Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 475

From the results of test arrangements LFJ1 to LFJ3, the dimensionless quantities

(15.19)

(15.20)

are calculated. From and the parameters m and n can be evaluated with the aid

of the charts for = 60°, respectively, given in Fig. 15.13 (center). In these

charts the combinations of m and n which lead to the same values of and are
indicated by green and blue curves. The intersection of the curves that correspond to
and calculated according to (15.19) and (15.20) yield the requested moduli ratios
m and n. Then C 1 can be determined from m and n using the diagram represented in
Fig. 15.13 (lower). As a result, E 1 , E 2 and G 2 can be determined as follows:

ϕ = 30° and ϕ

(15.21)

(15.22)

(15.23)

As an example, it is assumed that = 1.20 (thick green line) and = 0.70 (thick
blue line) are derived from the results of test arrangements LFJ1, LFJ2 and LFJ3 with
ϕ

= 30°. Then the moduli ratios m = 0.8 and n = 4, and thus C 1 = 0.7, result from the
intersection point of and (red point). The same result is obtained if test arrange-
ments LFJ1, LFJ2 and LFJ3 with

ϕ = 60° yield = 1.8 and = 0.85 (Fig. 15.13).

A precondition for obtaining reasonable results from this method is that in the three test
arrangements the loaded rock volume is close to the representative elementary volume
(REV) of the rock mass and that the investigated rock mass is homogeneous in the area
in which the three tests are located.
A method to determine both the elastic constants and shear strength of transversely
isotropic rock mass using fl at jacks was suggested by Kiehl (1985) and is also described
in Wittke (1990). In these so-called “biaxial fl at jack tests” specimens of a volume of
approximately 1 m3 are partially separated from the surrounding rock mass by sawn
slots and drilled slots, respectively, and loaded biaxially by means of two pairs of fl at
jacks. Two arrangements of these tests with respect to the isotropic plane are required,
here referred to as tests BFJ1 and BFJ2. Both tests require a rock wall that is oriented
perpendicular to the strike direction of the isotropic plane.
In test BFJ1, the slots are ordered parallel and perpendicular to the isotropic plane.
From the results of this test the elastic constants E 1 , E 2 ,

ν 2 can be determined.
For this purpose, the application of displacements of only a few mm are suffi cient to be
accomplished by LFJs (Fig. 15.14, upper left).

ν 1 and