Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 457

The modulus for initial loading, also referred to as “deformation modulus” E D , and the
modulus for unloading E are determined from the slopes of the linear portions of the
corresponding stress-displacement curves and then averaged over the measuring direc-
tions. The slope of the stress-displacement curves for initial loading in most cases will
be smaller than for unloading since during initial loading irreversible deformations due
to plastic deformations, such as on discontinuities, may occur. The example illustrated
in Fig. 15.3 (lower) yielded mean moduli of and , as-

suming a Poisson's ratio of = 0.25.

In this example, the stress-displacement curves of the two unloading branches on which
the evaluation were based had approximately the same slope, yielding a modulus of
. If this should not be the case the unloading moduli evaluated for each
branch should be averaged. In many cases the slope of the stress-displacement curves
for unloading and reloading can be regarded as approximately equal (Fig. 15.3, lower).
Otherwise, the reloading moduli should be specifi ed as well.

ν

Interpretation of the dilatometer test data beyond the limit of elasticity was also at-
tempted. The applied pressure p causes tangential tensile stresses around the borehole
which superpose the stress state already existing in the rock mass. If tensile strength is
exceeded during the test, radial tension cracks or a radially fi ssured zone may be formed
around the borehole. For this case, Rocha et al. (1966a) and Ladanyi (1976) derived
formulas for the determination of the rock mass deformation modulus from stress-dis-
placement curves measured in dilatometer tests. In these formulas it is assumed that
the rock mass is originally homogeneous, elastic and isotropic and is subjected to a
hydrostatic in-situ stress state. It is supposed that the tensile strength of the rock mass is
known and isotropic, but jointed rock is not normally subjected to a hydrostatic in-situ
stress state (see Chapter 9) and also tensile strength is usually not known and can rare-
ly be assumed to be isotropic. Therefore such formulas are of limited practicability
(Wittke 1990).
If in addition the unconfi ned compressive strength of the rock mass is exceeded during
the test, failure will be governed by compressive stresses, rather than by tensile stresses
only, and a crushed or fractured zone will be formed around the borehole (Ladanyi 1966).
Thus, the interpretation of dilatometer test data beyond the elastic limit requires that
the stress-strain behavior, the strength and the in-situ stress state of the rock mass are
known. Also, the location and strength of pre-existing discontinuities must be account-
ed for. All this information is not normally available.
From the stress-volume curve obtained in pressiometer tests in which the volume change
Δ

V due to a uniform radial pressure is measured, Young's modulus can be calculated as
follows, when the rock mass is assumed to be homogeneous and its stress-strain behav-
ior is elastic and isotropic (USACE 1982):

(15.2)

where p, that is, the volume of

the injected water, Vi i is the volume of the empty measuring cell, and

Δ V is the volume change of the measuring cell due to Δ