The development of Turkey’s deepest coal mine is currently underway to the proposed depth of 1200 meters (m) below surface at the Soma Coal Basin. An 11 m intersection forming an access way from one of the main declines was excavated at approximately 900 m below surface collapse within hours of completion even after being heavily supported by TH34 and cable bolts. It was discovered during the excavation failure investigation and rehabilitation work that the intersection was excavated below a water bearing stratification and faulted zone of the coal seam. The failure analyses concluded that the failure was contributed by a number of factors, mainly the overburden pressure of the overlying material along with the inrush of high-pressured groundwater with the very poor rock mass surrounding the excavation. This study investigates the possible reasons of the failure, numerical modelling of the intersection and alternative support suggestions for such squeezing rock conditions.
The case study area of the subject matter is located at Soma Coal Basin, west of Turkey in the region of the largest economic lignite coal bearing basin of the country. To date, the underground mine development work has been continuing for the deepest coal mine in the country to the proposed depth of approximately 1200 m below the surface. A summary of the stratification of the area is shown in figure 1. The coal-bearing stratum or Soma Formation starts with a basement grey conglomerate with fine-medium sized grains cemented with sand and silt. A hard, massive lignite zone having a thickness ranging between 15 m and 22 m overlays the basement formation with a bluish grey hard, massive marl covering the lignite zone [1].
A rockmass classification was carried out at the intersection of the case study along with the review of exploration drilling cores within the area of the incident to determine its quality (figures 2-3).
The rock mass quality of the area was classified based on the Rock Mass Rating (RMR89) and Tunnelling Quality Index (Q) systems proposed by Bieniawski [2], Barton et al., [3] along with the determination of the Geological Strength Index (GSI) for very poor jointed rock masses [4]. The uniaxial compressive strength of the rock mass was provided by the company of the case study based on the information from their database.