| In underground room and pillar mines, pillars are considered as the main elements to provide sufficient support for the roof and overburden layers. Pillars should be designed carefully for carrying the weight and loads that are transferred from overburden while maximizing the percent recovery of the ore. This research aims to study and determine the possibility of reducing of the pillars dimensions at Wille room and pillar mine located in Iowa. The level of factor of safety of the reduced size pillars and stability of the wider rooms are subjects of concern. Pillars consist of St. Peter sandstone, which is a very weak and fragile material, which makes underground extraction very challenging. The material has very high friction angle (65o), but very low and almost zero cohesion. Three-dimensional numerical simulations were performed to simulate the pillars response to changes in the geometry, in-situ stress conditions, and changes in cohesion of the sandstone. Due to the weak property of the St. Peters Sandstone, standard laboratory testing is almost impossible. Therefore, limited data is available, particularly the cohesion. Values of the cohesion were estimated by back analyses of stable pillars at Wille mine. Cohesion of 400 kPa was used as a typical value for this site. Cohesion of 120 kPa was also used in the analyses as the worst-case scenario. FLAC 3D V4 code was utilized for numerical simulations. The geometry of the model was built based on the dimensions of pillars and rooms and thickness of overburden layers at Wille mine. A single pillar was modeled to reduce the computational time. Due to the symmetry along the two perpendicular axes, only a quarter of a pillar, floor, and overburden layers with dimensions of 8.5 x 8.5 x 12 m (28 x 28 x 40 ft) and a half-room width of 5.25 m (17.5 ft) were simulated. All models were built under the deepest mining depth, which is 61 m (200 ft) as the worst-case scenario. Numerical simulations were performed under elastic, elasto-plastic, and strain-softening methods. Mohr-Coulomb and strain-softening models were used to simulate different cases. For elasto-plastic analyses, four different widths of pillars were used; W=17 m (56 ft), 16 m (52 ft), 15 m (49 ft), and 14 m (46 ft). The pillar height was kept constant; H=12 m (40 ft). By considering the results of the numerical simulations, under the case of c=400 kPa, pillars width can be reduced by 1 m (3 ft) and be changed from 17 m (56 ft) to 16 m (52 ft) without any significant stability problem. In the case of 15 m (49 ft) pillar, instabilities can be observed in pillars. Therefore, 2 m (6 ft) of reduction of the pillar width is not recommended. In case of c=120 kPa, even the 17 m (56 ft) pillars showed significant levels of overstressed and unstable conditions.;Keywords: Room and pillar mining, St. Peter sandstone, very low cohesion, reduced size pillars, numerical simulations, FLAC 3D, strain-softening method. |
