ABSTRACT: Coal strength based on scaled uniaxial compressive strength from the laboratory and back-calculations from in-mine observations of pillar stability indicate that material strength scaling rules are open to debate. Analyses presented indicate that the application of scaling factors lower than 0.5 provide a better correlation with field observations of pillar stability.


Pillar sizing based on empirically derived formulae has been performed since the early 1900's. The current art involves the scaling of laboratory-derived coal strength to values representative of the in-situ coal mass. The various pillar-strength formulae, incorporating conventionally accepted scaling concepts, were applied to size pillars at an underground coal mine located in Pennsylvania. Even for presently stable pillars at the mine, the calculated safety factors were below those deemed adequate. Back-calculations based on observed pillar instabilities yielded an in-situ coal strength ranging from 730 psi (5.0 MPa) to 1073 psi (7.4 MPa) depending on the particular pillar-strength formula utilized. These values are approximately of the same magnitude as the coal strength determined in the laboratory [700 psi (4.8 MPa) to 1500 psi (10.3 MPa)]. As a consequence, the application of published scaling relationships to these laboratory data produced predicted in-situ coal strengths much lower than those that apparently exist based on the observed in-mine conditions. In this paper, a series of typical steps leading to determination of a "safe/stable" pillar size are applied. Discussion includes details relevant to the laboratory as well as field investigations, that generated concerns related to the use of common pillar formulae.


Sample Material

Although sample material for three coal sems (referred to as S-1, S-2 and S-3) was prepared and tested, it should be noted that the pillar sizing discussion will ultimately refer to only one of these seams, namely seam S-3. The samples for all three seams, as received, averaged one foot wide, two feet long, and one foot thick (across bedding). The sample material was obtained from recently mined area and delivered to the Penn State Rock Mechanics Laboratory wrapped in heavy plastic bags and burlap.

Table I, Results of Uniaxial Compressive Strength Tests on Coal(available in full paper)

Preparation of Specimens

The lump samples were kept moist and wrapped until specimen preparation, since it was expected that oxidation and progressive cracking from the drying of the coal material would deteriorate the structural integrity of the lump severely. Initially, specimen preparation efforts concentrated on obtaining cylindrical specimens using a standard laboratory coring techniques. It became apparent, however, that the specimen preparation techniques had to be altered for coal material from each seam. The standard coring technique provided useable test specimens for seam S-1 only. The material from seams S-2 and S-3 was very friable and fractured readily as coring was performed. Consequently, cubical test specimens were prepared from seam S-2 and S-3 material, exclusively. A horizontal band saw equipped with a tungsten-carbide blade was used to successfully prepare these cubical specimens. The ends of all specimens (cylindrical and cubes) were finished by hand with 200-grit sandpaper. Following preparation, specimens were kept moist in an environmental chamber (100% humidity).

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