rock and tunnels in jointed rock: either columnar or
entablature. stress-fracturing (core-discing or rock-
bursting) in cases a, B, and c, but not in case D due to
stress re-distribution and greater deformation. Possible
σ θmax ≈ 140 MPa at 900 m depth. with failure initiation at
about 0.4 to 0.5 × Ucs. Barton (1986).

Figure 3. cohasset flow sampled at 920 m depth.

Drilling and stress measurements had indicated
strongly anisotropic stresses of approximately
60, 40 and 30 MPa, and some cores, presumably
drilled in the midst of columnar basalt, displayed
strong core discing ( Figure 3 ) . The likely perform-
ance of the planned disposal tunnels at the same
depth, was therefore of some concern. a concep-
tual image of excavation 'scale effects' is shown
in Figure 4, to illustrate the likely relative effects
of massive columnar basalt and the more jointed
and irregular entablature, on the performance to
be expected in planned disposal tunnels. Barton,
(1986), contract report.
king et al., (1986), performed an interesting
set of cross-hole seismic measurements, in a part-
flow-entablature part-columnar jointed basaltic
rock mass, at the BWiP site. The columns were
regular but sinuous, 0.15 to 0.36 m in thickness,
dipping 70 to 90°, with frequent low angle, discon-
tinuous cross-jointing. The measurements were
made between four horizontal boreholes drilled
12 metres into the wall of a drill-and-blasted
underground opening, at 46 m depth. The objec-
tive was to investigate the effect of blast damage
and stress redistribution, i.e., two of the assumed

Figure 5. The borehole layout for cross-hole seismic
measurements in the wall of a drill-and-blasted experi-
mental tunnel.

chief components of the eDZ or excavation
damage and disturbed zone.
The two diagonal seismic-ray paths #1-4 and
#2-4 showed, in contrast, almost identical seismic
velocities, with a plateau at about 5-5.5 km/s, and
reduction to about 3.6 to 4.4 km/s in the outer
2 to 3 m.