Experimental study on characteristics of grounawater fracture in coalmine overlying rock

The seepage characteristics of groundwater in the overlying fissures are the key factors to be considered in the later maintenance of coal mining and goaf, especially in the underground reservoir construction area of coal mines. It is also an important part of coal mine underground reservoir. In shallow groundwater-rich areas or under extreme summer rainfall conditions, underground reservoirs form vertical recharge, and analyzing the seepage characteristics of groundwater in overlying fissures provides a scientific basis for the safe operation of coal mine underground reservoirs and the protection of groundwater resources. In this study, the solid-liquid coupling similarity model test was used to obtain the development and stability of the overlying fissures in the mining area and the characteristics of groundwater seepage after the aquifer was connected. The analysis showed that the large abscission fissures and micro-fissures that extend far and wide are mostly water storage. , the vertical fissures running through multiple rock layers have strong water conductivity, and the hydraulic connection between the separation layer fissures is mainly formed by the vertical fissures in the two sides. Under the condition of vertical recharge, groundwater first infiltrates along the vertical fissures in an unsaturated manner, and is gradually saturated regionally from the upper and lower overlying fissures, finally forming a stable saturated infiltration form. On this basis, a mathematical model of groundwater seepage in saturated seepage state is established, and the numerical method is used to solve it. It is verified with similar simulation experiments that vertical fractures are the main water conduction channels, and their water conduction can account for up to 97%. At the same time, the migration speed of groundwater in vertical fractures is also many orders of magnitude higher than that in abscission fractures. Finally, through the sensitivity analysis, it is concluded that the vertical fracture seepage is positively correlated with the fracture development degree and the total water inflow, and negatively correlated with the rock permeability. The migration time of groundwater in overlying fissures is negatively correlated with the degree of fissure development, rock permeability and total water inflow

A Review of the Force-Transferring Mechanism of Entirely Grouted Cable Tendons Performed with Experimental Pull Tests

Entirely grouted cable tendons are commonly used in mining engineering. They have superior working ability in reinforcing the excavated rocks and soils. During the working process of cable tendons, the force-transferring ability and the corresponding mechanism are significant in guaranteeing the safety of underground openings. To further understand the force-transferring mechanism of entirely grouted cable tendons, this paper provided a literature review on the force transfer of cable tendons. First, the force-transferring concept of entirely grouted cable tendons was summarised. The force-transferring process and failure modes of cable tendons were illustrated. Then, the experimental test program used in testing the entirely grouted cable tendons was summarised. The advantages and disadvantages of various test programs were illustrated. After that, the working ability of entirely grouted cable tendons was reviewed. The effect of various parameters on the working ability of cable tendons was summarised and compared. These parameters include the rock stiffness, embedment length, cement grout property, resin grout property, modified geometry, borehole size, rotation and pre-tensioning. Last, a discussion was provided to elaborate the working ability and force-transferring mechanism of entirely grouted cable bolts. This literature review is beneficial for researchers and engineers, furthering their understanding of the working ability of cable tendons

The Mechanism and Application of Deep-Hole Precracking Blasting on Rockburst Prevention

The mechanism of preventing rockburst through deep-hole precracking blasting was studied based on experimental test, numerical simulation, and field testing. The study results indicate that the deep-hole precracking could change the bursting proneness and stress state of coal-rock mass, thereby preventing the occurrence of rockburst. The bursting proneness of the whole composite structure could be weakened by the deep-hole precracking blasting. The change of stress state in the process of precracking blasting is achieved in two ways: (1) artificially break the roof apart, thus weakening the continuity of the roof strata, effectively inducing the roof caving while reducing its impact strength; and (2) the dynamic shattering and air pressure generated by the blasting can structurally change the properties of the coal-rock mass by mitigating the high stress generation and high elastic energy accumulation, thus breaking the conditions of energy transfer and rock burst occurrence

Strength Damage and Acoustic Emission Characteristics of Water-Bearing Coal Pillar Dam Samples from Shangwan Mine, China

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam’s standing sustainability. In the paper, the coal pillar dam body of the 22 layers of coal in the Shangwan Coal Mine is studied (22 represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions

Strength Damage and Acoustic Emission Characteristics of Water-Bearing Coal Pillar Dam Samples from Shangwan Mine, China

Long-term erosion and repeated scouring of water significantly affect the technical properties of coals, which are the essential elements that must be considered in evaluating an underground reservoir coal column dam’s standing sustainability. In the paper, the coal pillar dam body of the 22 layers of coal in the Shangwan Coal Mine is studied (22 represents No. 2 coal seam), and the water content of this coal pillar dam body is simplified into two types of different water content and dry–wet cycle. Through acoustic emission detection technology and energy dissipation analysis method, the internal failure mechanism of coal water action is analyzed. This study revealed three findings. (1) The crest pressure, strain, and resilient modulus in the coal sample were inversely related to the water content along with the dry–wet cycle number, while the drying–wetting cycle process had a certain time effect on the failure to the sample. (2) As the moisture content and the dry–wet cycle times incremented, three features were shown: first, the breakage pattern is the mainly stretching fracture for the coal specimen; second, the number and absolute value of acoustic emission count peaks decrease; third, the RA-AF probability density plot (RA is the ratio of AE Risetime and Amplitude, and AF is the ratio of AE Count and Duration) corresponds more closely to the large-scale destruction characteristics for the coal samples. (3) A higher quantity of wet and dry cycles results in a smoother energy dissipation curve in the compacted and flexible phases of the crack, indicating that this energy is released earlier. The research results can be applied to the long-term sustainability assessment of the dams of coal columns for underground reservoirs and can also serve as valuable content to the excogitation of water-bearing coal column dams under similar engineering conditions

Direct fabrication of flexible strain sensor with adjustable gauge factor on medical catheters

The preparation of flexible sensors on medical catheters is primarily an indirect process using certain materials as substrates. Using medical catheters as flexible substrates is beneficial for directly detecting the strain and contact force of the catheters. In this study, a strain sensor is prepared by rubbing graphite on a silicone medical catheter with a minimum inner diameter of 1 mm × 2 mm. The gauge factor (GF) of the strain sensor is adjusted by changing the resistance during preparation. By precisely controlling the composite material seepage zone, the maximum achievable GF is 121 at a low strain of 1%, which can be used to detect low forces with a resolution of 0.0013 N. Liquid glue is used to encapsulate the sensor, which barely changes the conductive network of the sensor. The response time of the strain sensor is 50 ms, and the performance of the sensor remains unchanged after 1000 cycles, which renders it suitable for intestinal shape detection. Additionally, we demonstrate the feasibility of integrating four strain sensors on the medical catheter to detect its bending direction via vector synthesis, which allows the spatial shape to be examined. The results obtained indicate the potential of the friction method for preparing strain sensors with different GFs on medical catheters to be used in different medical applications

Creep Constitutive Model and Numerical Realization of Coal-Rock Combination Deteriorated by Immersion

Coal-rock combination refers to the coal and rock as a whole, and the failure of the whole structure of the combination is the main cause for the instability of the deep underground engineering. In deep underground engineering, the coal-rock combination is usually under certain hydrogeological conditions, and it is prone to seepage and rheological failure instability accidents due to the long-term action of water and stress. In this study, the creep constitutive model of coal-rock combination considering the influence of moisture content was established based on the Burgers creep model. According to the experimental results of triaxial creep of rock, the relationship between the moisture content and the parameter of the Burgers creep model was derived, and the correctness of the constitutive model in this study was verified. Then, through the C++ language, the core equation of the model was modified, and the numerical calculation of the model was realized by introducing the coal-rock combination creep model considering the influence of moisture content into FLAC3D numerical simulation software. Finally, the model was used to simulate and study the creep characteristics of coal-rock combination with different moisture contents under triaxial loading. The results showed that the stress environment and moisture content have significant effects on the creep characteristics of the coal-rock combination. Under the same stress state, with the increased of moisture content, the strain rate of the coal-rock combination exhibited a non-linear rapid increase in the constant-velocity creep stage, the limit creep deformation and the instantaneous elastic deformation increased, and the viscosity coefficient was significantly decreased. For example, when the axial stress was 5 MPa and the moisture content increased from 0% to 1.5%, the strain rate increased by 44.06%, the limit creep deformation increased by 20%, the instantaneous elastic deformation increased 10.53%, and the viscosity coefficient decreased by about 50%. When the moisture content is 0%, the axial stress increased from 5 to 14 MPa, and the limit creep deformation increased nearly four times. With the increase of moisture content, this value will further expand. The research conclusions can provide a certain reference basis for the long-term stability control of surrounding rock in underground engineering affected by the water