ABSTRACT
Although it was not the original intention in this book to explore in detail the ‘static’ deformability of rock masses, a superficial treatment is necessary, in view of the common use of seismic measurements to extrapolate nearsurface deformation or elastic modulus measurements. We saw in the last chapter how the loading of individual joints, or of multiple jointed columns of rock in the laboratory created changes in the P-wave and S-wave velocities and amplitudes. Detailed monitoring of in situ loading tests using sonic logging in boreholes beneath plate loading tests, such as shown in Chapter 3 (Figure 3.15), also shows correlation with the moduli obtained. However, due to the inevitable damage in the unloaded zone around the sites of the tests, the lowest moduli (and lowest velocities) are usually registered closest to the loading plates, flatjacks or pressure chambers, while higher velocities and moduli are registered at greater depth. This is probably due to the more uniform stresses outside the excavation disturbed zone, or EDZ, where shear stresses are less due to the radial stress (r) being higher. There is increasing discrepancy between dynamic and ‘static’ deformation moduli in the near-surface, low quality, weathered zone, that may unfortunately need to be used as a dam or large building foundation.
