Field Tests - Rock Mechanics Based on an Anisotropic Jointed Rock Model - page 486

Figure 15.19 Preparation of an in-situ direct shear test (Wittke 1990)

The arrangement of the hydraulic jacks should ensure that the shear force passes
through the center of the shear plane. Otherwise the specimen will be subjected to a
moment that can cause tensile stresses in the specimen. For this reason, FT T should be
applied to the specimen at an angle to the horizontal (Fig. 15.19, lower right, and Fig.

15.20, lower). According to Rocha (1964), tensile stresses can be avoided if the inclina-
tion angle

α

α fulfi lls the following condition:

(15.37)

where F N and F T are measured independently using load cells. Shear displacements,
lateral displacements and normal displacements are measured on the encapsulated
specimens using mechanical dial gauges or electrical displacement transducers (ISRM
1975a, DGEG 1980).