The kinetics of fluid-solid coupling during immersion is an important topic of investigation in rock engineering. Two rock types, sandstone and mudstone, are selected in this work to study the correlation between the softening characteristics of the rocks and moisture content. This is achieved through detailed studies using scanning electron microscopy, shear tests, and evaluation of rock index properties during exposure to different moisture contents. An underground roadway excavation is simulated by dynamic finite element modeling to analyze the effect of moisture content on the stability of the roadway. The results show that moisture content has a significant effect on shear properties reduction of both sandstone and mudstone, which must thus be considered in mining or excavation processes. Specifically, it is found that the number, area, and diameter of micropores, as well as surface porosity, increase with increasing moisture content. Additionally, stress concentration is negatively correlated with moisture content, while the influenced area and vertical displacement are positively correlated with moisture content. These findings may provide useful input for the design of underground roadways.
The physical and mechanical properties of rocks, especially rock strength measurement and classification, are fundamental to the design and engineering of rock structures such as underground roadways and tunnels. This is particularly true when such structures are built in mudstone and sandstone, which are two most widely distributed rock types encountered in underground mines. Such rock types are also frequently encountered in underground coal mining operations, which are increasing in depth and complexity. As such, fluid–solid interaction becomes important, as it influences these physical and mechanical properties, as well as the microstructure, of rock.
Approximately 90% of the rock slope failure is caused by groundwater flow in porous and fractured rock; 60% of the hazards in the coal mine are associated with groundwater and 30%–40% of the hydropower dam failure is due to the seepage of water [
The mechanical properties of rocks with different moisture contents have been widely studied. Van Eeckhout and Peng [
Okubo et al. [
Valès et al. [
Additionally, the analysis of microstructures has long been an effective method to study soil and rock properties. Gianelli et al. [
Chai et al. [
In most water-rock interaction studies, only mechanical property changes are considered and microstructural changes are typically ignored. Limited studies have been conducted on these changes on both macro- and microscopic scales. Therefore, this paper presents a series of experiments that investigate the process and characteristics of mudstone and sandstone during saturation. Not only is the relationship between mechanical properties and moisture content discussed, but we also analyze the microstructure changes under different moisture content. From this, detailed information about the influence of moisture content is obtained with regard to its influence on the softening characteristics of mudstone and sandstone.
The main study area is the 8214 working face of the Tongting Colliery, located in Huaibei City, Anhui, China (Figure
Location map of Tongting coal mine.
There is water that has remained in the goaf area of the 8212 working face, which is considered likely to enter the working face of 8214 and cause instability of both the roof and floor. There is also the possibility of hazardous water inrush. Therefore, the study of rock softening characteristics caused by different moisture contents and saturation is of great importance to the safety and operability of the Tongting Colliery.
The samples used in the experiments are typical undisturbed samples of mudstone and sandstone taken from roof and floor of the 8214 working face. To minimize moisture content changes during transportation, the undisturbed samples were first sealed in plastic bags and then wrapped in gunny bags immediately after being obtained. They were then transported to the laboratory and processed according to the test standards [
XRD is an analytical technique in which a prepared sample is bombarded with an X-ray beam at varying angles to determine its mineralogy [
Diffractograms of the sandstone and mudstone are shown in Figure
Minerals composition of mudstone and sandstone (%).
Rock type | Quartz | Calcium silicate | Kaolinite | Feldspar | Chlorite | Illite | Others |
---|---|---|---|---|---|---|---|
Mudstone | 27.70 | — | 56.10 | — | — | 11.20 | 5.00 |
Sandstone | 70.20 | 27.10 | — | — | — | — | 2.70 |
XRD analytical patterns of samples: (a) sandstone; (b) mudstone.
Specimens for rock mechanical tests were prepared in the laboratory using a core drilling machine; the core specimens were machined according to standards of the International Society for Rock Mechanics [
Shear tests were carried out in accordance with methods suggested by the ISRM [
Shear test samples: (a) sandstone; (b) mudstone.
Scanning electron microscopy is a well-established method for the characterization of surfaces in ultrahigh vacuum (UHV), high vacuum (HV), and low vacuum (LV) conditions in many different fields of research [
As described above, mudstone and sandstone samples were subjected to immersion testing. The different immersion times for the sandstone samples were 1, 2, 3, 4, 5, 6, and 8 days. Mudstone samples were saturated in water for: 1, 2, 3, 4, and 6 days.
The average moisture content of the mudstone and sandstone samples was then calculated for each immersion time and curves of moisture content and immersion time were generated (Figure
Curve of moisture content and saturation time.
It was found that, at the beginning of immersion, the moisture content of both mudstone and sandstone increased dramatically. The moisture content of mudstone increased to 4.59% after 3 days’ saturation, and that of the sandstone increased to 1.91% after 3 days. The rate of increase in moisture content becomes reduced with increasing immersion time, and the moisture content of mudstone and sandstone increased to only 4.97% and 2.27%, respectively, after 6 days’ and 8 days’ saturation.
The mechanical properties investigated in this paper include shear strength, cohesion, and internal friction angle. Changes in these mechanical properties at different moisture contents are shown in Table
The mechanical properties of rock samples under different moisture content.
Rock type | Moisture content | Shear strength/MPa | Cohesion/MPa | Internal friction angle/° |
---|---|---|---|---|
Sandstone | 1.27% | 64.95 | 21.68 | 31.93 |
1.85% | 45.01 | 15.54 | 29.20 | |
1.91% | 33.97 | 14.73 | 29.14 | |
2.16% | 30.14 | 13.06 | 29.16 | |
2.24% | 30.10 | 12.65 | 28.82 | |
2.29% | 30.04 | 12.64 | 28.62 | |
2.27% | 29.96 | 12.40 | 28.88 | |
| ||||
Mudstone | 1.97% | 15.44 | 5.95 | 28.18 |
3.41% | 9.38 | 4.98 | 27.20 | |
4.59% | 8.01 | 4.60 | 26.19 | |
4.84% | 7.75 | 4.36 | 26.14 | |
4.97% | 8.03 | 4.27 | 26.57 |
Regression analysis was carried out to investigate the relationship between moisture content and the mechanical properties. Curves of best-fit for the experimental data take the general form of
The expression between mechanical properties and moisture content.
Rock type | Mechanical properties | Expression | |
---|---|---|---|
Sandstone | Shear strength | | 0.93 |
Cohesion | | 0.94 | |
Internal friction angle | | 0.91 | |
| |||
Mudstone | Shear strength | | 0.94 |
Cohesion | | 0.92 | |
Internal friction angle | | 0.86 |
Curve of rock mechanical properties with moisture content: (a) compressive strength; (b) cohesion; (c) internal friction angle.
Figure
Figures
Stress–strain curves obtained during the experiments were analyzed to study the softening characteristics caused by different moisture contents. Stress–strain curves obtained at different moisture contents with a shear angle 45° are shown in Figure
Stress-strain curve: (a) mudstone; (b) sandstone.
It can be concluded from the curves that dramatic brittle failure occurs when the moisture content is low for both mudstone and sandstone. Shear stress drops suddenly after failure. With increasing moisture content, the properties of creep in the rock samples become increasingly important, as slow rupturing appears in both mudstone and sandstone with only moderate decreases in shear stress after rock failure.
The softening characteristics of rock samples can also be illustrated by the failure modes of mudstone and sandstone (Figures
Failure mode of mudstone under different moisture content: (a) 1.97%; (b) 3.41%; (c) 4.59%; (d) 4.97%.
Failure mode of sandstone under different moisture content: (a) 1.27%; (b) 1.91%; (c) 2.16%; (d) 2.29%.
Mudstone and sandstone block samples with different moisture contents were analyzed using SEM. The blocks were 1 × 1 × 0.5 cm in size, gold sprayed in a laboratory, and then fixed on the observation platform. The SEM analysis was conducted at the Advanced Analysis & Computation Center using a FEI QuantaTM250 instrument, with which 500, 2000, 4000, and 8000 multiple SEM images were captured. Only 4000 multiple SEM images were selected for analysis in this paper.
SEM images of the mudstone with different moisture contents are shown in Figure
Microstructure of mudstone with different moisture content: (a) 1.54%; (b) 1.87%; (c) 2.96%; (d) 4.31%.
Binary images of mudstone with different moisture content (white: micropores): (a) 1.54%; (b) 1.87%; (c) 2.96%; (d) 4.31%.
For the sandstone, only the initial state (moisture content of 0.91%) and final state (7-day immersion; moisture content of 2.18%) were analyzed. The SEM images and binary images are shown in Figures
Microstructure of sandstone with different moisture content: (a) 0.84%; (b) 2.10%.
Binary images of sandstone with different moisture content (white: micropores): (a) 0.84%; (b) 2.10%.
The parameters of the surface micropores, such as number, total area, and diameter, were simultaneously counted from the binary images and the surface porosity was calculated. Table
Parameter variations of micropores of sandstone and mudstone with different moisture content.
Binary images | SEM images | Number of micropores | Total area of micropores ( | Area of maximum micropore ( | Diameter of maximum micropore ( | Surface porosity (%) |
---|---|---|---|---|---|---|
Figure | Figure | 186 | 174.30 | 15.21 | 2.55 | 3.11 |
Figure | Figure | 275 | 195.33 | 11.46 | 7.43 | 3.49 |
Figure | Figure | 305 | 300.47 | 24.55 | 12.81 | 5.38 |
Figure | Figure | 394 | 579.26 | 27.50 | 10.79 | 11.4 |
Figure | Figure | 130 | 104.63 | 14.43 | 5.76 | 1.87 |
Figure | Figure | 297 | 418.31 | 25.46 | 11.15 | 7.46 |
Histograms and line charts of parameter changes of micropores of mudstone and sandstone extracted from SEM images with different moisture content: (a) surface porosity; (b) area of total micropores; (c) number of micropores; (d) area of maximum micropores; (e) perimeter of maximum micropores (1 mudstone with moisture 1.54%; 2 mudstone with moisture content 1.87%; 3 mudstone with moisture content 2.96%; 4 mudstone with moisture content 4.31%; 5 sandstone with moisture content 0.84%; 6 sandstone with moisture content 2.10%).
Overall, the number of micropores was observed to increase with increasing moisture content and, during immersion, water primarily entered the original pores and fractures. Different swelling properties of different minerals likely led to unbalanced stresses inside the rock, resulting in formation of new fractures. This not only caused an increase in the number of micropores, but also an increase in the total area and surface porosity. Also, seepage of water into the rock led to an increase in interconnectedness of the new micropores and fractures, forming larger ones, and thus increasing the area and diameter of the micropores.
The following equations describe the quantification of the various parameters required to develop the model of seepage and stress defined in further detail in Section
(1) Equilibrium equation
(2) Geometric equation
(3) Constitutive equation
(4) Seepage equation
(5) Seepage-stress relation equation
As shown in Figure
Calculation model of roadway excavation.
The physical and mechanical parameters of the rock mass are list in Table
Physical-mechanical parameters of surrounding rock (water content: 1.29%).
Type | Tensile strength/MPa | Compression strength/MPa | Cohesion/MPa | Frictional angle/° | Density/ (kg/m∧3) | Young’s modulus/GPa |
---|---|---|---|---|---|---|
Surrounding rock | 47.90 | 47.90 | 10.15 | 33.28 | 2600 | 10.17 |
Excavation alters the equilibrium state of in situ stress, resulting in the redistribution of in situ stresses around the roadway. Figure
Stress distribution of surrounding rock on different moisture content.
Stress concentration factor at the corner of roadway.
Stress concentration factors and stress nephograms at different moisture contents reflect not only the stress levels in rock mass but also the process of stress transfer and evolution. In general, both the center of the roof and floor undergo stress release after the roadway is excavated and form relief areas under and above the goaf, while the corners of the roadway concentrate stress and form pressurized areas.
When the moisture content is 0.5%, the stress concentration factor around the roadway is 3.4, which reduces to 2.54 when the moisture content is 2%. It can be concluded that pressure has been released around the roadway, but the influenced regions related to roadway excavation will extend with increasing moisture content.
Figure
Vertical displacement of surrounding rock on different moisture content.
Changes in roadway closure in the vertical direction are shown in Figure
Roadway closure on different moisture content.
This investigation was conducted to study the mechanical properties and microstructural changes in mudstone and sandstone. Rock samples were collected from Permian siltstone and sandstone of the Shihezi Formation at depths of 616.8–665.3 m in the Tongting Colliery. Fluid-solid coupling effects were studied in terms of roadway stability and compared with different moisture contents in an underground roadway excavation simulation. Based on the results of this investigation, the following conclusions can be drawn: Mudstone in the investigated areas consists of a wide range of clay minerals, with kaolinite being the main clay mineral (56.10%), while quartz and calcium silicate in the sandstone account for 70.20% and 27.10%, respectively. In all cases, the moisture contents of mudstone and sandstone increased rapidly at the beginning of immersion test. After 9 days of immersion, the moisture content of mudstone and sandstone reached 4.97% and 2.27%, respectively. All mechanical properties investigated showed a tendency to decrease with increasing moisture content. The general form As the seepage of water into the samples increased (increasing moisture content), new pores and fractures were created and the original pores and fractures linked up, resulting in an increase in the number, total area, and diameter of micropores and surface porosity. Stress will be redistributed after roadway excavation and pressurized areas will occur near the corner of the roadway. Stress concentration at the corner will decrease, indicating a release of stress with increasing moisture content. At the same time, the influenced region caused by roadway excavation will extend. Vertical displacement and the influenced region will increase with increasing moisture content. The roadway closure has a positive correlation with the moisture content and support pressure plays a vital role in stabilizing the underground roadway.
Uniaxial compressive strength
Scanning electron microscope
Energy dispersive X-ray analysis
X-ray diffraction
International Society for Rock Mechanics
Ultrahigh vacuum
High vacuum
Low vacuum
Pore water pressure
Biot constants
Kronecher constant
Permeability coefficient
Coupling coefficient
Shear modulus
Total stress
Effective stress
Total strain.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Thanks are due to National Key Research and Development Program (Project no. 2016YFC0600901); National Natural Science Foundation of China (Project no. 51574224); Natural Science Foundation of Jiangsu Province (Project no. BK20141130); Fundamental Research Funds for the Central Universities (Project no. 2014QNB27).