Report: APP CMHS Project 1

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3.3.8. Deep, Gassy and Multiple Seam Mining

3.3.8. Deep, Gassy and Multiple Seam Mining

Some coal mines extract coal from multiple seams with a mining depth of over 1,000 m. The main safety issues in mining deep and multiple seams include high gas emission, outburst risk, roadway stability and heat. Some advanced technologies have been developed in China to address such issues. They include pillarless mining with retained goaf edge roadway with ‘Y’ type ventilation system, strata control technology that maintains the goaf edge roadways, and gas drainage technology that extracts gas from the de-stressed multiple overlying and underlying seams.

Pillarless Mining with Retained Goaf Edge Roadway with ‘Y’ Type Ventilation System

Pillarless mining with retained goaf edge roadways has been used in major coal producing countries mainly to increase the recovery ratio of coal resources, reduce total gas and minimise the risk of spontaneous combustion. In deep seam mining, the key issue with pillarless mining is to retain goaf edge roadways. The combination of, ‘Y’ type ventilation system, advancement in gas drainage technology and pillarless mining, are successfully used to extract deep, gassy and multiple seams of low permeability in China.

Technology to Retain Goaf Edge Roadway

arge deformation of surrounding rocks and roof collapsing are two of the major hazards in deep mining. In recent years, advanced strata control technologies have been developed to retain goaf edge roadways. These technologies include mainly strengthened supports and fill materials. Their delivery system is to construct the sidewall of a retained goaf edge roadway as shown in Figure 47.

Figure 47 Pillarless mining

Strengthened Supports in Retaining Goaf Edge Roadways

The supports in the retained goaf edge roadway include “super” bolts, roof-truss type supports for rib control, cable bolt beam, and a self-moving hydraulic chock.

A “super” bolt assembly includes “three high” bolt (high load carrying capability, high stiffness and high pretension force), high torque nut and large plate installed into the roof, as shown in Figure 48. It should be noted that the pretension force plays a decisive role in roadway stability. Shear failure of the roadway is unavoidable under horizontal stress, pretension bolts can increase shear strength of surrounding rocks, forming pre-stressed structure and prevent the shear failure from moving further inside.

Figure 48 Strata support systems

A self-advancing hydraulic chock is designed as supplementary support in the roadway and is similar to the conventional hydraulic chock, but modified to consider the narrow width of the roadway and goaf edge support Figure 49.

Figure 49 Self advancing hydraulic support

To retain a goaf edge roadway and keep it stable, it is important to construct fill-in pack of high support performance with a highly mechanised and rapid fill operation system. Fill materials have high load-carrying capacity, controlled contraction, are inexpensive, and suitable for pump transportation. Fill systems are highly mechanised and suitable for safe and rapid underground operation, so that the fill-in pack can be constructed at the same pace as face retreat rate.

A fill material CHCT has been developed to meet the requirements of constructing the sidewall for retaining a goaf edge roadway. The materials are composed of cement, flyash, stone, sand, water and additives. Mass ratios of each component are 10 to 30 % cement, 7 to 40 % flyash, 15 to 40 % stone, 15 to 30 % sand, 10 to 30 % water and 2.5 to 7.0 % additives.

The fill operation system includes a surface-based system for mixing dry materials, a surface to underground delivery system, a delivery to pump feeding system, slurry making and pumping system, and a chock mould system, and gas management in deep seams.

Gas Drainage

In general, with increased mining depth, methane content and outburst risk increase and seam permeability decreases. Conventional methane control measures, such as inseam gas pre-drainage and surface goaf well drainage, are not effective, difficult to implement or very expensive. In deep and multiple seam mining, a new gas drainage technology with gas boreholes drilled from retained goaf edge roadways into methane-rich zones into roof strata, goaf and floor strata has been developed to replace the conventional methane drainage methods used in shallow coal seams. The principle and layout of this gas drainage technology are shown below in Figure 50.

Figure 50 Cross measure gas drainage holes

Application sites

The integrated approach to reduce multi hazards in deep, gassy and multiple seam mining has been successfully applied in some large coal mines in China, such as those in the Huainan, Yangquan and Songzao Groups.

3.3.8. Deep, Gassy and Multiple Seam Mining

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