Timber cribbing design for coal mine roof control

This study was made to investigate the strength characteristics of various timber crib configurations in underground coal mine openings. The study was accomplished by designing and making scale-models of two cribbing systems, one with an overhang and one without, to determine the strength characteristics of cribs under direct compressive loads. The strength characteristics were evaluated in terms of compressive stress, failure mode, dimensional tolerance, moisture content, and various crib configurations. Three tests on each of the following layer configurations, were evaluated: 2x2, 3x3, 4x4, and 6x6. The following moisture contents, 12%, 36%, 50%, and 70% were considered for each of the layer configurations. Three different scale-reduced specimens were used: 2x1-1/2x12 inches, 2x1-1/2x24 inches, and 4x3x24 inches. Each crib consisted of 10 layers, and failure mode was considered to be 50% reduction in height. It was found that a crib with an overhang has a greater strength under compression than a crib with no overhang, and variation of moisture content in wood does not have a dramatic effect on deformation. However, a cribbing system consisting of members with a 70% moisture content on the average produced a higher compressive strength. A graph of bearing area versus strain was provided for predictingmore » deformation of different layer configurations with different heights as well as specimen sizes. Vertical alignment of individual crib members in a system is important. Improper alignment introduces shear and bending moment stresses. 21 figures, 14 tables.« less