Evaluating Cemented Paste Backfill’s Strength Development at Early Age Using the Laboratory Vane Shear Test

The Unconfined Compressive Strength (UCS) test is the most widely used method in the underground mining industry to assess strength development during binder hydration. However, testing errors are common for weak materials (UCS less than 100 kPa, i.e., cemented paste backfill at early curing stage), and reliable test methods must be established for designs relying on early age strengths, particularly for continuous pours. Therefore, the standard geotechnical laboratory vane shear test is investigated as an alternative. A newly designed “cast-in-place” mold is used to avoid specimen damage arising from vane insertion. Parametric tests with vane rotation speeds of 30 to 120 degrees per minute show strength insensitivity to these shear rates. Vane shear strengths correlate linearly with UCS over the strength ranges used; however, the trend does not follow the usual assumption for clays in the undrained state, because the material does not shear at a constant volume as clays do. The influence of suppressed dilation is investigated using direct shear tests, but these results do not fully explain the discrepancy, suggesting other mechanisms also have an important influence. Therefore, vane shear tests can provide a useful complement to UCS tests not only at the laboratory scale but in the in situ condition for early strength characterization. It should be noted that the correlation to UCS must be considered empirical, and separate calibrations are required for each backfill material.