Civil Engineering Reference
In-Depth Information
Table 4.3
GSI values characterizing blocky rock masses on the basis of particle interlocking
and discontinuity condition
GEOLOGICAL STRENGTH INDEX FOR
JOINTED ROCKS (Hoek and Marinos, 2000)
From the lithology, structure and surface
conditions of the discontinuities, estimate
the average value of GSI. Do not try to
be too precise. Quoting a range from 33
to 37 is more realistic than stating that
GSI = 35. Note that the table does not
apply to structurally controlled failures.
Where weak planar structural planes are
present in an unfavorable orientation
with respect to the excavation face, these
will dominate the rock mass behavior.
The shear strength of surfaces in rocks
that are prone to deterioration as a result
of changes in moisture content will be
reduced if water is present. When work-
ing with rocks in the fair to very poor
categories, a shift to the right may be
made for wet conditions. Water pressure
is dealt with by effective stress analysis.
DECREASING SURFACE QUALITY
STRUCTURE
INTACT OR MASSIVE—intact
rock specimens or massive
in situ
rock with few widely spaced
discontinuities
90
N/A
N/A
80
BLOCKY—well interlocked un-
disturbed rock mass consisting
of cubical blocks formed by three
intersecting discontinuity sets
70
60
VERY BLOCKY—interlocked,
partially disturbed mass with
multi-faceted angular blocks
formed by 4 or more joint sets
50
40
BLOCKY/DISTURBED/SEAMY
—folded with angular blocks
formed by many intersecting
discontinuity sets. Persistence
of bedding planes or schistosity
30
DISINTEGRATED—poorly inter-
locked, heavily broken rock mass
with mixure of angular and
rounded rock pieces
20
10
LAMINATED/SHEARED—lack
of blockiness due to close spacing
of weak schistosity or shear planes
N/A
N/A
