Questions 6.1 -6.1 0:
intact rock
46.1 Predict how the volume of a specimen will change during a
uniaxial compression test, and sketch the variation of volumetric strain
versus axial strain. What physical processes are occurring in the sample
as the curve manifests significant gradient changes?
46.2 What causes a rock to break when it is compressed uniaxially
to failure? Is it when the stress reaches a certain value or when the
strain reaches a certain value? Or is it when some other parameter
reaches a critical value, such as the energy input per unit volume, or the
microcrack density per unit volume?
46.3 The geometry of the linear Mohr-Coulomb envelope is such that
a number of useful relations between strength parameters can be drawn
from it. Derive an expression for the uniaxial compressive strength of
rock in terms of the cohesion and angle of internal friction.
46.4 The linear Mohr-Coulomb envelope with a tensile cut-off sets a
definite limit on the maximal uniaxial tensile strength of a material. By
considering the largest uniaxial tensile Mohr circle that can be drawn,
determine this tensile strength limit in terms of a, and 4.
46.5 A firm whose judgement has been questioned on a previous
occasion has been entrusted with the strength testing of rock in a site
investigation project. During their first uniaxial compression test, the
equipment failed to measure the peak axial load, but the technician did
record that the specimen failed by the formation of a single fracture
inclined at 20" to the loading axis. In a subsequent triaxial test, as the
confining pressure was being increased before application of the axial
stress, the specimen failed prematurely when the confining pressure in
the Hoek cell was 85 MPa.
On the basis of these results, propose a failure criterion of the form
= au3 + b for the rock.