International Journal of Mining Science and Technology
A study of support strategies in deep soft rock: The horsehead crossing roadway in Daqiang Coal Mine
Introduction
Increasing mining depths bring increasing problems with soft rock, difficulties in retaining rock, and increasing roadway and chamber damage [1], [2], [3], [4], [5], [6]. Roadway forks have a larger cross section and damage is particularly serious there. Conventional supports cannot effectively control deformation and failure and this has brought great difficulties to coal mine safety [7], [8], [9], [10], [11], [12]. Therefore, the support of deep soft rock roadway cross-ways is particularly prominent. In this paper a deep air shaft in the Daqiang Coal Mine located in deep soft rock at a horsehead crossing point is discussed. The support issues related to anchor networks and cables used with soft layer coupling as the main truss supporting technology are studied.
Section snippets
Lithology
Daqiang Coal Mine level −550 (at a buried depth of 659 m) horsehead is located in the Mesozoic Jurassic formation and has a lithology consisting of coarse sandstone, siltstone, and a sandy conglomerate.
Lithologic analysis
Samples were taken from the air shaft at level −550 at the horsehead. They were analyzed with electron microscopy and X-ray diffraction, the results being shown here in Table 1, Table 2. The material contains clay minerals that swell. The clay minerals are mainly smectite, green I/S, kaolinite,
An analysis of the deformation mechanism
Theoretical analysis and field research of the study area lead to the conclusion that the deformation in the roadway occurs because of: (1) Swelling of montomorillonite clay. The Green I/S content is about 85% and this rock swells considerably upon absorbing water. This is a molecular expansion mechanism, type IA; (2) Large residual stresses. The roadway is complex and the deformation forces include self-weight and tectonic stresses. Construction disturbance around the shaft causes deviatoric
Support scheme
This analysis suggests the −550 roadway should be constructed from straight walls and semi-circular arches using spray and cable anchors with base angle bolts and soft corner coupling trusses.
Principles of coupling support
Coupled support technology is appropriate for deeply buried structures where non-linear behavior requires special treatment [15], [16], [17], [18], [19]. Engineering design uses large deformation mechanics and considers the local geology. This way, rock strength may be improved by the retaining structures
Analysis of the design
The deformation of the horsehead roadway was controlled by the spray and cable anchors, angle bolts, and corner coupled truss support. Monitoring of both sides of the horsehead showed the contraction of the two roadways, the roof subsidence, and the floor heave. Fifty days after beginning monitoring the maximum observed deformation was 24 mm. After 13 months the roadway was stable and the support system had provided good results.
Conclusions
A field investigation and theoretical analysis was used to determine the cause of damage to the −550 air shaft and horsehead roadway. A system was then designed that used spray, anchors, anchor bolts, and corner coupled truss supports. It was then implemented in the roadway. Observation of the resulting design in the deep rock showed that large deformations were controlled in an economic and practical way. This design can be used for similar roadways.
Acknowledgments
This study is supported by the National Basic Research Program of China (No.2006CB202200), the Ministry of Education Innovation Team Project (No. IRT0656), and the Central University Basic Research Special Fund Operating Expense (No.2009QL06), the New Century Excellent Talents Support Projects of Ministry of Education (No.NCET-08-0833), and the National Natural Science Foundation of China (No.41040027).
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