Page 165 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 10

Rock Mass Index

All things by immortal power near or far, hiddenly to each other are linked.
Francis Thompson

English Victorian Post

INTRODUCTION

There is no single parameter that can fully designate the properties of jointed rock
masses. Various parameters have different significance, and only in an integrated form
can they describe a rock mass satisfactorily.

Palmstrom (1995) proposed a rock mass index (RMi) to characterize rock mass
strength as a construction material. The presence of various defects (discontinuities)
in a rock mass that tend to reduce its inherent strength are taken care of in rock mass
index (RMi), which is expressed as

RMi ¼ qc Á JP ð10:1Þ

where qc ¼ the uniaxial compressive strength (UCS) of the intact rock material in MPa.
JP ¼ the jointing parameter composed of mainly four jointing characteristics, namely,

block volume or density of joints, joint roughness, joint alteration, and joint size. It is

a reduction coefficient representing the effect of the joints in a rock mass. The value
of JP varies from almost 0 for crushed rock masses to 1 for intact rocks ¼ sn Hoek

and Brown’s criterion (Eq. 13.6). RMi ¼ rock mass index denoting UCS of the rock

mass in MPa.

SELECTION OF PARAMETERS USED IN RMi

For jointed rock masses, Hoek, Wood, and Shah (1992) reported that the strength
characteristics are controlled by the block shape and size as well as their surface
characteristics determined by the intersecting joints. They recommended that these pa-
rameters were selected to represent the average condition of the rock mass. Similar ideas
have been proposed earlier by Tsoutrelis, Exadatylos, and Kapenis (1990) and Matula
and Holzer (1978).

This does not mean that the properties of the intact rock material should be disre-
garded in rock mass characterization. After all, if joints are widely spaced or if an intact
rock is weak, the properties of the intact rock may strongly influence the gross behavior
of the rock mass. The rock material is also important if the joints are discontinuous.
In addition, the rock description includes the geology and the type of material at the

Engineering Rock Mass Classification 133
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