Figure 8. Characteristic aspects of the shear behavior of dry joints; (a) evolution of the shear stress-shear displacement diagrams between the first and the last cycle of reversal loading; (b) compaction due to the wearing of the joint surface.

of rock joints is highly dependent on both roughness of the joint surface and the level of vertical pre-compression (Amadei et al. 1998; Huang et al. 2002, Misra 2002). As referred by these authors, in rough joints lower dilation is obtained at high normal stresses and for increasing shear displacements dilatancy tends to exhibit decreasing values.

Based on the shear stress-shear displacement diagrams, it is observed that the shear behavior of dry joints is characterized by an approximately constant stiffness followed by marked nonlinearity close to the peak load in the loading branches. On the other hand, the stiffness of the unloading branches exhibits always considerable high values when compared with the stiffness obtained in the loading and reloading cycles. The corrected displacement of the dry joint can be

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Shear displacement [mm]

Figure 9. Correction of the measured shear displacement-dry specimens.

obtained by removing the elastic deformation of the unit reading:

where umeas is the shear displacement given by the horizontal LVDTs, t is the shear stress for a given displacement and ku is the stiffness calculated in the unloading branches. It is possible to confirm that the elastic deformation of the units has a minor role in the total shear displacement of dry joint, see Figure 9.

The shear behavior of dry joints is thus characterized by significant non-linear deformations in the pre-peak stage and perfect plastic deformations after peak stress resulting from the characteristic sliding failure mode. The former characteristic of the shear behavior of dry joints was already pointed out by Lourengo & Ramos (2004). Apart from the nonlinear-ity in the pre-peak regime, the envelop of the diagrams is also in good agreement with the shape of the shear stress-shear displacement diagrams indicated by Lee et al. (2001), also for smooth sawn-cut granitic joints.

3.3 Shear strength-normal stress diagrams

Figure 10 shows the relationships between the values of the shear strength obtained in the monotonic tests and in the first cycle of the cyclic tests for dry and saturated conditions as a function of the normal stress. For both specimens, an expressive linear correlation was attained between normal and shear stress, which confirms the initial assumption that the shear strength is well described by Coulomb's friction law.

The slight decrease on the shear strength obtained on saturated specimens is here reflected by the lower value of the friction coefficient, being 0.65 and 0.60 the values that were achieved for dry and saturated

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