Questions and answers: in situ rock stress 5544.10 The plan below (Sugawara and Obara, 19997) shows a ho-
rizontal section through a rock mass. Stress measurements were
made from the gallery along the borehole line Ox using the
Japanese CCBO technique. The measured principal stresses in the
horizontal plane are plotted on the plan. What are the main conclu-
sions that you can draw from the stress variations?
20 MPa
Stress scale \IA4.10 An initial interpretation is as follows.
(a) The stress field is not homogeneous throughout the rock mass, as
both the magnitudes and orientations of the principal stresses vary
along Ox.
(b) The stress field near the excavation is as expected: the principal
stresses should be parallel and perpendicular to the excavated rock
surface, and the principal stress perpendicular to the excavation at
measuring point 1 should be very low.
(c) The fault appears to be having an effect on the stress field because
the maximum principal stress is rotating anticlockwise along the
borehole from the gallery towards the fault, but then clockwise from
the fault further into the rock mass.
(d) The fault appears to be transmitting stress; if it were an open fault,
the maximum principal stress at measurement points 6 and 7 would
be more sub-parallel to the fault.
(e) The changes in principal stress orientation in the regions to the left
of the fault and to the right of the fault follow reasonably consist-
ent trends (compared to random orientations at each measurement
point). This implies that there is a consistent regional stress field
which has been locally affected by the gallery excavation and the
fault, and that there is a primafacie case for the measurement tech-
nique being credible.
(f) The magnitudes of the principal stresses in the region of points
9-16 are lower than those at points 7, 8 and 17, 18 indicating a
relatively destressed region, probably caused by more than one fault
or excavation in the location.'This diagram is from the ISM Draft Suggested Method. Sugawara K. and Obara Y.
(1999) In situ stress measurement using the compact conical-ended borehole overcoring
(CCBO) technique. Int. 1. Rock Mech. Min. Sci., 36,3, 307-322.