RI 3654 Use of Subaudible Noises for the Prediction of Rock Bursts, Part 2

"INTRODUCTION 'The investigation reported herein is part of a comprehensive research program being conducted by the Metal Mining Research Section, Mining Division, Bureau of Mines, on pressure problems in underground mines. This program was initiated in 1937 and for the first 3 years was concerned primarily with measurement of the pressure on mine pillars and supports and their strength. 4/ 5/ During this period, investigations were conducted in both the laboratory and the field in the comparatively shallow load and zinc mines of Missouri and Oklahoma. As a result a seismic method of determining the pressure on mine pillars was developed.Before this research program was initiated, Chas. F. Jackson, chief of the Mining Division, suggested that rock under pressure might give rise to noises inaudible to the unaided ear - that is, subaudible noises. As a result of this suggestion, listening tests were made concurrently with the routine pressure measurements to see if any rock noises were present; with one exception, it was found there were no rock noises in any of these shallow mines. At the same time, pressure measurements disclosed that, with one exception, pillars were not under pressure amounting to more than 10 percent of the crushing strength of the rock. The one exception was a pillar in the Big John mine of the Eagle Picher Mining & Smelting Co. in the Tri-State district, which showed some physical evidence of failure, such as small cracks and spalling; it was reported that the pillar had been heard to crack or snap at intervals. Listening tests in this pillar disclosed that subaudible noises of comparatively small intensity were present and occurred at 1- to 2-minute intervals, on the average. Unfortunately, owing to the shortness of this pillar it was impossible to make any pressure measurements."