Engineering Rock Mechanics

(Jacob Rumans) #1

1 34 Rock masses: deformabilify, strength and failure


0

270 90

0

270 90

140.0.-^0

90

180 ltk
One fracture set Two fracture sets
0 0

270 ' 90 no 90

180 180
Three fracture sets Four fracture sets

presence of fractures has a significant effect on the rock mass strength.
Moreover, the fractures determine the location and orientation of the
failure surfaces. In a particulate soil, there are effectively weaknesses at
all locations and at all orientations, so the failure location and orientation
are often governed not by the soil structure, but by the geometry of the
stress field.


2 140
120
c, 100
C VI 80

Rock mechanics - intact rock plus
fractures, anisotropic, failure
location and orientation governed by
pre-existing weakness in the rock
mass
Soil mechanics - more isotropic and
weaker material, failure location and
z 20 orientation governed by stress field
Bo ::o LL2zzLl 30 60 90
Inclination of discontinuity normal to
major principal stress axis

It is useful to bear these ideas in mind when dealing with a weak
rock or strong soil, such as shale and chalk, or severely weathered rocks.
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