Waste Rock Behavior at High Pressures: Dimensioning High Waste Rock Dumps - Multiscale Geomechanics: From Soil to Engineering Projects - page 94

Figure 4.8. CID triaxial test results with crushed waste rock
4.4.4.2. Undrained tests
Figure 4.9 presents the results of the CIU and CID triaxial tests on leached waste
rock for an initial grain size distribution cut under 0.2 m (8”). For both imposed
confining pressures, 1.7 MPa and 2.5 MPa, we observe that the tendency of the
granular skeleton to contract during shearing is translated, under an undrained
compression loading, by a strong increase in the pore water pressure (ΔU), reaching
values of about 1.5 MPa and of 2.3 MPa, respectively [BAR 07b]. Compared to
the drained condition, this leads to a strong reduction of the shear strength, as the
[q - ε 1 ] and [q - p'] plots of this figure makes clear. However, the shear strengths
reached in these tests are situated on the strength envelope determined at low
confining pressures, characterized by the biggest internal friction angle of the
material. Moreover, this strong reduction of the mean effective stress also leads to a
weaker change in the grading of the material.