Page 161 - Rock Mechanics For Underground Mining

FACTORS INFLUENCING THE IN SITU STATE OF STRESS

sub-surface point be given by

p zz = z (5.1)
where is the rock unit weight, and z is the depth below ground surface.
Failuretosatisfythisequilibriumcondition(equation5.1)inanyfielddetermination
of the pre-mining state of stress may be a valid indication of heterogeneity of the stress
field. For example, the vertical normal stress component might be expected to be less
than the value calculated from equation 5.1, for observations made in the axial plane
of an anticlinal fold.
A common but unjustified assumption in the estimation of the in situ state of stress
is a condition of uniaxial strain (‘complete lateral restraint’) during development of
gravitational loading of a formation by superincumbent rock. For elastic rock mass
behaviour, horizontal normal stress components are then given by

p xx = p yy = p zz (5.2)
1 −
where is Poisson’s ratio for the rock mass.
If it is also assumed that the shear stress components p xy , p yz , p zx are zero, the
normal stresses defined by equations 5.1 and 5.2 are principal stresses.
Reports and summaries of field observations (Hooker et al., 1972; Brown and Hoek,
1978) indicate that for depths of stress determinations of mining engineering interest,
equation 5.2 is rarely satisfied, and the vertical direction is rarely a principal stress
direction. These conditions arise from the complex load path and geological history
to which an element of rock is typically subjected in reaching its current equilibrium
state during and following orebody formation.

5.2 Factors influencing the in situ state of stress

The ambient state of stress in an element of rock in the ground subsurface is deter-
mined by both the current loading conditions in the rock mass, and the stress path
defined by its geologic history. Stress path in this case is a more complex notion than
that involved merely in changes in surface and body forces in a medium. Changes in
the state of stress in a rock mass may be related to temperature changes and thermal
stress, and chemical and physicochemical processes such as leaching, precipitation
and recrystallisation of constituent minerals. Mechanical processes such as fracture
generation, slip on fracture surfaces and viscoplastic flow throughout the medium,
can be expected to produce both complex and heterogeneous states of stress. Conse-
quently, it is possible to describe, in only semi-quantitative terms, the ways in which
the current observed state of a rock mass, or inferred processes in its geologic evolu-
tion, may determine the current ambient state of stress in the medium. The following
discussion is intended to illustrate the role of common and readily comprehensible
factors on pre-mining stresses.

5.2.1 Surface topography
Previous discussion has indicated that, for a flat ground surface, the average verti-
cal stress component should approach the depth stress (i.e. p zz = z). For irregular

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