Mechanical and Acoustic Emission Characteristics of Coal-like Rock Specimens under Static Direct Shear and Dynamic Normal Load.

In underground engineering, shear failure is a common failure type in coal-rock mass under medium and low strain-rate disturbance loads. Analyzing the shear failure mechanical properties of coal-rock mass under dynamic normal load is significant. In order to reveal the influence of disturbance load on the shear mechanical properties of coal rock, a dynamic and static load coupling electro-hydraulic servo testing machine was used to conduct the shear tests of coal-like rock materials under dynamic and constant normal load. The amplitude of dynamic load is 10 kN and the frequency is 5 Hz. The damage process of the specimens was detected by the acoustic emission (AE) detection system. The results imply that the shear failure process of coal-like rock materials under constant normal load can be divided into four stages. The normal disturbance decreased the shear strength of the specimens and increased the shear modulus of the specimens. With the increase in normal load, the influence of disturbance on the shear strength of the specimen decreased. By analyzing the AE parameters, it was found that the dynamic load made the internal damage of the specimen more severe during the shear failure process. The damage variable was calculated by AE cumulative energy, and the damage evolution was divided into three stages. The shear failure mechanism of the specimen was judged by RA (rise time/amplitude) and AF (average frequency). It was found that from the elastic deformation stage to the unstable development fracture stage, the proportion of shear fracture increased. When the dynamic normal load was 10 kN and 30 kN, the fracture was mainly shear fracture; When the dynamic normal load was 50 kN, the fracture was mainly tensile or mixed fracture. The dynamic normal load affects the shear strength and failure mechanism. Therefore, the influence of disturbance load on coal-rock mass strength cannot be ignored in underground engineering.

Study on Mechanical Properties and Weakening Mechanism of Acid Corrosion Lamprophyre.

In order to study the weakening mechanism and mechanical behaviors of hard lamprophyre of Carboniferous Permian coal-bearing strata in China's mining area, lamprophyre samples were subjected to static rock dissolution experiments with pH values of 0, 2, and 4. The acid corrosion mechanism of lamprophyre was revealed from the weight changes of samples, characteristics of solution ion concentration, and macro-mechanical properties. The experimental results show that reaction occurred between lamprophyre and acid solution. With the increasing concentration of H+, the reaction was more intense, the degree of acid etching was higher, and the weight loss was greater. The internal damage induced by acid etching results in the slow extension of the compaction stage of stress-strain curve of uniaxial compression, and the obvious deterioration of mechanical properties of the lamprophyre. The uniaxial compressive strength of the lamprophyre in the dry state is 132 MPa, which decreased to 39 MPa under the acid etching condition, showing significant mudding characteristics. Dolomite (CaMg(CO3)2 with 19.63%) and orthoclase (KAlSi3O8 with 31.4%) in lamprophyre are the major minerals constituents involved in acidification reaction. Photomicrograph recorded from SEM studies reveals that the dissolution effect was directly related to the concentration of H+ in the solution. The dissolution effect was from the surface to the inside. The small dissolution pores became larger and continuously expanded, then finally formed a skeleton structure dominated by quartz. The content of K+, Ca2+, and Mg2+ in the solution after acid etching reaction indicates that the acidified product of orthoclase is colloidal H2SiO3, which adhered to the surface of samples during acid etching and hinders the further acidification of minerals. The dissolution of dolomite and orthoclase under acidic conditions directly leads to the damage of their structure and further promotes the water-rock interaction, which is the fundamental reason for the weakening of the mechanical properties of lamprophyre.

Vertical stress and stability of interburden over an abandoned pillar working before upward mining: a case study.

Upward mining of the residual coal seam over an abandoned pillar working is one of the effective measures to alleviate the contradiction between limited resources and increased consumption. Interburden stability over an abandoned pillar working plays a significant role in guaranteeing the safety of upward mining; however, it has not yet been extensively studied and understood. In this study, the vertical stress of the interburden over an abandoned pillar working was first investigated. The mechanical model of the interburden was established and the damage conditions were analysed. Then, the stability of the interburden over 38502 abandoned workings in Baijiazhuang coal mine was determined by mechanical analysis and field monitoring. The results show that: (i) Vertical stress of the interburden over abandoned mining zones is clearly lower than the initial stress, indicating the existence of a de-stressed effect. Moreover, vertical stress of the interburden over residual coal pillars is greater than the initial stress, which is the evidence of a stress concentration effect. (ii) The interburden over an abandoned pillar working should be regarded as an elastic rectangular plate supported by generalized Kelvin bodies in mechanical modelling. (iii) The interburden over abandoned mining zones may experience two damage stages. In the first stage, initial plastic damage appears at the central region of interburden. In the second stage, the plastic damage evolves from the central point to the surrounding areas. (iv) The mechanical analysis and field monitoring both indicate the initial damage occurred at the central region over 38502 abandoned workings in Baijiazhuang coal mine before upward mining. Related rock control measures should be implemented in that region to guarantee the safe mining of the residual coal seam.

Failure analyses of unconfined CCWBM body in uniaxial compression based on central pressure variation.

Cemented coal waste backfill material (CCWBM) is made of coal gangue, fly ash and cementitious materials. It has been widely used in the field of backfill mining to control surface subsidence and protect the environment. A large number of unconfined backfill bodies without lateral support are formed in partial backfill mining. To study the failure characteristics of unconfined CCWBM body in partial backfill, the stress-strain curves of the CCWBM were obtained by uniaxial compression tests at different ages (1-28 d). The central pressure was measured by the embedded pressure sensors. The failure characteristics of the specimen were monitored by acoustic emission (AE) positioning technique. Three observations can be made. 1. The central pressure variation curves lag behind the mean stress change curves. The central pressure curve can be divided into three stages: slow increase stage, rapid growth stage and decline stage. It has two pressure manifestations: early appearance and peak appearance. They can be as the failure precursor and instability critical, respectively. 2. The specimen forms a central elastic bearing area in the process of compression. The plastic area develops to the inner side with the increase of pressure, and an upper and lower compound cone-shaped residual area is finally formed. 3. The embedded pressure sensor can be used to monitor the instability of the unconfined backfill body. The research results can provide guidance for the in-situ stability monitoring and reinforcement of unconfined CCWBM body in partial backfill.

A B3LYP and MP2(full) theoretical investigation on the cooperativity effect between hydrogen-bonding and cation-molecule interactions and thermodynamic property in the 1: 2 (Na⁺: N-(Hydroxymethyl)acetamide) ternary complex.

The cooperativity effects between the O/N-H∙∙∙O hydrogen-bonding and Na⁺∙∙∙O cation-molecule interactions in the 1: 2 (Na⁺: N-(Hydroxymethyl)acetamide) systems were investigated at the B3LYP/6-311++G**, MP2(full)/6-311++G** and MP2(full)/aug-cc-pvtz levels. The thermodynamic cooperativity calculations were also carried out for two pathways of the ternary-complex formation. The result shows that, in most ternary complexes, the O/N-H∙∙∙O and Na⁺∙∙∙O interactions are weakened in comparison with those in binary systems, leading to the anti-cooperativity effects, in particular in the complexes in which only the Na⁺∙∙∙O interactions exist. Shifts of electron density confirm the existence of anti-cooperativity. The increase of favorable enthalpic contribution leads to the positive cooperativity effect with negative ΔG(coop.) on forming the ternary complex by initial N-(Hydroxymethyl)acetamide dimer followed by addition of Na⁺. In forming the ternary complex by Na⁺∙∙∙N-(Hydroxymethyl)acetamide with the second N-(Hydroxymethyl)acetamide unit, the large unfavorable entropy change leads to the negative cooperativity effect with positive ΔG(coop.). The ternary complex is more easily formed by the pathway in which Na⁺ binds to N-(Hydroxymethyl)acetamide dimer.