A safe working environment can be provided for underground coal miners through implementing new ground control technology. To devise a practical method for stability design of underground rock structures in coal mines, the Bureau of Mines has examined the time- independent behavior of coal measure rocks in uniaxial and triaxial compression tests. These rocks exhibit work hardening before failure. Their stress-strain relationship can be expressed by a hyperbolic equation similar to that for soils. The behavior of rocks at failure can be regarded as the limit of work hardening. To simplify the stability design for temporary rock structures, rock behavior may be idealized as perfectly plastic for stability analysis of roof, ribs, and floor in underground coal mines, although such idealization does not strictly represent the real inelastic behavior of rocks.