The failure mechanisms in hard rock pillar foundations loaded by squat rib pillars is investigated by means of theoretical and experimental studies. A conservative approach for determining foundation strength using elasticity and Mohr-Coulomb failure criterion is described and the important effect of the horizontal field stress on foundation strength is discussed. The models used for experimental and numerical studies approximate the geometry of a 72 per cent extracted mine layout with 28 m wide rib pillars. Observations made on sections taken from biaxially tested specimens showed that considerable damage can occur to pillar foundations at stress levels somewhat below the maximum stress that can be carried by the pillar. The load shedding due to fracturing in the pillar foundations is investigated using the finite difference computer code FLAC and compared with the results obtained from the laboratory model tests. When average stresses acting on pillars are approximately 2,8 times the uniaxial compressive strength of foundation rock, fracturing of the foundations can soften the pillar system by approximately 50 per cent compared to an intact pillar system.
It is well established in the literature that rib pillars with a width to height ratio greater than ten, which will be referred to as squat pillars in this paper, can be extremely strong (Madden 1984, Brummer 1987). However, under very heavy loading, the foundation of these pillars may fracture, or even fail, resulting in large stope closures and reduction in the load carried by the pillars. In the literature, failure mechanisms in hard rock pillar foundations are inferred from the results of Punch tests conducted in the laboratory (Wagner & Schumann 1971, Cook et al 1984). According to these studies, the onset and development of fracturing below a cylindrical steel punch resting on a semi-infinite rock mass take place as illustrated in Figure 1. At stress levels equal to the uniaxial strength of the foundation rock, tensile fractures occur near the edges of the punch. As the punch is further forced into the rock, tensile fracturing grows but at the same time rock starts to fail in compression and shear along the edges of a half circle, the diameter of which approximately equals the diameter of the punch. With further increase in punch stresses, the fractures developing from the edges of the punch meet eventually at the vertical centreline, resulting in "pillar foundation failure". In deep level hard rock tabular deposit mining, the average pillar stress at which "pillar foundation failure" occurs is commonly known as the "pillar foundation strength" and has been used in many cases of squat pillar layout as the main design criterion. However, practice has shown that prior to foundation failure, fractures that occur in pillar foundations can also result in undesirable effects. For example, fractures developing around the pillar edges can adversely affect the stability of on-reef developments driven alongside the pillars, such as gullies. Moreover, any damage to pillar foundations "softens" the pillar systems, thereby reducing their load carrying capacity.
