|
Empirical verification of voussoir beam approach to the risk of bolted mine roof failure |
|
|
|
|
Empirical verification of voussoir beam approach to the risk of bolted mine roof failure Author: A. Kidybiński
In this paper a two-section voussoir beam analogue of mine roof strata was defined as based on its stability equation and the safety factor of bolted roof in the Polish copper mines (Lubin-Głogów Copper District) was analysed using roof-falls statistics. The equation contains both known values (such as S - maximum roof span, Y - specific weight of rocks, d -length of bolts being equivalent to roof strata beam depth, and Rc- uniaxial compressive strength of rocks) as well as unknown values (such as Z - height of compression arch, N - coefficient by thickness of compression arch, wp - coefficient of articulated joints' strength and Wk - coefficient by stiffness of bolting system). All unknown values were merged within joint factor K and a roof falling risk equation was formulated containing all values mentioned as well as the mass / intact strength ratio of rocks. A total of 136 short sections of openings (galleries) were selected in three copper mines (Lubin, Rudna and Polkowice-Sieroszowice) both with considerable roof failure experienced (82 sections) and long term stable roof strata (54 sections), the latter being chosen at the closet distance to the nearest section with roof failure - in order to review stable v. unstable conditions in comparable geological environment. Detailed data for each roof failure site were taken from Roof Fall Cards prepared by mine staff for each case roof fall and containing information about local geology (especially position and size of local faults), type and spacing of roof bolts (primary and secondary- if any), strength of rocks, geometry of an openings or crosscuts, area and thickness of fallen roof strata, history of local seismicity and roof falling precursory phenomena observed. Stable roof sections of galleries (54) were described in similar Roof Stability Cards. Taking"into account all such information, probable risk of roof failure was calculated for all 136 sites at time lapse immediately preceding a real roof fall occurrence - as a reciprocal of asafety factor. Then, minimum approximation error approach for the whole set of data was used to find K factor value for each mine taking a risk value equal to 1.00 as a boundary between stable and unstable openings (Fig. 8). The final formula for assessment of roof failure probability allows for 94 percent accuracy for three mines. It is possible therefore to use it for predictions of roof stability at questionable locations. Finally, a table of recommendations is presented for each class of calculated roof failure risk.
|
2011