Borehole Breakouts as Stress Indicators
Extensive field evidence and laboratory
experiments suggest that borehole breakouts,
defined as borehole cross-section elongations
resulting from preferential rock failure, is a direct
consequence of the in situ stress in the rock. One
of the early observations of breakouts was in the
quartzite and conglomerates of the Witwatersrand
gold mine in South Africa (Leeman, 1964). The
spalling was observed to occur at diametrically
opposed points on the borehole wall perpendicular to the direction of the
maximum principal stress.
The most publicized observation of breakouts was in the 3 m diameter drift at
420 m level in the Underground Research Laboratory (URL), Canada. Two
diametrically opposed breakouts were approximately aligned with the vertical
stress, which is the overall least principal stress at URL.
The breakout phenomena was also observed in the non-welded Paint Brush tuff
along the spring line of the Exploratory Studies Facility approximately 250 m
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below the Yucca Crest, Nevada. The measured stress
regime reveals that both horizontal stresses are
smaller than the vertical stress, suggesting the overall
maximum principal stress is the vertical stress.
Three different modes of failure have been suggested
as the mechanism leading to the breakout.
Extensile failure model
(V. Maury, 1987; Haimson and Herrick, 1986; Lee and Haimson, 1993; Song and
Haimson, 1997)
A family of subparallel microcracks was induced without any obvious shear
displacement behind the borehole wall in the two zones aligned with the
minimum horizontal stress direction. The cracks are densely spaced and
subparallel both to the borehole wall and the maximum horizontal stress
direction. Progressive spalling of detached flakes bounded by these extensile
cracks leads to deep and pointed breakouts.
Shear failure model
(Zoback et. al, 1985)
Cracks initiates at the borehole wall, were intergranular and propagated along a
pth of high shear stress. Moreover breakouts were formed by the intersection of
two distinct conjugate fractures, rather than by clusters of flakes between
micorcracks.
Compaction band model
(B.C. Haimson, in Phys. Chem. Earth(A), Vol. 26, no.1-2, pp. 15-20, 2001)
In high porosity granular rock, breakouts initiate at the borehole wall in the two
zones aligned with the minimum horizontal stress direction. In previous models,
propagation of breakouts was limited by the diminishing length of the flakes
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(extensile failure model) or by the domain defined by crossing conjugate
fractures. However, in highly porous (approximately 20 to 25% or more) rock,
initiation of breakouts provides seed for the propagation of anti-mode I fracture in
the form of compaction band. The tip of the breakout or anti-mode I fracture
advances orthogonally to the maximum horizontal stress. The typical shape of
the breakouts created under this condition is in the form of long fracture with
compaction band ahead of the fracture tip.
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