Page 99 - Rock Mechanics For Underground Mining

ROCK MASS CLASSIFICATION

J a istheJointAlterationNumberrepresentingtheconditionordegreeofalteration
of the structures in the rock mass, varying from 0.75 for wall-wall contact in unaltered
rock or for joints containing tightly healed, hard, non-softening, impermeable filling
to 20 for structures with thick fillings of clay gouge;
J w is the Joint Water Reduction Factor representing the groundwater conditions,
varying from 0.05 for exceptionally high inflows or for water pressure continuing
without noticeable decay to 1.0 for dry conditions or minor inflows; and
SRF is the Stress Reduction Factor which is a coefficient representing the effect
of stresses acting on the rock mass, varying from 0.5 for high stress but tight structure
conditions in good quality rock to 400 for heavy squeezing rock pressures or heavy
rock burst conditions and immediate dynamic deformations in massive rock.
The three quotients in equation 3.11 may be taken to represent the block size,
the inter-block frictional shear strength and the “active stress”, respectively. The
details of how the six parameters in the Q system are determined are given by Barton
et al. (1974), Hoek and Brown (1980), Priest (1993) and Barton (2002), for example.
Except for some changes to the SRF parameter introduced to account for rockburst
conditions, the original Q system has remained essentially unchanged since it was
first developed. Possible Q values range from 0.001 to 1000 on a logarithmic scale.
The system defines nine geotechnical classes of rock mass ranging from exceptionally
poor (Q ≤ 0.01) to exceptionally good (Q ≥ 400). The application of the Q system in
underground mining rock mechanics will be discussed at various points in this book.

It should be noted that some applications use the parameter Q which is the value of
Q with the active stress term J w /SRF, put equal to unity.
3.7.4 Geological strength index (GSI)
As part of the continuing development and practical application of the Hoek-Brown
empirical rock mass strength criterion to be discussed in section 4.9.1, Hoek (1994)
and Hoek et al. (1995) introduced a new rock mass classification scheme known as
the Geological Strength Index (GSI). The GSI was developed to overcome some of
the deficiencies that had been identified in using the RMR scheme with the rock mass
strength criterion.
The GSI was developed specifically as a method of accounting for those properties
of a discontinuous or jointed rock mass which influence its strength and deformability.
As will become apparent in Chapter 4, the strength of a jointed rock mass depends on
the properties of the intact pieces of rock and upon the freedom of those pieces to slide
and rotate under a range of imposed stress conditions. This freedom is controlled by
theshapesoftheintactrockpiecesaswellasbytheconditionofthesurfacesseparating
them. The GSI seeks to account for these two features of the rock mass, its structure
as represented by its blockiness and degree or interlocking, and the condition of the
discontinuity surfaces. Using Figure 3.30 and with some experience, the GSI may be
estimated from visual exposures of the rock mass or borehole core.
It will be noted that the GSI does not explicitly include an evaluation of the uniaxial
compressive strength of the intact rock pieces and avoids the double allowance for
discontinuity spacing as occurs in the RMR system. Nor does it include allowances for
water or stress conditions which are accounted for in the stress and stability analyses
with which the Hoek-Brown criterion is used. Although the origin and petrography
of the rock are not represented in Figure 3.30, the rock type will usually constrain the
range of GSI values that might be encountered in rock masses of that type. Marinos
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