Chapter I
INTRODUCTION
1.1 Preambles
Australia ranked fourth in the world for total coal and lignite production; behind China, America, India and Indonesia. Australia is the world’s largest exporter of metallurgical coal and the second largest exporter of thermal coal. As one of the biggest countries in coal energy production and export, Australia’s coal production amounted to 72% of total energy production in 2017. Australian coal production is dominated by Queensland and NSW. In 2018, the gross output of coal production in Queensland reached 287.4 million tonnes (Queensland government, 2018) and 248.6 million tonnes in NSW (Coal service NSW, 2018), which was totally 97% of the production of Australian. The remaining coal was mined in Western Australia, South Australia and Tasmania.
The current development of contemporary science and technology has ushered in the enormous changes in coal mines performance. Improvement in mechanization, automation mining systems have all contributed to improvement in mine production and productivity. There has been a steady increasing of production of coal since 1990’s.
However, with the age increment of coal mine, there are several new challenges facing coal operators and mining engineers. For example, the increased depth of mining is accompanied by a more complex environment, such as higher temperatures and confining pressure. To deal with these situations and to ensure the safety production should always be the primary concern of operators and engineers. The requirement for stability of rock strata at surface slopes open pit and underground openings was adopted in modern collieries. Ground control constitutes one of the important factors in maintaining continuity of production in underground operations. Australian underground coal mines are developed with rectangular shaped roadways and these roadways are reinforced mainly by rock bolting with wire mesh lining, supplemented with cable bolting as secondary supports. Research on tendon technology (rock bolting and cable bolting) is vital for improvement of underground operations. This study
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embraces both experimental and numerical characterisation of cable strength behaviour.
Increasing the mechanical properties and reducing the deformability of the rock mass are two methods to improve the working conditions. Rock support systems involve the application of techniques and devices in contact with an excavation face to supply reaction force. Such devises, include timber, wooden and steel props, steel arches, concrete pillars, mesh and sprayed material. Rock support and rock reinforcement measures should control the deformation and stabilise the rock mass adjacent to an excavation (Li et al., 2016). It is fair to say that the number of rock bolts and cable bolts used in Australian mines may amount to 500 million for primary support and 0.7 million cable bolts for secondary support. Rock reinforcement has been utilised to improve the rock properties. Reinforcement is achieved by placing rock bolts cable bolts and ground anchors into boreholes drilled within surrounding material (Windsor, 1997). These three reinforcement devices deal with different scales of rock mass and the relationship is listed below:
1. Rock bolt and rock bolting (less than 3 m in length);
2. Cable bolt and cable bolting (in the range from 3 m to 15 m);
3. Ground anchors and ground anchoring (longer than 10 m).
In general, the tensile strength of a reinforcement element is relative to its length, called
“length-capacity relation”, which also links with primary, secondary and tertiary reinforcement. According to the reinforced object, the reinforcement was classified in to three levels. Primary reinforcement is applied to maintain overall stability and secondary reinforcement is installed to stable medium to large blocks, subordinating to primary reinforcement. Tertiary reinforcement is used to prevent surface loosening and degradation (Windsor, 1997).
There are two types of mining excavations, according to Brady and Brown (2013):
firstly service openings, including mine accesses, ore haulage drives, airways, crusher chambers and underground workshop spaces, and secondly production openings, including ore sources or stopes, drill headings, stope accesses, ore extraction and service ways. The service openings would be support and reinforced in the mining life of the orebody and the production openings would be discarded with the finality of the stope,
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resulting in a periodic rock controlling. Hence, the reinforcement is classified as permanent reinforcement and temporary reinforcement according to the service life, correspondingly the reinforcement in service openings and production openings is considered separately.
When excavating in the underground environment, the openings are affected by the ground stress, and surrounding rock around the opening has the potential to separate from the excavation boundary, causing a safety hazard in the field. The rock mess in the post- excavation stress field results in either the failure of blocks or slippage in weakness plane. The deformation of surrounding rock along the direction of reinforcement installation caused the axial tensile on elements. If larger magnitude displacement and slippage of bedding planes occurs around reinforcement drill holes, complex loadings are imposed on the element, including pure shear, pure tension and a combination of tension and shear. The combination of tension and shearing would be the main reason leading to the failure of reinforcement elements. In other cases, the rotation of block applies more complex loadings on these elements. To control the movement, deformation and rotation around openings, the ground support scheme should be projected to prevent displacement and control discontinues.
The understanding of strata control and movement limitation in the reinforcement system of rock bolts and cable bolts generally was gained by connecting a stable zone to the material undergoing deformation and discontinues. The whole system of reinforcement is comprised of four components according to Thompson et al. (2012):
the rock, the reinforce element, internal fixture and external fixture. Hence, the load transfer between the interactions of these components and typical interaction could be concluded into four types, including the rock and the internal fixture, the reinforce element and the internal fixture, the reinforce element and the external fixture and the external fixture and the rock. These quality interactions have a vital influence on the performance of reinforcement and the effectiveness of strata control. However, if these interactions failed before the reinforcement element broken, leading loss of partly or all the ability of single reinforcement element in the system, the phenomenon of de-bonding occurred.
With the increasing use of cable bolts as secondary support in coal mines, attention has been drawn to the cable bolts behaviour and performance in different ground conditions.
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The performance of cable bolts has been scrutinised in terms of:
1. The strength, both in tensile and in shear;
2. The quality of the installed cable bolt anchorage;
3. The nature of their construction with respect to the roughness of the outer wires surface, both smooth and indented;
4. The behaviour of the cable in different ground rock mass formations.
Recent studies on the evaluation of the shear strength properties of cable bolts in shear and in tension have enabled a better characterisation of cable bolts and their installation in different ground conditions. Based on field observations of the retrieved tendons from collapsed ground above excavations and improved knowledge and understanding about the behaviour of cable bolts in different shape and configurations, opinions are differing significantly on how cables bolts should be installed in the ground for effective stabilisation and better ground control. Currently, there are two schools of thought on the installation and encapsulation of cable bolts for a given ground condition. One school of thought is advocating the use of smooth wire cables for ground reinforcement allowing the cable to be de-bonded at the anchorage so that the strata surrounding the excavation is able to deform in a controlled manner. Another school advocates the use of indented cable bolts for a minimum of ground movement.
The benefit of allowing the cable to gradually de-bond, particularly in soft formations, results in segmental shearing of the ground along different length of the cable bolt, as evident from Figure 1.1, which clearly demonstrates segmental cable anchorage at the sheared zones, thus making cable bolts to be held anchored in rock at various sheared zones. The variations of thoughts have come about because of:
1. Lack of effective studies undertaken to evaluate the shear characterisation of cables;
2. Lack of understanding on the reasons for increased roughness of the cable wires;
3. The nature of cable shearing with respect to the competency of the host rock layers, and their failures or snapping in different rock /strata strength.
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Thus, there remains a significant and important factor that has not been dealt with in the past studies on cable strength properties, in particular the nature of cable shear failure in different rock strength conditions, these factors and others will be the subject of the study to be reported in this thesis.
Figure 1.1 Failure cable bolt from underground coal mine (From Brian McGowan of Glencore Australia)