Direct shear tests on cemented paste backfill–rock wall and cemented paste backfill–backfill interfaces

This paper presents the results of the shear strength (frictional strength) of cemented paste backfill-cemented paste backfill (CPB-CPB) and cemented paste backfill–rock wall (CPB-rock) interfaces. The frictional behaviors of these interfaces were assessed for the short-term curing times (3 d and 7 d) using a direct shear apparatus RDS-200 from GCTS (Geotechnical Consulting & Testing Systems). The shear (friction) tests were performed at three different constant normal stress levels on flat and smooth interfaces. These tests aimed at understanding the mobilized shear strength at the CPB-rock and CPB-CPB interfaces during and/or after open stope filling (no exposed face). The applied normal stress levels were varied in a range corresponding to the usually measured in-situ horizontal pressures (longitudinal or transverse) developed within paste-filled stopes (uniaxial compressive strength, σc ≤ 150 kPa). Results show that the mobilized shear strength is higher at the CPB-CPB interface than that at the CPB-rock interface. Also, the perfect elastoplastic behaviors observed for the CPB-rock interfaces were not observed for the CPB-CPB interfaces with low cement content which exhibits a strain-hardening behavior. These results are useful to estimate or validate numerical model for pressures determination in cemented backfill stope at short term. The tests were performed on real backfill and granite. The results may help understanding the mechanical behavior of the cemented paste backfill in general and, in particular, analyzing the shear strength at backfill–backfill and backfill-rock interfaces.