Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 459

may produce tension cracks and rock fracturing. Thus, the rock mass may no longer
behave elastically. This can lead to a considerable underestimation of Young's modulus
calculated according to (15.4), even when the steel plates adapt to the borehole wall
(Drodz et al. 1970, Heuze & Salem 1977). The use of borehole jacks in weak rock there-
fore cannot be recommended either.
Figure 15.4 Borehole jack test, reasons for an underestimation of determined moduli
Compared with other in-situ tests for determining deformability of the rock mass,
which will be described in the following sections, borehole expansion tests have the
advantage that many tests can be conducted because of their relatively low costs and
time required. It is thus possible to obtain statistically relevant data for the distribution
of rock mass deformability. Further advantages are that these tests can be conducted
under water and at great depths and that high pressures can be applied (Rocha et al.
1966a). These are the reasons why borehole expansion tests are the field tests for deter-
mining deformability of the rock mass that are most frequently carried out in practice.