Engineering Geology

Chapter 2
Table 2.5. Classification of rock quality in relation to the incidence of discontinuities
Fracture
Velocity
Quality frequency ratio
classification RQD (%) per metre Mass factor (j) (V cf /V cl )
Very poor 0–25 Over 15 0.0–0.2
Poor 25–50 15–8 Less than 0.2 0.2–0.4
Fair 50–75 8–5 0.2–0.5 0.4–0.6
Good 75–90 5–1 0.5–0.8 0.6–0.8
Excellent 90–100 Less than 1 0.8–1.0 0.8–1.0
This particular difficulty can be overcome by using the fracture spacing index. This simply
refers to the frequency per metre, with which fractures occur within a rock mass (Table 2.5).
The effect of discontinuities in a rock mass can be estimated by comparing the in situ compressional
wave velocity, V cf , with the laboratory sonic velocity, V cl , of an intact core sample
obtained from the same rock mass. This gives the velocity ratio V cf /V cl . The difference in
these two velocities is caused by the discontinuities that exist in the field. For a high-quality
massive rock with only a few tight joints, the velocity ratio approaches unity. As the degree
of jointing and fracturing becomes more severe, the velocity ratio is reduced (Table 2.5). The
sonic velocity is determined for the core sample in the laboratory under an axial stress equal
to the computed overburden stress at the depth from which the rock material was taken, and
at a moisture content equivalent to that of the in situ rock. The field seismic velocity is determined
preferably by uphole or crosshole seismic measurements in drillholes or test adits,
since by using these measurements it is possible to explore individual homogeneous zones
more precisely than by surface refraction surveys.
An estimate of the numerical value of the deformation modulus of a jointed rock mass can be
obtained from various in situ tests (see Chapter 7). The values derived from such tests are
always smaller than those determined in the laboratory from intact core specimens. The more
heavily the rock mass is jointed, the larger the discrepancy between the two values. Thus, if
the ratio between these two values of deformation modulus is obtained from a number of
locations on a site, the engineer can evaluate the rock mass quality. Accordingly, the concept
of the rock mass factor, j, was introduced by Hobbs (1975), who defined it as the ratio of
deformability of a rock mass to that of the intact rock (Table 2.5).
Recording Discontinuity Surveys
Before a discontinuity survey commences, the area in question must be mapped geologically
to determine rock types and delineate major structures. It is only after becoming familiar with
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