Page 165 - Rock Mechanics For Underground Mining

METHODS OF IN SITU STRESS DETERMINATION

Figure 5.3 Relation between fault
geometry and the field stresses caus-
ing faulting.

and of significance in site investigation, relates to the problem of spatial variability of
the field stress tensor. A fracture field in a rock mass is usually composed of members
which are variably oriented. It is inferred that a mechanically compatible stress field
may also be locally variable, in both magnitudes and orientations of the principal
stresses. A heterogeneous stress field is thus a natural consequence of an episode of
faulting, shearing or extensive slip, such as occurs between beds in parallel folding.
Successive episodes of fracturing, where, for example, one fault set transgresses an
earlier set, may be postulated to lead to increasing complexity in the stress distribution
throughout the medium.
It is clear from this brief discussion that the ambient, subsurface state of stress in a
rock mass presents prohibitive difficulty in estimation ab initio. Its direct determina-
tion experimentally also presents some difficulty. In particular, the spatial variability
of the stress tensor suggests that any single experimental determination may bear
little relation to volume averages of the tensor components. In the design of a mine
excavation or mine structure, it is the average state of stress in the zone of influence of
the structure which exerts a primary control on the post-excavation stress distribution
in the excavation near-field rock. The requirements for successful definition of the in
situ state of stress are a technique for a local determination of the stress tensor, and a
strategy for integration of a set of observations to derive a representative solution for
the field stress tensor throughout the sampled volume.

5.3 Methods of in situ stress determination

5.3.1 General procedures
The need for reliable estimates of the pre-mining state of stress has resulted in the
expenditure of considerable effort in the development of stress measurement devices
and procedures. Methods developed to date exploit several separate and distinct prin-
ciples in the measurement methodology, although most methods use a borehole to gain
access to the measurement site. The most common set of procedures is based on de-
termination of strains in the wall of a borehole, or other deformations of the borehole,
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