Non-destructive testing of rock bolt fastening as an element of monitoring the state of mine workings

Abstract

Purpose. To substantiate the informative parameter, to develop the method and equipment for non-destructive testing (NDT) of rock bolt fastening, as an element of geomechanical monitoring of the mine workings state.Methods. Analytical and bench experimental testing of shock-wave processes in the "rock bolt -bonding layerrock massif" system.Findings. It has been established, that the non-destructive shock-wave method is the most satisfying for monitoring the geomechanical state of the "rock bolt -bonding layer -rock massif" system. This method is based on the registration and analysis of rock bolt oscillations, caused by non-normalized hit to the rock bolt end. It has been confirmed, that the most informative parameter is the relaxation time, that is the time over which the amplitude of oscillations decreases by "e" times. It has been shown, that clamping of the resin-grouted rock bolt in massif and its tensioning affects the relaxation time of damped oscillations. Furthermore, the relaxation time is inversely proportional to the degree of the rock bolt clamping, and is linked linearly with an increase in the rock bolt tensioning. The spectral composition of wedge shaped rock bolts self-oscillations, which are caused by impact excitation, depends both on the nature of clamping and on the rock bolt length, and makes it possible to identify the rock bolts by their length.Originality. The existence of a special type of damped longitudinal oscillations of the rock bolt in viscoelastic medium of the bonding layer has been established. The relaxation time of the specified type of oscillations was chosen as an informative parameter. An analytical link has been established between the informative parameter of shock-wave method of monitoring and the main indicators of rock bolt fastening quality: the degree of adhesion with massif and tension value.Practical implications. The method has been improved and equipment has been developed for non-destructive testing of rock bolt fastening, which makes it possible to control in-situ the rock bolt fastening quality, including the geomechanical state monitoring of mine workings.

Physical modeling of the rock bolt interaction with the rock massif

“…The theoretically obtained [20] inversely proportional dependence of the relaxation time of the rock bolt oscillations on its clamping degree has been experimentally confirmed. It has been shown, that by the proposed method, the rock bolt fastening in a rock massif with high reliability is controlled in the range from the minimum to 60% of the maximum possible.…”

“…The denser the clamping points are the more high-frequency components will be in the spectrum. The presence of a special type of longitudinal oscillations of the rock bolt in a viscoelastic medium is explained in [20]. A feature of such oscillations is high initial amplitude.…”

“…Theoretical calculations [20] showed the existence of a deterministic inversely proportional relationship between the parameter p, which characterizes the rock bolt clamping degree, and the free oscillations relaxation time τ: τ = В/р. Here B is a coefficient that characterizes a specific rock bolt.…”

“…To study the rock bolt free oscillations with different clamping degree, two test benches were made. Their designs are described in [20,21]. The first test bench -with discrete partial clamping of the rock bolt ( Fig.…”

Method for optimizing the protecting pillars parameters in underground coal mining

“…The operational state of mine workings depends on measures aimed to protect them from the rock pressure manifestations, which are provided for at all stages of the mine working existence: design, construction, operation, and abandonment [6,7]. Unfortunately, the planned and implemented measures sometimes do not make possible to maintain a stable state of mine working [8,9]. On the other hand, the experience of coal mining enterprises exploitation indicates the situations arising where, according to well-founded design decisions, the stability of mine workings is excessive and is ensured by the unnecessary waste of resources, primarily mineral resources, for example, within natural protecting objects, the so-called pillars.…”