Study on failure characteristics and control technology of roadway surrounding rock under repeated mining in close-distance coal seam

In this study, taking the Sheng’an coal mine as an engineering background, the failure characteristics of the surrounding rock of a roadway under repeated mining in a close-distance coal seam is comprehensively illustrated through field measurements (e.g., drilling imaging), theory analysis and numerical simulation (finite difference method (FDM)). The results show that although the return airway 10905 remains intact, the apparent failure of the roadway’s roof and the coal pillar can be observed. In addition, the expression of floor failure depth caused by upper coal seam mining is obtained through elastic-plastic theory. Meanwhile, the deformation of the surrounding rock of the roadway increases with the increase of repeated mining times, especially for the horizontal displacement of the roadway on the coal pillar side. Moreover, the cracks’ evolution of surrounding rock in the roadway can be observed as asymmetric characteristics. Finally, the stability control technology of “asymmetric anchor net cable + I-steel” is proposed to prevent potential mining disasters, and the feasibility of this support scheme is verified by numerical simulation and field practices. It can meet the requirement of safe mining and provide guidelines to effectively solve the failure of a roadway in close-distance coal seam mining

A Cone-Beam Computed Tomographic Study on Mandibular First Molars in a Chinese Subpopulation.

The purpose of this study was to conduct a cone-beam computed tomographic (CBCT) investigation on the root and canal configuration of the mandibular first molars, especially the morphology of the disto-lingual (DL) root, in a Chinese subpopulation. A total of 910 CBCT images of the mandibular first molars were collected from 455 patients who underwent CBCT examinations as a preoperative assessment for implants or orthodontic treatment. The following information was analyzed and evaluated: tooth position, gender, root and root canal number per tooth, root canal type of the mesial root(s) and distal root(s), angle of the DL root canal curvature, distance between two distal canal orifices in the teeth with DL root, and angle of disto-buccal canal orifice-disto-lingual canal orifice-mesio-lingual canal orifice (DB-DL-ML). Most of the mandibular first molars (64.9%, n = 591) had two roots with three root canals, and most of the mesial root canals (87.7%, n = 798) were type VI. The prevalence of the DL root was 22.1% (n = 201). The right side had a higher prevalence of DL root than the left side (p<0.05). Additionally, the curvature of the DL root canal were greater in the bucco-lingual (BL) orientation (30.10°±14.02°) than in the mesio-distal (MD) orientation (14.03°± 8.56°) (p<0.05). Overall there was a high prevalence of DL root in the mandibular first molars, and most of the DL roots were curved in different degrees. This study provided detailed information about the root canal morphology of the mandibular first molars in a Chinese subpopulation

The angle of the DB-DL-ML.

<p>The three pictures showed the angles of different degrees. D, distal side; M, mesial side; L, lingual side; B, buccal side.</p

The angle of the DB-DL-ML.

<p>The three pictures showed the angles of different degrees. D, distal side; M, mesial side; L, lingual side; B, buccal side.</p

The angles of the DL root canal curvature in the mandibular first molars in the BL and MD orientations.

<p>The angles of the DL root canal curvature in the mandibular first molars in the BL and MD orientations.</p

The correlations between the prevalence of DL root and gender.

<p>Level of significant was set at 0.05 (<i>p</i> = 0.229).</p><p>The correlations between the prevalence of DL root and gender.</p

The angle of the DL root canal curvature in the MD orientation.

<p>The three pictures displayed curvatures of DL root in the MD orientation of different degrees: a, straight root canal; b, moderately curved root canal and c, severely curved root canal. M, mesial side; D, distal side.</p

The root and root canal number in the mandibular first molars.

<p>The root and root canal number in the mandibular first molars.</p