Three-dimensional finite element analysis of an undermined shaft at the Hustas mine, Turkey
Underground coal production at the Hustas mine, west of the village of Eynez, was from the KM2 seam, which lies at an average depth of 200 m and ranges from 15 m to 25 m in thickness with an average extractable thickness of about 18 m. The underground lignite mining methods employed in Turkey are mostly non-mechanized longwall methods. At the Hustas mine, the method used is a manual double-slice longwall method. The face area is maintained at about 2 m high using hydraulic steel props and wooden posts. Coal is extracted from the face by conventional manual methods and the additional lignite thickness above the supports is recovered by allowing it to cave behind the face. The process of caving, combined with an average extraction thickness of 18 m and a relatively shallow depth, results in significant damage to the strata immediately above the extraction area, which extends all the way to the surface. Mine infrastructure, surface or underground, located within the subsidence limits, can suffer major damage. Pillars left to ensure the stability of major underground coal mine infrastructure (shafts, drifts) often contain substantial quantities of coal. Recovery of this coal is an efficient use of the resource, but often presents safety hazards; pillar extraction destabilizes the protected structure, and can make its use dangerous. In this study, the stability of an active shaft at an underground coal mine situated in the Soma-Manisa region of Turkey was investigated during the recovery of the coal in the shaft safety pillar. The lining of the shaft is 50 MPa concrete with a thickness of 50 cm containing double-deck wire mesh for reinforcement against tensile stresses.A proprietary three-dimensional finite element analysis model (ANSYS) was used. An elastic-plastic material model utilizing the Drucker-Praeger failure criterion was employed. The extraction of the safety pillar was modelled in eight steps, with intermediate results obtained from the model after each step.Stresses and deformations formed around the shaft support were estimated using the model and the results were verified by in situ observations. The results show that stress distributions around the shaft and the subsidence of the shaft support are influenced by the location of the extraction front and the break angle of the overlying material. When the extraction had reached a distance of 30 m from the shaft centre (after mining step 3) the maximum modelled axial stress and strain in the shaft lining were 7.7 MPa and 1.59 mm/m, respectively. These values are not expected to damage the reinforced concrete shaft lining. In subsequent mining stages, tensile stresses and strains develop in the shaft lining, increasing as the extraction proceeds to a position about 30 m beyond the shaft centre (after mining step 5) to maximum values of 9.09 MPa and 0.64 mm/m, respectively. Stresses in the lower part of the shaft are difficult to interpret due to the fractured nature of the rock associated with the caving of the immediate roof behind the extraction face. No fractures or cracks were observed in the shaft lining in the field, but the shaft did collapse after the face had passed 30 m or so beyond it. Although the shaft ultimately collapsed, the modelled stresses and strains were generally too small to cause damage to the shaft lining. The reason for this may be that the shaft ends above the coal seam, so that stresses and strains were not transmitted from the extraction horizon along the shaft, and that the bottom of the shaft is in the immediate roof which is fractured by the caving method and therefore less able to transmit stresses and strains to the shaft lining.In this study, it was found that an effective area of break angle was formed when the shaft ended at a level above the coal seam. Compressive stresses and the compactions in the shaft support are transformed into tensile stresses and the expansions after the face advanced in the effect area of break angle and prior to the advance of the panel at the centre of the shaft during the recovery of safety pillars.