Environmental Engineering Reference
In-Depth Information
TABLE 3.24
Deformation Characteristics of Intact Rock in Uniaxial Compression to Failure
a
Deformation Stress vs.
Rock Type
Failure Form
Strain
I. Elastic
Fine-grained, massive, Basalt, quartzite,
Sudden, explosive, brittle
diabase dolomite, some strong limestones
II. Elastic-plastic
Fine-grained sedimentary. Softer
Plastic yielding
limestones, siltstone, tuff
III. Plastic-elastic
Sandstone, granite, some diabases, schist
Brittle fracture — stiffness increases as
cored parallel to foliation
microfissures close
IV. Plastic-elastic-plastic
Metamorphic — marble, gneiss
Inelastic yielding — stiffness increases
as microfissures close
V. Plastic-elastic-plastic
Schist cored perpendicular to foliation
Inelastic yielding — stiffness increases
as microfissures close
VI. Elastic-plastic creep
Rock salt
Inelastic yielding and continuous
creep
a
After Miller, R.P., Ph.D. thesis, University of Illinois, Urbana, 1965.
Creep
deformation occurs in some rocks over long time intervals at stress levels substan-
tially less than those required to cause short-term deformation or failure. In rock masses,
the problem has most practical significance in soft rocks such as halite or in overstressed
rocks (high residual stresses), where relaxation occurs along joints (Hendron, 1969).
Soils
are essentially nonelastic, usually plastic, and occasionally viscous. They demon-
strate pseudo-elastic properties under low stress levels, as evidenced by initial stress-
strain linearity. “Elastic” deformation, however, is immediate deformation, and in many
soil types does not account for the total deformation occurring over long time intervals
because of consolidation.
Elastic Constant Values
Typical values for the elastic constants for a variety of materials are given in
Table 3.25.
Typical values for various rock types are also given in
Figure 3.92.
Poisson's ratio
for soils is evaluated from the ratio of lateral strain to axial strain during
a triaxial compression test with axial loading. Its value varies with the strain level and
becomes constant only at large strains in the failure range (Lambe and Whitman, 1969). It
is generally more constant under cyclic loading: in cohesionless soils it ranges from 0.25 to