On the evaluation of rock integrity around mine workings with anchorage by the shock-spectral method

The goal of this study is to investigate the integrity of gypsum-containing rocks during underground mining using non-destructive testing (NDT) anchors by the shock-spectral method. Studies were conducted in-situ at a gypsum deposit in the roof of the transport drift and cleaning chamber of a mine. Steel-polymer anchors were studied. The anchors were fastened in the hole with polymer resin and were fastened outside the hole with washers and nuts. The anchors were free in the middle. The vibrational response after striking the protruding end of an anchor was recorded, a spectrum was calculated, and the frequency (F) of the spectral maximum and the acoustic quality factor (Q) were determined. Q was calculated as the ratio of F to the frequency band at the 1/2 level of the spectrum maximum. With a tensioned anchor, it was determined that at the free length of the anchor, half of the wavelength was placed (F is high) and the outer layer of roof rocks determined the measured data. With a weakened anchor, a quarter wavelength was placed on its length (F is low) and the inner layer of roof rocks determined the measured data. All anchors were divided into two groups. When F ≤ 1050 Hz the inner layers determined the Q-factor, and at F > 1050 Hz the external layers determined the Q-factor. The average value of Q-factors of tensioned anchors (outer layers) were below the average value of Q-factors of weakened anchors (inner layers). Large Q values were assumed to correspond to higher rock integrity and strength. The ratio of the average value of the Q-factors of the external layer to the average value of the Q-factors of the inner layer was used to assess roof rock integrity. Smaller values of this ratio were characterized by lower rock integrity and a tendency to collapse and fall.