ABSTRACT ABSTRACT: Cemented backfill sill mats are an artificial pillar used to improve recovery by replacing an ore pillar by an artificial equivalent. In the design of a sill mat pillar, one should account for the driving forces in terms of the self weight of the sill mat plus the load of unconsolidated backfill used to fill the mined stope above it that results upon undermining the sill mat below. The resisting forces are largely governed by the strength properties of the cemented sill mat. This paper presents stability charts derived using numerical codes incorporating variable stope dip angles, sill mat thicknesses and strength properties of the cemented backfill and relating the results to limit equilibrium analytical equations presently in use. 1 INTRODUCTION A cemented rockfill sill mat design should optimize the materials’ requirements to achieve a stable condition, while meeting safety guidelines and minimizing mining costs. This optimum is mainly driven by the cement cost which is the largest material cost component of backfill. Therefore, strategies intended at reducing cement content are of the highest benefit to mining operations. This paper defines and compares the relevant analytical relationships and numerical modeling results of backfill behavior on sill mat stability. The analysis is done via observations of practitioners and researchers working in the mining field. 2 LOAD OF BACKFILL One of the key factors in assessing cemented rockfill (CRF) stability is quantifying the vertical stress acting on the top of the sill mat. In situ measurements and analytical derivations by numerous authors imply that arching of unconsolidated rockfill (URF) material decreases the vertical load on the sill by transferring part of the total vertical load to either rock walls. Methods to reliably determine the true load lead to more reliable and cost effective solutions in the design of sill mats.