Page 368 - Engineering Rock Mass Classification_ Tunnelling, Foundations and Landslides

Chapter 26 Geological Strength Index 323

TABLE 26.3 Rating for Joint Alteration Factor (JA)

Term Description JA
0.75
Rock wall Healed or “welded” joints Clear joints 1
contact (unweathered) 2
Softening, impermeable filling
(quartz, epidote, etc.) 4

Fresh rock walls No coating or filling on joint 3
(unweathered) surface, except for staining 4
4
Alteration of joint wall: The joint surface exhibits 6
slightly to moderately one class higher alteration 8
weathered than the rock 8–12

Alteration of joint wall: The joint surface exhibits
highly weathered two classes higher alteration
than the rock

Coating or thin filling

Sand, silt, calcite, talc, etc. Coating of frictional material
without clay

Clay, chlorite, talc, etc. Coating of softening and
cohesive minerals

Filled joints Sand, silt, calcite, etc. Filling of frictional material
with partial Compacted clay materials without clay
or no Soft clay materials
contact Swelling clay materials “Hard” filling of softening
between and cohesive materials
the rock
wall Medium to low over-
surfaces consolidation of filling

Filling material exhibits
swelling properties

Sources: Palmstrom, 1995; Cai et al., 2004.

GENERALIZED STRENGTH CRITERION

Hoek, Carranza-Torres, and Corkum (2002) suggested the following generalized
Hoek-Brown strength criterion for undisturbed rock masses:

s1 ¼ s3 þ qc½mb s3 þ sŠn ð26:6Þ
qc

where s1 ¼ maximum effective principal stress, s3 ¼ minimum effective principal
stress, qc ¼ uniaxial compressive strength (UCS) of rock material (intact) for standard
NX size core (see Table 8.13 after Palmstrom, 2000), mb ¼ reduced value of the material

constant mr, and

GSI À 100
mb ¼ mrÁ exp 28 À 14D ð26:7Þ