In the Central Mining Institute the construction of hydraulic borehole penetrometer has been developed; it allows to perforate the wall of borehole and to determine on the basis the penetration resistance /strength/ of individual rock layers. Next, the laboratory tests have been carried out on bed separation of different sedimentary rocks / tensile strength measured perpendicularly to stratification / and on penetration resistance in the same cross-sections. On the grounds the empiric correlation between both the values has been worked out allowing to measure bed separation resistance with penetrometer directly in the borehole made in the roof. Then a number of strength profiles of roof rock were made in the mines. The initial criteria of roof fall were given determined by bed separation resistance, the distance from the exposed roof surface and the width of the roof exposure. The tests in question have been ordered to the Institute by US Bureau of Mines and have been conducted with the aim of common utilization in the Polish and American coal mines.

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