Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 493

Flow rate Q and excess pressure p are recorded continuously during each stage. In Fig.

15.24 (lower right) Q related to the length of test section is plotted versus

Δ

Δ p. For lam-

inar fl ow the same linear relationship between Q/ and Δ p is achieved for both upward

and downward measurements.
The evaluation of a Lugeon test for rock mass permeability is based on the assumption
that laminar fl ow takes place radially, perpendicular to the borehole, and that a steady
state condition has been achieved. In a Lugeon test the rock mass permeability normal
to the borehole integrated over the test section length known as “transmissibility”

(15.40)

is measured, where k RM is the rock mass permeability coeffi cient of an equivalent ho-
mogeneous, isotropic aquifer of infi nite extent that can be calculated as follows (ISRM
1977a):

(15.41)

where γ w is the unit weight of water, r 0 is the radius of the test hole and R is the radius

of infl uence that needs to be estimated. This formula is only valid if the test section is
located entirely below the water table in saturated rock. Otherwise unsaturated fl ow
needs to be considered for test interpretation.
Radial fl ow implies that water fl ow through the bottom of the test hole is neglected.
Double packer tests, in which the lower end of the test section is sealed by a second
packer, better comply with the boundary condition of radial fl ow. However, double
packer tests are more susceptible to bypassing of water around the packer than sin-
gle packer tests. Various multiple packer systems comprising two and more test sec-
tions have been developed to reduce or avoid water losses (Maini et al. 1972, Heitfeld &
Krapp 1981, Langer et al. 1989, Wittke 1990).
In a rock mass with moderate permeability the single packer test, as illustrated in
Fig. 15.24, is the best method for avoiding water losses (Wittke 1990). But in low per-
meability rock mass, where bypassing of packers is not expected, double packer tests are
widely used (Section 15.8.2).
The l ow rate in l/min related to a test section of length l = 1 m and an injection pressure
of

p = 10 bar is referred to as a “Lugeon” (Fig. 15.25). This unit is often used to assess
whether sealing measures such as grouting are necessary in the rock foundation of a
concrete or rock-fi ll dam. Figure 15.25 shows the relationship between k RM and the
Lugeon value for an NX test hole (2r 0 = 76.2 mm) and assumed radii of infl uence of
R = 10 m (blue line) and R = 100 m (red line), respectively (DGEG 1984c). As a result,
1 Lugeon roughly corresponds to a permeability coeffi cient of k RM ≈ 10 -7 m/s.

Δ