Soft Seam Research Articles

Promoting Gas Extraction Technology with Screen Pipe for Long Borehole Protection in Soft Seam

The inherent properties of soft coal seams and their mechanical environment make long boreholes susceptible to issues like collapse, deformation, and blockages. These problems shorten the service life of the boreholes and hinder extraction efficiency. This paper tackles these challenges by analyzing the deformation and damage patterns of long boreholes in soft coal seams. It examines the stress distribution and deformation characteristics around both protected and unprotected boreholes at different burial depths. Additionally, it recommends using screen pipe protection technology to improve gas extraction and mining operations, as demonstrated in Changping Coal Mine. The results show that screen pipe protection substantially improves the stress distribution and deformation stability of coal seam boreholes. The flow attenuation coefficients of two boreholes equipped with protection technology decreased by 48% and 61%. After 50 days of extraction from boreholes with a protection rate exceeding 90%, gas concentration remained above 50%, which is 2.59 times higher than that of unprotected boreholes. This technology effectively addresses the frequent accidents, poor extraction performance, and inefficiency of long boreholes in soft coal seams, ensuring the mine’s safe and efficient production.

Impact of Stitch Density on Tactile Comfort Properties of Seams

The impact of stitch density and seam type on tactile properties of seams has been investigated. Lapped seam 1 (LSa-1) using stitch class 605 and superimposed seam (SSa-2) using stitch class 514 (ASTM 6193) were constructed with four different stitch densities, namely, stitches per inch (SPI) 10, SPI 14, SPI 18 and SPI 22, and the tactile properties of seams examined by evaluating the characteristics of seams such as seam compression, seam thickness, seam bending behaviour and surface friction of seams. From the study, the optimized stitch density has been identified as SPI 10 for producing soft seam. Seam class and stitch density play a vital role in determining seam comfort properties.

Research on Mechanism and Control Technology of Rib Spalling in Soft Coal Seam of Deep Coal Mine

In order to solve the difficult problem of coal wall spalling of soft seam in deep mining, the occurrence mechanism of coal wall spalling in deep mining is studied based on the limit equilibrium theory of D‐P criterion. Through the analysis of the formula of plastic zone, it can be concluded that the coal wall spalling of soft coal seam is related to mining depth, mining height, stress concentration factor, internal friction angle of coal, cohesion, and the compressive strength of coal body. Also, the change of uniaxial compressive strength of soft coal with different moisture content is studied through laboratory experiments. The experimental results show that the uniaxial compressive strength of soft coal reaches the maximum value when the moisture content is about 3.3%. The comprehensive research shows that the cohesion and strength of coal mass can be improved by increasing the working resistance of support, water injection in advanced deep‐hole coal seam of two roadways, and pre‐grouting in shallow hole of coal wall. The control measures can effectively control the coal wall spalling of soft coal seam. The research results have important guiding significance for safe and efficient mining of soft coal seam.

Seismicity induced by longwall coal mining at the Thoresby Colliery, Nottinghamshire, U.K.

The U.K. has a long history of deep coal mining, and numerous cases of mining-induced seismicity have been recorded over the past 50 years. In this study we examine seismicity induced by longwall mining at one of the U.K.’s last deep coal mines, the Thoresby Colliery, Nottinghamshire. After public reports of felt seismicity in late 2013 a local seismic monitoring network was installed at this site, which provided monitoring from February to October 2014. This array recorded 305 seismic events, which form the basis of our analysis. Event locations were found to closely track the position of the mining face within the Deep Soft Seam, with most events occurring up to 300 m ahead of the face position. This indicates that the seismicity is being directly induced by the mining, as opposed to being caused by activation of pre-existing tectonic features by stress transfer. However, we do not observe correlation between the rate of excavation and the rate of seismicity, and only a small portion of the overall deformation is being released as seismic energy. Event magnitudes do not follow the expected Gutenberg-Richter distribution. Instead, the observed magnitude distributions can be reproduced if a Truncated Power Law distribution is used to simulate the rupture areas. The best-fit maximum rupture areas correspond to the distances between the Deep Soft Seam and the seams that over- and underlie it, which have both previously been excavated. Our inference is that the presence of a rubble-filled void (or goaf) where these seams have been removed is preventing the growth of larger rupture areas. Source mechanism analysis reveals that most events consist of dip-slip motion along near-vertical planes that strike parallel to the orientation of the mining face. These mechanisms are consistent with the expected deformation that would occur as a longwall panel advances, with the under- and over-burdens moving upwards and downwards respectively to fill the void created by mining. This further reinforces our conclusion that the events are directly induced by the mining process. Similar mechanisms have been observed during longwall mining at other sites.

The influence of viscoelastic surfactant fracturing fluids on gas desorption in soft seams

Hydraulic fracturing is an effective method of increasing the permeability of coal seams, which has gradually come into use in underground coal mines. Fracturing fluid is a key factor affecting the improvement. In this study, the way in which viscoelastic surfactant fracturing fluids affect gas desorption in soft seams was analyzed using the zeta potential and scanning electron microscope (SEM)-based methods. The results obtained with the zeta potential method indicated that due to the components of viscoelastic surfactant fracturing fluids, the potential at the coal surface jumped from −16.3 mV to 48 mV, the interaction energy between the fracturing fluid and the coal surface increased, the adsorption potential of the gas decreased and gas desorption increased. The SEM results demonstrated that the number of gas transport channels increased due to the effective dissolution of cement in the acidic viscoelastic surfactant fracturing solution. The cumulative pore volume of the coal samples processed with viscoelastic surfactant fracturing fluids was 0.001 cm3/g, which was 1.8 times the cumulative pore volume of coal samples processed with water. The porosity and connectivity of the coal pores both increased, which was beneficial to gas desorption. Comparative experiments using water and viscoelastic surfactant fracturing fluids were conducted in underground coal mines. The results demonstrated that the amount of gas extracted increased by 26.1% when viscoelastic surfactant fracturing fluids were used. Gas desorption in soft seams was promoted by viscoelastic surfactant fracturing fluids. The new data reported in this study can be used to optimize the use of fluids for underground hydraulic fracturing in Chinese coal mines.

Hydraulic Fracturing Mechanical Mechanism Analyses for Soft Seams Considering Strain Softening Character

Mathematical model based on elasticity is not suitable for soft seam hydraulic fracturing mechanism study because its intensity is small, Poisson's ratio is relatively large, and its prone to plastic deformation. Based on plastic mechanics, the theory of large deformation and fracture mechanics theory, hydraulic fracturing of soft coal seam is divided into three phases, namely, coal bed compaction, fracture initiation and crack propagation from the view of the deformation mechanism, the occurring and developing mechanism. The initiation pressure of soft seams considered strain softening character after plastic deformation is obtained on the basis of above. The result shows that the initiation pressure is related to elastic modulus, Poisson's ratio, the angle of internal friction and residual strength. Elastic modulus is inversely proportional to the initiation pressure, the greater its value, the smaller the initiation pressure; but Poisson's ratio, the angle of internal friction and the residual strength and fracture initiation pressure is directly proportional relationship, the greater its value, since the smaller the crack pressure.

Experimental Study on Mutual Effects of Rock Tunneling Positions with High Underground Pressures

Using the mould pattern contrasting experiments the carrying capacities and deformation effects of underground tunnels within soft bed and top or floor strata are studied under high ground pressures in this paper. Through measuring mould strengths and deformation with different stratum and space position the relations of tunneling positions in different strata with carrying capacity and strains are obtained. It is shown that the soft seam tunneling under high ground pressure can bring about higher secondary stresses and have more acting effects than top or floor tunnels. In the same pressure conditions the carrying capacity of having top tunneling above soft tunnel decreases near 30% with that of having floor tunnel. The mutual effects with double horizontal spaces of tunnels decrease 25%. The experiment conclusions are important significance of conducting and applying for reasonable designing underground tunneling engineering.

Investigation into the deformation of a large span roadway in soft seams and its support technology

We investigated the deformation failure mechanism of surrounding rock from the aspect of engineering support for a roadway in seams with soft roofs and soft floors and observed the large displacement of the roadway in these soft seams. The result shows that the deformation area is quite large, and settlement of the roof is evident and displacement of the side walls is also obvious. We considered rock bolt-cable coupling for roadway support in seams with soft roofs and floors, in which the cable should be fixed at key positions. As well, we designed an optimal scheme to support a roadway in soft seams of the Shizuishan Second Mine in Ningxia, China. Field monitoring results show that bolt-cable coupling support has achieved the aims of roadway stability control and minimizes deformation.

Analysis on the Influence of Coal Strength to Risk of Outburst

Abstract The lower the coal strength is, the bigger the width of stress released zone is in front of the tunneling face. Enhancement of the stress released zone width may increase the resistance of outburst and reduce the outburst hazard; in fact, most outbursts take place in the soft seam. At first, the author analyzed the relation of coal mass strength and the width of stress released zone in front of the tunneling face; then, state that the stresses make change of the gas which is in the edge of the stress released zone; to analysis the relation between coal mass strength and the quantity of gas desorption per minute. According to the research result, the coal mass strength is lower, the width of stress distressed zone is bigger, and the quantity of gas desorption per minute is bigger; but, the latter is more dominant than the earlier ones. Finally, by making use of the research result, explained the gas abnormal phenomenon that is a sign of outburst premonition, and analyzed the mechanism of outburst prevention in coal seam infusion.

A new method of drilling long boreholes in low permeability coal by improving its permeability

When drilling holes for gas pre-extraction from soft coal seams having low permeability, phenomena such as hole collapse and drill pipe sticking can occur. These problems are due to the coal's low intensity and high gas pressure, both of which severely limit the efficiency of gas extraction. Based on an analysis of borehole collapse in soft seams, and on a study of the pore-formation mechanism in stiff layers, this paper proposes a new drilling technique that uses a high-pressure pulsed water jet. This technique can be used to drill holes in stiff layers of the roof/floor until the soft layers of the coal seam are reached. This method can improve the gas extraction rate. According to damage models for rock, a theoretical analysis and numerical simulation was performed for a high-pressure pulsed water jet. This analysis examined the instantaneous dynamic loads, the damage features when using a flexible impact regime, the changes in fracture field patterns, and the high-pressure pulsed water jet's oscillatory effect upon the gas flow of coal-mass. The impact and vibration effects of a high-pressure pulsed water jet could effectively contribute to crushing of the coal, to improvement of the coal's cracking and connectivity rate and to improvement in the permeability of the coal. Additionally, the high-pressure pulsed water jet's dynamic pressure and flow rate must be calculated for breaking rock during drilling and slotting. The developed system was applied in the Sanhui mine of Sichuan Province, China. The hole drilled by the new method is 2.7 times deeper than that of the hole drilled in the coal seam. In the Yutianbao mine, the average length was 148 m when drilling between the roof and the seam, and this value was more than 2.4 times deeper than that previously drilled in the coal seam. The gas flow was about 1.06 m3/min per meter, which is more than 5 times that of a traditional hole.

Effects of a Westphalian channel on coal-seam geometry: a reappraisal of the ‘Dumb Fault’ of north Derbyshire

SUMMARY The ‘Dumb Fault’ is a belt of Westphalian A (Langsettian) disturbed strata, that has been re-examined in an opencast mine in Derbyshire. The Deep Hard Seam is absent beneath the ‘Dumb Fault’, and anomalously thick coals occur on either side of the belt. A swilley, consisting of an elongate depression infilled in its lower part with impure coal, is present in the overlying Deep Soft Seam, and anticlinal folds occur in underlying seams. The ‘Dumb Fault’ is not a fault, but a minor channel system, that was subject to episodes of bank collapse which removed and displaced peat masses, forming the anomalously thick coal. Abandonment of the channel left an unplugged reach, which was filled with plant debris forming the swilley. Burial compacted the thickened displaced peat masses, with the anticlines in the underlying seams developing to accommodate the compaction. The potential adverse effects on mining of the features described include loss of coal tonnage, problems of excavation, highwall failure, difficulties in digging clean coal and changes of seam gradient.

Unlined Tunnels of the Southern California Edison Company

The design, construction, and operating experience with eleven unlined power tunnels sited in granitic rock in the. Sierra Nevada Range in Central California, are summarized. The tunnels vary in age from less than 5 yr to more than 50 yr and vary in bore from 7ft to 24 ft. The advantages and disadvantages of unlined tunnels are compared with those for unlined tunnels, and the need for careful preproject geologic studies is emphasized. The results of tunnel friction loss tests are presented, and the effect of invert treatment on hydraulic and structural performance is reviewed. Case histories of leakage and rockfalls as well as the remedial work done to correct the problems are described. The need is emphasized for careful geologic examination of a tunnel interior and care in gunite treatment of soft seams and joints. It is concluded that under favorable geologic conditions, an unlined tunnel is a very economical type of tunnel to construct and maintain.