Flat jack stress determinations in selected colliery roadways have revealed a range of measured stress from 330-p.s.i. tension to 3,750 p.s.i._x000D_
compression. Photoelastic examination of loaded plastic and actual rock mo.odel mine openings has shown that the primary horizontal stress required to attain these measured stresses would be from one to three times the superincumbent vertical load._x000D_
Examination of roof movement with a. borehole oxtensumeter and a series of anchors at various horizons to 30 feet in a. vertical roof borehole showed that when a standing face waa.s advanced the roof movement was practically confined to the lower two feet. The ma.j ority of this movement occurred during the 36 hours after mining at this face commenced._x000D_
The use of a. radioactive isotope and a. scintillation counter in a._x000D_
pair of parallel bcreholes promises to be sensitive and accurate method of roof bedding plane crack detection.