Stress Measurements - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 517

Figure 16.8 shows how even small discontinuities can considerably influence the stress
state measured by a triaxial cell. More serious problems are malfunctions of the data
logger and debonding of strain gauges. Figure 16.9 clearly shows strains recorded dur-
ing overcoring using a malfunctioning data logger. Oscillations, drifts and spikes are
observed. Such measurements cannot be considered as reliable and should be rejected
as invalid.
Also, imperfect rock and strain gauge behavior resulting from core damage can lead
to deviations from the regular shape of recorded strains (Fig. 16.5) when using triaxial
cells in weak or ductile rocks.
Owing to these various potential sources of defects a sufficient large number of tests
should be carried out.
The evaluation of stress measurements should be conducted using suitable software by
the supplier of the cell. In any case both the recorded strains and the results of stress
calculations must be subjected to critical assessment and plausibility check by the geo-
technical consultant. This also applies to all stress measuring methods described below.
Figure 16.8 Influence of discontinuities on the local stress state measured by means of a triaxial cell