Influence of Load Path and Effective Stress on One-Dimensional Deformation of Cemented Paste Backfill (CPB) During Deposition and Curing

Pore water pressure and effective stress development within cemented paste backfill (as one of the most popular local and regional underground mining supports) depends on rates of change of hydraulic conductivity and stiffness, which in turn are functions of cement hydration and backfilling rates. Previous laboratory studies attempted to investigate these interactions; however, the loading conditions they used are not representative of effective stress paths recorded in field monitoring programs during mine backfilling. In this work, typical effective stress paths occurring in mining operations are characterized in terms of an initial period of zero effective stress ranging from 3 to 48 h, and subsequently developed effective stress rates ranging from 5 to 20 kPa/hr. Servo-controlled consolidation machines apply the prescribed stress paths with adjustments every minute, thereby achieving an essentially continuous stress rate. The stress paths are applied to samples with 3.0%, 5.3% and 7.5% binder contents, and electrical conductivity monitoring on control samples is used to correlate the stress levels to stages of cement hydration. The secant constrained modulus is used to quantify the degree to which different stress paths may damage developing hydration products, resulting in softer backfill. For instance, the secant constrained modulus at 2.5% axial strain of samples with 3.0% binder content and loaded at the fastest rate with 48 h delay time was almost half of the ones loaded at the slowest rate and 12 h delay time after 65 h of curing indicating cement hydration products damage due to faster loading rate. The test samples' void ratios are compared with similar CPB’s in-situ void ratios and stress paths. Void ratios obtained from these experiments were very close to the average in-situ values under similar loading conditions. The test results help interpret the as-placed CPB’s bulk properties and will lead to better sample preparation procedures for other tests intended to determine CPB's engineering properties.