Effect of loading frequency on the deformation behaviours of sandstones subjected to cyclic loads and its underlying mechanism

https://www.research.manchester.ac.uk/portal/files/101162653/R2_manuscript_to_ENGEO_MS_TC.pdf

Influence of moisture on crack propagation in coal and its failure modes

A new energy-dissipation method to identify crack initiation and propagation thresholds is introduced. Conventional and cyclic loading–unloading triaxial compression tests and acoustic emission experiments were performed for coal specimens from a 980-m deep mine with different confining pressures of 10, 15, 20, 25, 30, and 35 MPa. Stress–strain relations, acoustic emission patterns, and energy evolution characteristics obtained during the triaxial compression tests were analyzed. The majority of the input energy stored in the coal specimens took the form of elastic strain energy. After the elastic-deformation stage, part of the input energy was consumed by stable crack propagation. However, with an increase in stress levels, unstable crack propagation commenced, and the energy dissipation and coal damage were accelerated. The variation in the pre-peak energy-dissipation ratio was consistent with the coal damage. This new method demonstrates that the crack initiation threshold was proportional to the peak stress (σ
 p) for ratios that ranged from 0.4351 to 0.4753σ
 p, and the crack damage threshold ranged from 0.8087 to 0.8677σ
 p.

Estimation of Crack Initiation and Propagation Thresholds of Confined Brittle Coal Specimens Based on Energy Dissipation Theory

The effects of water on the strength of black sandstone in a brittle regime

The presence of water strongly affects rock properties and would be related to a series of geological disasters. To understand water saturation effects on the mechanical behavior of different rock types and interpret the underlying mechanisms of differences in water sensitivity, three kinds of rocks, namely sandstone, granite and marble, were selected for tests. Uniaxial compression experiments were conducted on specimens under oven-dried and water-saturated conditions. Acoustic emission (AE) techniques were also applied to monitor and record AE signals during tests. Experimental results reveal that water weakens the mechanical parameters of the three tested rocks, such as uniaxial compressive strength (UCS), elastic modulus and critical strain. The sandstone undergoes the greatest weakening with the addition of pore water, the mechanical properties of the granite exhibit relatively minor reductions, while the marble is the least affected by water saturation. The water-weakening degree of rock properties depends on the porosity as well as the mineralogy, especially the proportion of quartz and swelling clays. Moreover, after water saturation, the failure pattern of the sandstone and the granite tends to transform into the shear-dominant mode from the tensile one in dry state, probably due to frictional reduction. However, the water presence does not change the failure mode of the marble.

https://www.mdpi.com/2076-3417/9/20/4450/pdf

Water-Weakening Effects on the Mechanical Behavior of Different Rock Types: Phenomena and Mechanisms

Studying the mechanical properties of and crack propagation in coal after water intrusion is necessary to tackle a number of geological engineering problems such as those associated with underground water storage in collieries and support for underground roadways in coal mines. To study the mechanical properties and crack development, 12 coal samples with moisture contents of 0, 2.37, 3.78 and 5.29 % were prepared for acoustic emission tests under uniaxial compression. Over about 6 days, the coal samples absorbed moisture from a humidifier in three different phases. In this period, uniaxial tests show that the peak stress, elastic modulus, strain softening modulus and post-peak modulus decreased with rising moisture content in the samples while the peak strain increased. It was further found that, by analysing the relationship between the stiffness and stress and the accumulated acoustic emission counts, all the phases of crack development can be evaluated. This is useful for studying the effect of water intrusion on crack propagation and for calculating the mechanical properties of the coal such as the elastic modulus. This investigation also quantifies the percentage of the stress thresholds for crack closure, crack initiation, and crack damage that constitutes the peak stress. These stress thresholds do not change with moisture content. Our results are of great significance for water storage in coal mines, for determination of pillar dimensions in coal mines, and for expanding the knowledge base of the mechanical properties of coal and the characteristics of crack propagation.

Effects of Water Intrusion on Mechanical Properties of and Crack Propagation in Coal

The invention provides a simulated stability testing device and method for a water storage dam of a goaf. The device comprises the dam body with fill and gangue and a server jack fixing device, the upper portion of a left side plate of the dam body is provided with a pressure regulating valve, a lead of the pressure regulating valve is connected with a signal receiver of a water pressure sensor, the lower portion of the left side plate with a water inlet/outlet, a front panel of the dam body is a hollow channel board with fillers, multiple stress sensors are distributed in the hollow channel board, the stress sensors are connected with a signal receiver of the stress sensors via leads, and the signal receiver of the stress sensors, the signal receiver of the water pressure sensor and a signal sensor arranged in a pressure pump of a server jack are connected with a computer. The hydraulic jack is loaded via servo control, and the stability of the water storage dam in the goaf under the effects of covering rocks and moving/static water pressures is researched in the simulated manner. The stress sensors and the water pressure sensor monitor the change rules of the stress and the water pressure of the water storage dam of the goaf, and the stability of the dam under the effects of covering rock deadweight and moving/static water pressures is calculated.

Liu Yapeng

Papers

Effects of Water Intrusion on Mechanical Properties of and Crack Propagation in Coal

Simulated stability testing device and method for water storage dam of goaf