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ABSTRACT: New Concept Mining (NCM) has implemented the Dynamic Impact Tester (DIT) to conduct laboratory based dynamic testing on rock bolts. The DIT allows NCM to move rapidly through the R&D cycle for new rock bolts. This allows both a shorter time to market as well as comprehensive understanding of the performance of rock bolts. In addition to these benefits, the DIT is being used in several exciting ways to improve the understanding in the mining industry of the performance of dynamic ground support. An example is given where the dynamic testing database has been used to back analyze the quantitative performance of a Vulcan Bolt during an underground seismic event.
As underground mines are going deeper, the stresses in the rock mass are approaching, and in some cases exceeding the strength of the rock mass. Supporting excavations in this type of ground comes with a unique set of challenges that go beyond the ability of regular ground support. In order to serve this need in the industry several companies have developed rock bolts and surface support solutions. New Concept Mining (NCM) is one of these ground support companies that has done extensive research and development to better understand and develop these ground support requirements. In order to understand these ground support solutions, NCM has built the Dynamic Impact Tester (DIT). The purpose of this DIT is to dynamically test rock bolts to quantitively interrogate the response of the support tendon, to a high strain rate axial event, as an approximation of the loading expected during a seismic event.
2. DYNAMIC ROCK BOLT TESTING LANDSCAPE
There are several testing machines around the world capable of testing dynamic ground support to one degree or another. SWERIM has recently developed a momentum transfer style dynamic testing machine out of Luleå, Sweden. The Western Australia School of Mines (WASM) also has a momentum transfer style dynamic testing machine out of Kalgoorlie, Australia (Player, et al, 2008). The Central Mining Institute out of Katowice, Poland has developed a unique dynamic testing machine aimed at qualifying rock bolts to support coal bursting (Pytlik, et al, 2015). Canmet has an impact based dynamic testing machined on which the ASTM D7401-08 (ASTM D7401-08) is based, out of Ottawa, Canada (Li, et al, 2011). Sandvik has developed a rig that can be used underground to dynamically test installed rock bolts at the proximal end (Darlington, 2014). Geobrugg has been involved in developing a testing machine that is likely the closest approximation to a system test, this is based in Walenstadt, Switzerland (Saner, et al, 2016). NCM has commissioned the DIT (Knox, et al, 2018a) in June 2017, it is an impact based dynamic testing machine complying with ASTM D7401-08 (ASTM D7401-08 – 03). The DIT is housed at the NCM testing facility in Johannesburg, South Africa.
The problem of assessing the state of stress in mine workings is vital for safe underground mining. The main source of information about a local stress field is usually obtained from measurements on the walls of underground galleries or in boreholes. An alternative to these methods are non-destructive short-term geophysical methods calibrated with a small amount of information from drilled wells and used to monitor stress conditions. Of this category, the greatest attention was paid to the method of acoustic emission and the method of electromagnetic radiation. Both of these phenomena are caused by the rock fracturing. This paper considers the physical basis for the application of acoustic emission in underground conditions based on a modem understanding of the phenomenon and its features.
The release of high concentration of stress in the form of collapsed rocks towards the underground galleries is the main cause of accidents during mining (Zhang et al. 2017a), since the natural distribution of underground stress is significantly altered by the additional stress superimposed by the mining operations, and substantial decrease in strength of rocks (He et al. 20017). Hence, the problem of assessing the state of stress in mine workings is vital for safe mining. The manifold methods have been utilized to monitor the potential for the stress concentration (Zhang et al. 2017b). Hereafter we merely concern the local short-term methods. The main source of information about a local stressed field is usually obtained from measurements the walls of underground galleries or in drilled wells. An alternative to these methods are non-destructive short-term geophysical methods, i.e. acoustic emission (AE) or/ and electromagnetic radiation (EMR) caused by rock fracturing. Acoustic emission (micro-seismic) has been used since the 1920s to assess the underground stress conditions in Poland, South Africa, Canada, the U.S., Australia, China, etc. (He et al. 2017 and references therein). Numerous studies of the AE phenomenon make it possible to understand that the signals AE are a small-scale phenomenon whose properties are analogous to the large-scale radiation of an elastic wave caused by rock-bursts and earthquakes (Kuksenko et al. 1985; Lockner and Rehez 1994; Guha 2001 and references therein; Lei et al. 2003; Thompson et al. 2009; John-son et al. 2013; Goebel et al. 2014; McLaskey and Lockner 2016 and references therein). The formation of cracks and/or macroscopic fractures in rocks is the main cause of the AE exciation, whose energy and spectrum depend on the mechanisms of their generation and rock properties (Michlmayr et al. 2012). Monitoring of AE excitation is applicable for the detection of crack sources, hazard assessment in mines, tunnels, etc (Kim et al. 2015; Kuksenko and Makhmudov 2017 and references therein). It was shown that the AE time-sequence parameters correspond well with the evolution of the rupture events of the rock material at different stages and hence can provide the precursor for the rock damage (Kuksenko et al. 1985; Lockner and Rehez 1994; Rehes 1999; Liang et al. 2017). For example, Cai et al. (2001) characterized rock damage near excavation using AE monitoring. It was shown that the combined AE and EMR methods it possible to accurately assess the risk of high stress accumulation (Frid and Vozoff 2005; Lu et al. 2015; Jiang et al. 2016). The experimental results obtained at the San Pietro gypsum mine, Prato Nuovo, underscore the close correlation between AE and EMR activity, while it was noted that both types of emissions preceded a failure event for approximately one day and 3—4 days, respectively (Carpinteri and Borla 2017). Henceforth, we consider several aspects of the phenomenon of AE, which are important for its application for local short-term stress assessment while features of EMR phenomenon were examined in detail by Frid and Mulev (2018 and references therein).
Prioul, Romain (Schlumberger Cambridge Research) | Musil, Martin (Schlumberger Cambridge Research) | Drinkwater, Nick (Schlumberger Cambridge Research) | Signer, Claude (Schlumberger Cambridge Research) | Tetzlaff, Dan (WesternGeco)
ABSTRACT: Seismic events near the stope face of six longwalls at Western Deep Levels Limited are discussed in terms of their relation to blasting time. First, the time period is established during which seismic events can be regarded as directly blasting-induced. Later, the different seismicity levels of the six longwalls are discussed. Finally, the ratio of the number of seismic events, which occurred during the blast and after the blast is evaluated for several magnitude categories. Longwall geometry, geological discontinuities of large and small scales did however result in anomalies such as abnormal concentrations of rockbursts during and outside blasting time. It is also shown that seismicity levels of longwalls, which advanced at an oblique angle to geological features, were strongly reduced when compared with other longwalls. The potential of production blasts to trigger impending seismic events under certain conditions becomes apparent.
The gold mine Western Deep Levels Limited (WDL) is situated in the Witwatersrand Basin approximately 75 km south-west of Johannesburg. The lease area totals nearly 45 km2, extending 11km on strike and about 4km on dip. Mining operations began in 1957 with shaft sinking operations. The first gold pour took place in 1962. Two economic gold-bearing reefs, the Ventersdorp Contact Reef (VCR) and the Carbon Leader Reef (CLR), are extracted (Fig. 1). The VCR is worked between 1500m and 2300m below surface, dipping on average 21 degrees South-east and sub-outcrops in the north-west. About 900 m below the VCR lies the CLR horizon which is continuous over the whole lease area. The VCR consists of a conglomerate with a great variation in pebble sizes and channel width Which exceeds 2,5 m in places ("reef roll"). The CLR is formed by a narrow, carbon-rich conglomerate with a channel width of a few centimetres.
For the exploitation of the VCR both, No.2 and No.3, shafts were sunk in the northern part of the lease area to 1930 m below surface. To gain access to the CLR sub-vertical shafts were
(Figure in full paper)
sunk down to 2975 m below surface. Further development in the form of tertiary vertical shafts was necessary to enable mining in the lower sections of the CLR-horizon.
The mine adopted a longwall mining system in which approximately 75 % of alliongwalls are protected by systematic stabilizing pillars. Mini-longwalls consisting of six panels, with a total length of about 200m on dip, are separated by 40 m wide strike stabilizing pillars. Stabilizing pillars were introduced in 1980 to address the eminent rockburst problem of the late 70's. Since 1987 backfill in the form of classified tailings is added in some areas to improve the regional support.
The reef is extracted conventionally by drilling and blasting. The broken rock is scraped to boxholes which lead to haulages in the footwall. These haulages ("follow-behinds") are developed some distance behind the actual face-position to avoid high field stresses.
Both horizons are intersected by a number of dykes which are mainly north east - south west orientated.