Roof control is a most important phase of underground mining. When the roof strata is weak, supporting the roof often is the largest single mining cost. Mines or parts of mines have been abandoned because of poor roof conditions which have imposed such a large cost on the overall mining operations. In addition, as mining personnel are well aware, unstable roof conditions present a working hazard that requires special protective measures to provide safety for the workmen. Support of the roof is almost always required in coal mining for two reasons: 1. Protecting men and facilities from falls, crushes, bumps, and other roof, face, and rib failures. 2. Keeping working places open for the desired length of time required for coal extraction, ranging from a few hours to as long as 50 years or more. The Problem Falls of roof and coal are a natural threat to the safety of all individuals in a coal mine, and with the introduction of mechanization, this threat has now developed into the No. 1 safety problem for the industry. Annually, more fatal injuries result from falls of roof, face, and ribs than from the combined total of all other hazards. Considering the roof-fall problem in the coal mining industry on a national basis, the following tabulation proves conclusively the necessity for a solution: Table 5 shows the record for a 10-year period of roof-fall accidents that caused fatal injuries to more than one person in single accidents for the nation. Investigations following the roof-fall accidents in the United States for 1968 revealed that the 98 fatal injuries caused by falls of rock and coal were the result of failure on the part of some one to perform correctly one or more ordinary functions of mining. Table 6 lists the primary causes of the roof-fall accidents in addition to the method of coal extraction. Mine Roof, Its Evaluation and Testing The importance of knowing the true characteristics of mine roof and properly handling situations uncovered in its evaluation are of primary importance in roof control, even surpassing in importance the adoption of a standard roof-control plan. While it is true that a standard plan for controlling roof eliminates much guesswork on the part of miners and is responsible for a reduction in roof-fall accidents in addition to being a necessary requirement for modern mining, quite often it will be mistakenly considered as the total support necessary for a working face, or maximum, instead of the bare minimum without any regard for supplementary support. A standard roof-control plan is a plan developed for normal operating conditions, whatever that may be at the individual mine or section, and any adverse change in the roof requires additional support. human element enters the picture at this point and the amount of supplementary support required for a given situation is dependent upon the judgment of the individual examining the working place. The judgment of the individual performing the roof evaluation will be influenced by several factors, such as experience, attitude, type of instrument used in sounding the roof, and the characteristics of the immediate roof. The Modern mining requires the adoption of standard roof-control plans for use under normal roof conditions. A standard roof-control plan will also include provisions when abnormal conditions are encountered, and it is this phase of roof control, the detection of abnormalities, that this section will concentrate its efforts in addition to stressing the necessity for following the standard plan. No positive method has been devised for determining mine-roof conditions that are not visible. Considerable research has been conducted on developing some practical means for use by mining personnel in which unseen abnormalities can be detected, but as yet, new techniques, as for example, the sonic process, are still in the development stage. Mining personnel must still rely on the methods in which the personal-factor processes of the individual are used, such as observation and the sound-and-vibration technique for detecting faults, fissures, rolls, concealed kettle bottoms, and the many other hidden sources of roof-fall accidents. The geology of coal-mine roof has been studied extensively, and in most instances, the bedding planes are horizontal, as indicated in Figure 1, with layers of clay, shale, sandstone, or other coalbeds. If the bedding planes were to remain regular in thickness and the other physical characteristics remain unchanged, the problem of |