Experimental Study on the Mechanical Properties of Tectonic Coal Using Reconstituted Coal Specimens

Tectonic coal, an aggregate of coal particles formed by compacting pulverized coal, has been developed extensively in China. Currently, reconstituted coal specimens are widely adopted to investigate the mechanical properties of tectonic coal, but they have a low compaction degree compared to the tectonic coal in the field. Therefore, the current understanding of the mechanical properties of tectonic coal is not accurate. Herein, a new high–pressure–resistant mold was developed, and a heavy press was developed to prepare highly compacted reconstituted coal specimens. Based on the reconstituted coal specimens and the intact coal specimens obtained through coring, the mechanical properties of tectonic coal and intact coal were measured and compared systematically. The results show that the compaction degree of reconstituted coal specimen can be improved significantly by increasing the external force. For Sijiazhuang coal, the compaction degree of the reconstituted coal specimen almost reaches that of the tectonic coal in the field when the external force is increased to 550 KN. Moreover, the tectonic coal exhibits a low elastic modulus and low strength but high stress sensitivity. The elastic modulus and cohesion of tectonic coal are 22.08% and 43.47% of the corresponding values for intact coal. However, with the increase in the confining pressure from 5 to 20 MPa, the elastic modulus of tectonic coal increases by 1.14 times, while that of the intact coal increases just by 8.70%. In addition, tectonic coal and intact coal present different post-peak failure modes under the triaxial compression stress path. Typical shear failure occurs in the intact coal, while multiple shear failure occurs in the tectonic coal.