Laboratory Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 417

sumed, tangential strain ε r of the specimen are equal and the volu-

metric strain of the specimen can be calculated as

ε t and radial strain

(14.23)

Alternatively, the measurement of axial strain ε r is suffi cient to cal-

culate the volumetric strain of a specimen with isotropic deformability:

ε a and radial strain

(14.24)

As a measuring device, strain gauges may be glued onto the lateral surface of the spec-
imen as described in Section 14.4.1. Alternatively, the specimen's strains can be deter-
mined by displacement measurements using displacement transducers (Fig. 14.3).
The specimen is fi rst subjected to a hydrostatic load (p = σ 1 = σ 2 = σ 3 ) at constant stress

rate. Subsequently, the axial stress σ 1 is increased at constant axial strain rate or stress

rate and constant cell pressure p until failure. The strain rate or stress rate, respectively,
should be selected in such a way that the duration of test is not less than 5 minutes. For
intact rocks with pronounced time-dependent behavior and for saturated porous rocks
where a build-up of pore pressure may take place, the strain rate and stress rate, respec-
tively, should be selected as small as possible (DGEG 1979b).
The described testing procedure in which failure of the specimen is initiated by an in-
crease in axial stress leading to a stress state of σ 3 at failure is approved as the

standard testing procedure (DGEG 1979b, ISRM 1983). In the less common “triaxial
extension test” failure is achieved by either increasing confi ning stress p or decreasing
axial stress

σ 1 > p =

σ 3 .
If the volumetric strain of the specimen during hydrostatic compression is measured,
the bulk modulus K IR of the intact rock can be calculated from the increments of ap-
plied stress p and volumetric strain

σ 3 . In both cases the specimen's stress state at failure is p = σ 1 >

ε V :

(14.25)

The elastic constants of the intact rock can be determined from the stress-strain curves
obtained during deviatoric compression. For isotropic stress-strain behavior and if, in
addition to the axial strain ε a , the tangential strain ε t of the specimen is also measured,

the elastic constants E IR and ν IR can be calculated from the increments of deviatoric

stress σ = σ 1 - σ 3 , axial strain ε a and tangential strain ε t , as illustrated in Fig. 14.4, ac-

cording to Equations (14.14) and (14.15).

To separate the elastic, reversible strains from inelastic, irreversible strains a testing pro-
cedure including loading and unloading cycles is required (Fig. 14.4). In addition, at
the points of load reversal the specimen may be held under constant load in order to
investigate the time-dependent behavior (Fig. 14.5).