Abstract: Locked sand mined in Pattison mine has very unique properties: very high friction angle and very low cohesion. Wilson theory on pillar design takes properties of pillar material into consideration. Analysis shows that friction angle has great effect on strength of pillar. Based on the analysis of problems of Wilson theory, improvements are made on the calculation of yield zone width and the load which pillar can take. Results calculated with parameters of Pattison mine show that the width of yield zone is 0.2m, the load which pillar can take is 7.4×109 N, while the load which pillar will take is 6.8×108 N. This means that the size of pillar still has a room to be minimized in order to raise the extraction ratio.

Key words: Locked sand, Friction angle, Confining pressure, Yield zone, Pillar strength