Quality testing of bolt support in deep roadways of mining districts

Abstract

The bolt support quality of coal roadways is one of the important factors for the efficiency and security of coal production. By means of a self-developed technique and equipment of random non-destructive testing, non-destructive detection and pre-warning analysis on the quality of bolt support in deep roadways of mining districts were performed in a number of mining areas. The measured data were obtained in the detection instances of abnormal in-situ stress and support invalidation etc. The corresponding relation between axial bolt load variation and roadway surrounding rock deformation and stability was summarized in different mining service stages. Pre-warning technology of roadway surrounding rock stability is proposed based on the detection of axial bolt load. Meanwhile, pre-warning indicators of axial bolt load in different mining service stages are offered and some successful pre-warning cases are also illustrated. The research results show that the change rules of axial bolt load in different mining service stages are quite similar in different mining areas. The change of axial bolt load is in accord with the adjustment of surrounding rock stress, which can consequently reflect the deformation and stability state of roadway surrounding rock. Through the detection of axial bolt load in different sections of roadways, the status of real-time bolt support quality can be reflected; meanwhile, the rationality of bolt support design can be evaluated which provides reference for bolting parameters optimization.

1. Introduction

Many coal seams more than 800 m deep are being mined in eastern China. Roadways of large cross-section are being developed in coal seams more than 600 m deep in western China. Moreover, these roadways are commonly supported by roof bolts and have encountered many support problems, including great depth, soft rock strata, abnormal tectonic stress, and intense mining disturbance. The stability of the surrounding rock is difficult to maintain and the roof bolt often fails before mining. Therefore, it is imperative to develop a non-destructive testing technology and equipment for the support quality in deep roadways as well as the pre-warning technology for support invalidation. Based upon non-destructive testing theory, technique and equipment of bolt support quality in coal roadways, non-destructive testing on bolt support quality in deep roadways of coal mines and pre-warning analysis were conducted. The measured data of abnormal in-situ stress and support invalidation etc. were obtained and analyzed in this paper. Meanwhile, some successful pre-warning cases were also illustrated.

2. Status quo of bolt support quality detection

During installation of roof bolts, installation quality is heavily affected by installation equipment and tools, field conditions, and operators` skills. The effective anchorage length of bolts and the reliability of anchorage quality are directly influenced if the drill hole is too deep, or the hole is not cleaned completely, or the resin cartridge is not mixed completely. If the drill hole is too shallow, the bolt head end protrudes from the roof surface and is excessively long. The pre-tension of bolts is difficult to generate from the torque applied on the bolt head, and may even break the bolt. As a result, the initial anchoring force of bolts is too small and the effective supporting range shrinks, which in turn leads to the failure of the anchored rock horizon. After the completion of bolt installation, as the surrounding rocks deform, the axial bolt load gradually increases until it becomes relatively stable. When retreat mining begins, the axial bolt load increases dramatically in the front abutment zone, sometimes exceeding the bolt strength and causing accidents. In spite of the advantages offered by roof bolting, e.g., low cost and rapid installation, it can easily cause a sudden roof accident by the bolts being pulled out or failing when the anchorage length and the initial anchoring force of bolts are insufficient or the axial load exceeds the ultimate strength of the bolt.

Since the installed roof bolting system is not observable, it is difficult to effectively detect and troubleshoot its support quality problems. The problems discussed in the previous section directly influence the stability and safety of the roadway support structure.

At present, the detection of bolt support quality employs mainly the pull-out test to determine the ultimate anchorage capacity. In the test, a torque wrench is used to apply the pre-tension, and a hydraulic pressure cell or bolt dynamometer is used to monitor the axial bolt load. These tools damage or change the bolting system or the state of the bolt support system. The present testing techniques require the installation of test sensors at the fixed measuring points selected in advance. It is difficult to carry out random detection for a large area since each bolt load is different, even though the bolts are installed in the same support cross-section.

With an increase in bolt load or its service life lengthened, the hydraulic pressure cell or bolt dynamometer is easily damaged and may fail. It cannot effectively evaluate the working status of the overall bolts in the region near the fixed monitoring instruments (usually each set of 50 m) through the observation of the axial load of individual bolts. At the same time, it is unable to detect the abnormal section of roadways or recessive danger such as hidden faults.

The above difficulties result in roof accidents and high repair rates of roadways. On the other hand, the economic loss due to the use of stronger-than-required bolts is excessive. Therefore, it is very important to adopt a kind of random, non-destructive, easy, fast and effective detection technique to monitor bolt quality and troubleshoot roadway safety problems.