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Recovery of sill pillars is often associated with mining under existing backfill. The backfill stability in such cases is of primary concern due to risks associated with personnel safety, equipment loss, and ore dilution. Two key aspects that control the stability of exposed backfill are the fill strength and the size of the fill exposure. Use of stronger fill will allow for increased excavation size but will come at a higher cost because of higher binder content requirements. A mine operator, therefore, must decide what combination of excavation size and fill strength is appropriate. In this paper we present the results of a study for a base metal mine in Canada aimed at estimating the strength requirements for cemented rockfill (CRF) being undercut. Numerical simulations utilising bonded particle models were used to relate the size of the undercut to the CRF strength requirements. The results were expressed as the depth of fill failure and as ore dilution, allowing the operator to select appropriate fill strength.