The impact of discontinuities within paste fill stopes and practical paste exposure design

Discontinuities formed during the placement of cemented paste backfill introduce planes of weakness within the fill mass. These discontinuities can influence exposure stability, yet their mechanical properties remain poorly constrained. This study combines laboratory testing and three-dimensional numerical modeling to quantify the strength of paste fill discontinuities and evaluate their influence on horizontal exposure stability. Direct tensile and shear testing of laboratory-prepared discontinuities demonstrates that their strength is variable and dependent on formation mechanisms, challenging the common assumption that cold joints possess zero cohesion and tensile strength. Three-dimensional numerical modeling of horizontal stope exposures incorporating discontinuities shows that stability is governed by discontinuity persistence, spacing, and position relative to the exposed span, as well as interaction with the surrounding rock mass through sidewall convergence and stress redistribution. For the scenarios investigated, backfill beam heights greater than approximately 20%–30% of the exposed span generally maintained stability, whereas beam heights below approximately 10% were prone to failure. Models incorporating discontinuous or nonpersistent joints predicted considerably lower unconfined compressive strength requirements than models assuming fully persistent discontinuities to maintain stability. Although specific paste fill strength requirements are site dependent, the results presented demonstrate that reliable paste exposure design requires laboratory-calibrated material properties combined with three-dimensional modeling that accounts for the excavation and filling sequence and rock mass interaction. The framework presented provides practical guidance for evaluating backfill exposure stability while reducing dilution risk and optimizing binder consumption in underground mining operations.