Determination of pillar strength by full scale pillar tests in the Laisvall mine

In the Laisvall Mine, Boliden Mineral AB, Northern Sweden, a full scale test was conducted on 9 pillars. They were subjected to increasing stresses by reducing their width by 0,8 m per step in 6 steps. The purpose of the test was to determine if failure will occur due to punching of the pillars into the roof or floor or due to pillar failure; further it was required to determine the stress level at failure to serve as input to pillar design and long term planning. Both types of failure occurred. The uniaxial compressive rock mass strength of the pillars was determined to about Gcm = 20 ± 1,4 MPa, giving a scale factor for strength in comparison to drill core strength of ≈ 1 : 10. Pillar strength is given by σρ = 1.17 × σ cm (0.778 + 0.222 W/H) where σcm = uniaxial compressive strength of the rock mass for the scale of W. The load bearing capacity of the shale roof at initiation of failure is σbm = 25 MPa. The implications of testing pillars with decreasing size in terms of loading and equilibrium conditions are described by applying the concept of the ground reaction curve to room and pillar mining. One point of the post failure curve of one pillar was determined by stress measurement and compared with the ground reaction curve of the test stope. Both pillar and hanging wall/footwall were in the postfailure stage at the end of the test. The validity of applying test results from a narrow to a wide mining zone is discussed.