# Investigation of the Microstructure Characteristics and Deformation Mechanisms of the Carbonaceous Slate under Hydromechanical Coupling

Authors: Hucheng Yang; Shengrui Su; Peng Li; Jianxun Chen
Journal: Geofluids
 (2023)

---

## Abstract

Carbonaceous slate inevitably possesses microscopic pores, microcracks, cleavages, and bedding planes in complex geoenvironment during the diagenesis processes. The microstructure of carbonaceous slate changes apparently under the effects of underground water infiltration, tectonic stress, and engineering disturbance, which induces the large deformation of rock mass and influences the stability of geotechnical engineering projects. To investigate the microstructure characteristics and reveal deformation mechanisms of carbonaceous slate under the influence of water pressure and stress, the variations of the pore size distribution(PSD), connectivity of pores, and porosity of samples during water injection and triaxial compression were studied using multiple methods. The results indicated that voids include plate-like micropores and microcracks, which are discontinuous without external stress. The micropores with a size of less than 1μm dominate in number. The flaky particles were extruded and bent at a confining pressure, which caused the intermediate pores to form and altered the PSD of the samples. Hence, the connectivity dramatically improved and resulted in increased permeability. Water with dissolved clay minerals and small particles could move between micropores and microcracks during the water injection process. The elastic deformation of the particles recovered, and the intermediate pores disappeared when the imbalanced forces on two sides of the particles were narrowed. Pore water pressure affected the effective stress state and decreased the cohesion and stiffness of the rock. In the lower stress state, the porosity had a certain range of decrease (about 0.2%) mainly due to micropore compression, while the microcrack sprouted and expanded with the increase of compressive stress, resulting in the extension of porosity. The interaction of the stress and water seepage on the slate reduced the rock strength and favored the deformation, leading to a large macrodeformation of the soft rock in the long run.


---

## Body

## 1. Introduction

Carbonaceous slate is a typical soft rock with plenty of weak planes (bedding planes, microcracks, fissures, and so on). Due to its low strength and brittle nature, carbonaceous slate incurs serious difficulties and great costs for the construction of projects in western China. The large deformation and failure of the surrounding rock readily occurred under high geostress during the tunnel construction, such as the Muzhailing railway tunnel [1] and Tianpingshan tunnel [2]. Hydromechanical coupling processes widely exist in geological environments and significantly affect the stability of rock masses in rock engineering. In order to fully understand the strength and deformation mechanisms, many researchers analyzed the anisotropic deformability and strength through a series of macroscopic mechanical tests using slate specimens and numerical simulations to reveal the relationship among stress, bedding angle, porosity, and permeability [3–7]. The stress variations have a direct impact on microcracks and micropores, which were the seepage channels inside the rock mass, thereby changing the porosity and permeability of the rock mass. Meanwhile, the evolutions of porosity and permeability also have a vital influence on the seepage pressure distribution, effective stress distribution, and rock deformation [8, 9]. The mineral component, arrangements of particles, and microcracks together controlled rock mechanical behaviors, especially the microcrack played a dominant role in rock failure [10, 11]. Under the action of water and stress, the mechanical properties of soft rocks deteriorated significantly, the microstructure was damaged by the swelling and disintegration of clay minerals and water, and the connection between particles degraded due to pressure dissolution and other effects [12]. Under the effect of hydration, mineral particles of slate generated volume expansion and cementation became loose, the internal bonding strength was reduced, the rock was softened, and volume expansion was more likely to occur in the direction of vertical laminae [13]. Hydromechanical interaction invoked by excavation activities leads to the evolution of rock’s physical properties and strength and results in the development of considerable rock mass deformation, fracture displacement, and pressure fluctuation in fractured rocks [14, 15]. As we all know, the deformation of rock mass is mainly caused by the deformation of the skeleton structure, micropore, and crack, whereas the changes of micropore and crack are more sensitive to stress changes. At present, most of the studies are on the macroscopic mechanical behavior of sandstone under hydraulic coupling, and there are fewer studies on the microscopic pore characteristics of slate, especially carbonaceous slate, during the stress change.The pore structure of the rock has been studied using a variety of methodologies and techniques, such as the scanning electron microscope (SEM), computed tomography (CT) scanning, nuclear magnetic resonance (NMR), and mercury injection porosimetry (MIP) [16, 17]. The combination of various technologies to characterize the pore structure is the main research idea nowadays; the use of appropriate methods to research the rock mass’s microstructure properties can reveal the rock mass’s strength and deformation mechanism [18]. NMR provides a fast, noninvasive, and nondestructive method for characterizing pore fluid due to its sensitivity to fluid components saturated in sedimentary rocks [19, 20]. The pore characteristics, such as the pore type and pore connectivity, as well as the calculation of total porosity and pore size distribution (PSD), can be researched by analyzing the features of the relaxation time distribution. The relaxation time contains three different relaxation mechanisms: volume relaxation, surface relaxation, and diffusion relaxation [21, 22]. During the relaxation phase, a CPMG sequence [23, 24] can acquire the entire original signal for transverse relaxation, and the relaxation time distribution curve can be obtained by inverting the signal attenuation data. The data inversion theory can be found in the research of Song and Ravinath [25]. The homogenous magnetic field and the CPMG pulse sequence are used for measurement to minimize the effect of diffusion relaxation; therefore, diffusion relaxation and volume relaxation can be ignored when the echo time is short enough [26, 27]. Therefore, the relaxation time of the fluid mainly depends on the surface relaxation, which is the result of fluid protons interacting with magnetic field electrons on the pore wall; the expressions are as follows [28]:
(1)1T2=1Ts=ρ2SV,(2)SV=Fsrc.Among them,S/V is the surface to volume ratio for the pores, Fs is the pore shape factor (slit shape is 1, column shape is 2, and spherical shape is 3) [28], rc is the pore throat radius of the rock, and ρ2 is the surface relaxivity. The surface relaxivity is an inherent parameter of rock properties that makes low-field NMR a useful tool for obtaining pore size distribution of geological materials [19, 22]; its value is related to physical characteristics such as the surface, the pore types, the mineral component, and the wettability of the rock surface [29, 30]. The distribution of relaxation times can be converted into the PSD using Equation (1) under the assumption that all pores have the same surface relaxivity [31]. The peak value and area of the curve represent the pore volume of the rock, and the curve peak’s continuity reflects the pores’ connectivity characteristics. The surface relaxivity was determined by combining the relaxation duration and the PSD which were acquired using different methods [19, 31–33]. Yao and Liu compared the specimen’s T2 spectrum of NMR with the PSD of MIP and demonstrated that specimen T2 spectrum is superior in practicability and evaluation accuracy and can be used to study the distribution of pore throat structure [34]. MIP is based on the gradual injection of liquid mercury into an evacuated pore system with external pressures to obtain the pore size distribution. This pressure can then be converted to an equivalent pore radius. The pore throat structure can be obtained using Equation (3) to transform the mercury injection pressure curve [29]:
(3)Rc=−2γcosθPc,where Pc is the absolute injection pressure (MPa), Rc is the pore radius when mercury enters at the pressures, γ is the interfacial tension, and θ is the contact angle between mercury and pore inner surface; for mercury injection, γ=0.48N/m and θ=140° [18].The macroscopical mechanical behavior and deformation characteristics of the slate under the hydromechanical coupling have been analytically and experimentally studied, achieving fruitful results. In addition, traditional NMR was widely used to investigate the microstructure of bulk materials, but most of them focused on the sandstone and obtained the microstructural data by implementing static scans. The research on the carbonaceous slate from a microscopic point of view is still far from satisfactory. From the microstructure point of view, a well understanding of deformation mechanisms under the interaction of the groundwater and stress is highly crucial for the large deformation control of the soft rock. Here, an advanced NMR technique combined with SEM-EDS and MIP was applied to systematically investigate the microstructure variation of carbonaceous slate under the hydraulic and stress coupling, which can realize systematical analysis of the pore characteristics under the interaction of water and stress. The NMR technique can obtain real-timeT2 spectral data and images during the processes of saturating the rock sample by water injection and applying the triaxial loading to the specimen. It could provide a theoretical basis for revealing the macroscopic deformation mechanism of the carbonaceous slate and controlling the large deformation measures in construction.The macromechanical behavior and deformation characteristics of the slate under the hydromechanical coupling have been analytically and experimentally studied, achieving fruitful results. In addition, traditional NMR was widely used to investigate the microstructure of bulk materials, while most of them focused on the sandstone and obtained the microstructural data by implementing static scans. The research on the carbonaceous slate from a microscopic point of view is still far from satisfactory. From the microstructure point of view, a well understanding of deformation mechanisms under the interaction of the groundwater and stress is highly crucial for the deformation control of the soft rock. Here, an advanced NMR technique combined with SEM-EDS and MIP was applied to systematically investigate the microstructure variation of carbonaceous slate and reveal the deformation mechanisms from the particle distortion and pore characteristics under the hydraulic and stress coupling. The NMR technique can obtain real-timeT2 spectral data and images during the processes of saturating the rock sample by water injection and applying the triaxial loading to the specimen. It could provide a theoretical basis for revealing the macroscopic deformation mechanism of the carbonaceous slate and controlling the large deformation measures in construction.


## 2. Materials and Methods

### 2.1. Geological Background

The experimental samples were taken from the Muzhailing highway tunnel located in Dingxi City, Gansu Province, China. Muzhailing; the watershed between the Zhanghe and Taohe rivers in the middle mountain area of West Qinling is a deep-cut valley landform of denuded structure medium-height mountain. These areas were affected by multiple tectogenesis, the stratum lithology changed greatly, the geological structure was complex, and the stratum was relatively fragmented. The in situ stress is more than 20 MPa; it belongs to the extremely high geostress tunnel. The tunnel groundwater is abundant and generally appears as gushing water during construction. The special lithology with large deformation is formed under the complex geological environment of the diagenetic process, as well as the influence of multiple factors such as structure, faults, high-ground stress, and groundwater. Parts of the rock mass are mainly consisting of highly weathered carbonaceous slate, which is a kind of metamorphic rock with variable argillaceous structure.Muzhailing highway tunnel is one of the significant and challenging tunnels in the Wei (Yuan) Wu (Du) highway, which is a part of the Lanzhou-Haikou national highway (G75); the location is shown in Figure1. The surrounding rock of the tunnel is a monoclinic bedding structure of the lower Permian, which is the gray-black carbonaceous slate and sandy slate interbedded. The tunnel emerged with large deformation (the cumulative amount was more than 1000 mm in water-rich areas) and poor stability in the construction, and rheological properties are apparent. The selected samples were buried at a depth of about 556 m, the thickness of the carbonaceous slate was about 30–150 mm, and the slate was dipped towards 5° with a dip angle of 78°, as shown in Figure 2.Figure 1
The location of Muzhailing highway tunnel.Figure 2
Surrounding rock and the NMR specimen.


### 2.2. Preparation of Samples

In this research, NMR, SEM-EDS, and MIP are comprehensively used to examine the pore size distribution, mineral component, morphology, and pore characteristics of the rock. The same rock mass was used to create three different types of samples. First, two cylindrical samples measuring approximately 25 mm in diameter and 75 mm in length were drilled paralleling to the bedding planes for an NMR test. Subsequently, four samples with the10×10×10mm cubic rock blocks with significant layers and one sample with 10×10×6mm were sampled from the rest small sections for the SEM-EDS test and MIP, respectively. Before the SEM-EDS experiment, the samples were broken along the layers and formed a flat fresh surface.


### 2.3. The Test Presentation

In this study, an advanced nuclear magnetic resonance (NMR) technique was applied to investigate the variations of microscopic pore characteristics of carbonaceous slate during water-injected saturation and axial compression under certain surrounding pressure. To explain the deformation mechanism scientifically and rationally, the supplementary tests, SEM-EDS and MIP, were utilized to characterize the mineral composition, surface morphology, and pore size distribution of carbonaceous slate.SEM-EDS is widely used methodology for examining the surface morphological characteristics and chemical makeup of materials. SEM can observe microcracks with extremely high resolution, and acquired images can visually reflect the pore morphology, pore size, and connectivity of microsized pores, which could qualitatively characterize the microstructure. The SEM-EDS experiment was performed using a Quanta 200 scanning electron microscope/energy-dispersive X-ray spectroscopy from FEI, USA. Four samples were scanned and imaged with this instrument, 4-6 tiny areas were selected for each sample, and 6-8 images were acquired for each area at different magnifications. In order to identify the pore size distribution of the soft rock, the MIP experiment was conducted using a MicroActive AutoPore V 9600 (Micromeritics, USA) on a random sample. The intrusion and extrusion curves of the MIP were measured, and pore size distributions were obtained. Both conventional tests are often utilized to character the bulk material, and the specific test procedures will not be described in detail.Macro MR12-150H-I low-field real-time NMR test system produced by Suzhou Niumai Analytical Instrument Co., Ltd. was used, as shown in Figure3. The systems are mainly composed of an NMR imaging analyzer, a magnet, a core holder, a dual-cylinder constant pressure and current pump, and a confining pressure pump. The NMR radio frequency pulse frequency was in the range of 1-30 MHz, the maximum sampling bandwidth was 2000 kHz, and the magnetic induction intensity of the magnet was 0.52 T. The main part of the core holder was a polyether ether ketone (PEEK) resin material without nuclear magnetic signals, which tightly wrapped around the sample to achieve various axial and confining pressure by injecting different amounts of fluorocarbon oil outside. The maximum confining pressure can reach 40 MPa. The seepage is provided by a double-cylinder constant pressure and constant current pump, the water pressure range from 0 to 40 MPa, and the double pumps flow continuously with a flow rate of 0-50 mL/min and ±0.5% (1 mL/min).Figure 3
Schematic view of the experimental setup.Two samples were selected for the NMR test. According to the depth of the tunnel, the distribution of fissure water, and the geostress of surrounding rock, the water displacement pressure was determined to be 0.5 MPa, and the maximum compressive stress was 20 MPa. The confining pressure was also set at 0.5 MPa to keep the oil and water from mixing. The specific test procedures are formulated as follows:(1)
Measure the weight and size of the core in the natural state, observe the microcracks on the surface with a magnifying glass, and take photos to record it(2)
Wrap the whole specimen and part of the holder with the PEEK heat shrinkable tube, which is a nonnuclear magnetic signal. Then, use the hot air gun to heat the tube to fix it accurately and ensure complete sealing. Finally, put it into the corresponding probe box(3)
To prevent leaks during the experiment, carefully connect the pipes of the probe box to the double-cylinder constant pressure and constant current displacement pump. To ensure the stability of pressurization, inject fluorocarbon oil into the pipeline, pressure chamber, and pump body cavity. Then, run the pump to remove air from the pipeline. At last, connect the port of the probe box to the signal acquisition system(4)
To confirm that the nuclear magnetic signal and the acquisition system are in a good state, check the associated magnet probe and precisely set the center frequency, pulse width, and other pertinent parameters(5)
To prevent water from penetrating the cavity and mixing with fluorocarbon oil, it is necessary to set confining pressure for the core and ensure that the internal pressure of the tube is not greater than the outside during the injection process. After gathering the sample’s NMR signal in its natural state, the confining pressure pump is used to apply 0.5 MPa of confining pressure. Water is injected at a constant pressure of 0.5 MPa, and the NMR signals are gathered at various intervals.T2 curves are compared until the signal volume of the two adjacent times is almost unchanged, and a constant flow of water is discharged from the outlet; the core is considered saturated(6)
Axial pressure of 2 MPa is applied based on the saturated core with a confining pressure of 0.5 MPa, the signal is collected after 10 minutes of pressure for each level, and theT2 spectrum data is examined. To ensure the experiment’s safety, the maximum experimental axial pressure is set at 20 MPa, roughly half the strength of the triaxial compression test(7)
The sample is disassembled, the weight and geometric parameters of the sample are recorded, and then, the test is finalized


## 2.1. Geological Background

The experimental samples were taken from the Muzhailing highway tunnel located in Dingxi City, Gansu Province, China. Muzhailing; the watershed between the Zhanghe and Taohe rivers in the middle mountain area of West Qinling is a deep-cut valley landform of denuded structure medium-height mountain. These areas were affected by multiple tectogenesis, the stratum lithology changed greatly, the geological structure was complex, and the stratum was relatively fragmented. The in situ stress is more than 20 MPa; it belongs to the extremely high geostress tunnel. The tunnel groundwater is abundant and generally appears as gushing water during construction. The special lithology with large deformation is formed under the complex geological environment of the diagenetic process, as well as the influence of multiple factors such as structure, faults, high-ground stress, and groundwater. Parts of the rock mass are mainly consisting of highly weathered carbonaceous slate, which is a kind of metamorphic rock with variable argillaceous structure.Muzhailing highway tunnel is one of the significant and challenging tunnels in the Wei (Yuan) Wu (Du) highway, which is a part of the Lanzhou-Haikou national highway (G75); the location is shown in Figure1. The surrounding rock of the tunnel is a monoclinic bedding structure of the lower Permian, which is the gray-black carbonaceous slate and sandy slate interbedded. The tunnel emerged with large deformation (the cumulative amount was more than 1000 mm in water-rich areas) and poor stability in the construction, and rheological properties are apparent. The selected samples were buried at a depth of about 556 m, the thickness of the carbonaceous slate was about 30–150 mm, and the slate was dipped towards 5° with a dip angle of 78°, as shown in Figure 2.Figure 1
The location of Muzhailing highway tunnel.Figure 2
Surrounding rock and the NMR specimen.


## 2.2. Preparation of Samples

In this research, NMR, SEM-EDS, and MIP are comprehensively used to examine the pore size distribution, mineral component, morphology, and pore characteristics of the rock. The same rock mass was used to create three different types of samples. First, two cylindrical samples measuring approximately 25 mm in diameter and 75 mm in length were drilled paralleling to the bedding planes for an NMR test. Subsequently, four samples with the10×10×10mm cubic rock blocks with significant layers and one sample with 10×10×6mm were sampled from the rest small sections for the SEM-EDS test and MIP, respectively. Before the SEM-EDS experiment, the samples were broken along the layers and formed a flat fresh surface.


## 2.3. The Test Presentation

In this study, an advanced nuclear magnetic resonance (NMR) technique was applied to investigate the variations of microscopic pore characteristics of carbonaceous slate during water-injected saturation and axial compression under certain surrounding pressure. To explain the deformation mechanism scientifically and rationally, the supplementary tests, SEM-EDS and MIP, were utilized to characterize the mineral composition, surface morphology, and pore size distribution of carbonaceous slate.SEM-EDS is widely used methodology for examining the surface morphological characteristics and chemical makeup of materials. SEM can observe microcracks with extremely high resolution, and acquired images can visually reflect the pore morphology, pore size, and connectivity of microsized pores, which could qualitatively characterize the microstructure. The SEM-EDS experiment was performed using a Quanta 200 scanning electron microscope/energy-dispersive X-ray spectroscopy from FEI, USA. Four samples were scanned and imaged with this instrument, 4-6 tiny areas were selected for each sample, and 6-8 images were acquired for each area at different magnifications. In order to identify the pore size distribution of the soft rock, the MIP experiment was conducted using a MicroActive AutoPore V 9600 (Micromeritics, USA) on a random sample. The intrusion and extrusion curves of the MIP were measured, and pore size distributions were obtained. Both conventional tests are often utilized to character the bulk material, and the specific test procedures will not be described in detail.Macro MR12-150H-I low-field real-time NMR test system produced by Suzhou Niumai Analytical Instrument Co., Ltd. was used, as shown in Figure3. The systems are mainly composed of an NMR imaging analyzer, a magnet, a core holder, a dual-cylinder constant pressure and current pump, and a confining pressure pump. The NMR radio frequency pulse frequency was in the range of 1-30 MHz, the maximum sampling bandwidth was 2000 kHz, and the magnetic induction intensity of the magnet was 0.52 T. The main part of the core holder was a polyether ether ketone (PEEK) resin material without nuclear magnetic signals, which tightly wrapped around the sample to achieve various axial and confining pressure by injecting different amounts of fluorocarbon oil outside. The maximum confining pressure can reach 40 MPa. The seepage is provided by a double-cylinder constant pressure and constant current pump, the water pressure range from 0 to 40 MPa, and the double pumps flow continuously with a flow rate of 0-50 mL/min and ±0.5% (1 mL/min).Figure 3
Schematic view of the experimental setup.Two samples were selected for the NMR test. According to the depth of the tunnel, the distribution of fissure water, and the geostress of surrounding rock, the water displacement pressure was determined to be 0.5 MPa, and the maximum compressive stress was 20 MPa. The confining pressure was also set at 0.5 MPa to keep the oil and water from mixing. The specific test procedures are formulated as follows:(1)
Measure the weight and size of the core in the natural state, observe the microcracks on the surface with a magnifying glass, and take photos to record it(2)
Wrap the whole specimen and part of the holder with the PEEK heat shrinkable tube, which is a nonnuclear magnetic signal. Then, use the hot air gun to heat the tube to fix it accurately and ensure complete sealing. Finally, put it into the corresponding probe box(3)
To prevent leaks during the experiment, carefully connect the pipes of the probe box to the double-cylinder constant pressure and constant current displacement pump. To ensure the stability of pressurization, inject fluorocarbon oil into the pipeline, pressure chamber, and pump body cavity. Then, run the pump to remove air from the pipeline. At last, connect the port of the probe box to the signal acquisition system(4)
To confirm that the nuclear magnetic signal and the acquisition system are in a good state, check the associated magnet probe and precisely set the center frequency, pulse width, and other pertinent parameters(5)
To prevent water from penetrating the cavity and mixing with fluorocarbon oil, it is necessary to set confining pressure for the core and ensure that the internal pressure of the tube is not greater than the outside during the injection process. After gathering the sample’s NMR signal in its natural state, the confining pressure pump is used to apply 0.5 MPa of confining pressure. Water is injected at a constant pressure of 0.5 MPa, and the NMR signals are gathered at various intervals.T2 curves are compared until the signal volume of the two adjacent times is almost unchanged, and a constant flow of water is discharged from the outlet; the core is considered saturated(6)
Axial pressure of 2 MPa is applied based on the saturated core with a confining pressure of 0.5 MPa, the signal is collected after 10 minutes of pressure for each level, and theT2 spectrum data is examined. To ensure the experiment’s safety, the maximum experimental axial pressure is set at 20 MPa, roughly half the strength of the triaxial compression test(7)
The sample is disassembled, the weight and geometric parameters of the sample are recorded, and then, the test is finalized


## 3. Results and Discussion

### 3.1. Surface Morphology and Composition of the Slate

The surface morphology and structural composition of carbonaceous slate were observed through SEM at the magnification of 1600–5000 times. At lower magnification (see Figures4(a) and 4(d)), small amounts of granular quartz and flaky mica are visible in the section, which are surrounded by a large number of carbonaceous and clay minerals that are filled in and cemented into groups of flaky particles of varying sizes. The mica-enriched areas form laminated surfaces with a silky luster. Multiple particles are aligned in a directional stack and create clusters. The thickness of the cluster is considerably smaller than the other dimensions. As seen in Figures 4(b) and 4(c), the microcrack that forms when the particles are snapped by tectonic stress can be seen clearly. The intergranular micropores are narrow slit-type pores that are too small to be seen clearly. It inferred that they were created by the shrinkage of clay minerals due to water loss in the process of diagenesis. Analyzing the energy spectrum curves from the surface energy-dispersive X-ray spectrum in the experiment indicated that the carbonaceous slate elements mainly contained carbon, oxygen, silicon, calcium, aluminum, and other elements (in Figure 4(e)). There was no hydrogen element and almost no iron element. The element distribution of samples was uneven, and the amount was various.Figure 4
SEM-EDS images in samples. (a) The grains and particles. (c) Micropores in the fracture section. (b) The microcracks in the surface. (d) The particles and clusters. (e) The energy spectrum of elements and its location.
(a)(b)(c)(d)(e)The results revealed that the carbonaceous slate was the finest-grained foliated metamorphic rock and presented a dense foliated texture leading to low permeability. Its slaty cleavages were orientated in planes perpendicular to the direction of metamorphic compression. The presence of well-developed slaty structure cleavage, lamination, or bedding planes in carbonaceous slate makes its physical, mechanical, and hydraulic properties highly anisotropic or transversely isotropic [35].


### 3.2. The Interaction of the Hydraulic Action and Stress on the Slate

NMR technology detects the hydrogen-containing fluid in the internal pores of the slate to analyze the structural pore size distribution, porosity, and so on. NMR transverse relaxation time (T2) distribution is consistent with the pore size distribution, and the T2 peak position on the relaxation time axis is proportional to the pore size. The test data was accurate and dependable since the EDS results revealed that carbonaceous slate contained no hydrogen and little iron, the hydrogen signal found in the T2 spectrum was entirely derived from water, and the iron had little impact on the magnetization throughout the test.The water injection test was a process in which the natural sample reached the saturation state under the constant injection pressure of 0.5 MPa; the infiltration amount and distribution position in the core during the water injection process were characterized by collecting the various NMR signals. The fluid and pore volumes were equal when the rock was saturated, and the signals from two adjacent times were nearly constant. Then, from 0 to 20 MPa, axial forces every 2 MPa were applied while being contained by pressures of 0.5 MPa. The microstructural characteristics of the core, such as the pore size distribution, porosity, and other parameters in those processes, were measured by collecting theT2 curves of the water by NMR technology. The relationship of the layered planes with the stress and water pressure in the 3D coordinate is shown in Figure 5, the schistosity planes were orthogonal to the x-axis, and the z-axis was parallel to the height of the core. The injected water flowed along the cleavage between the bedding planes from the top to the bottom of the core.Figure 5
The core with the pressures in orthogonal coordinates.
(a)(b)(c)As shown in Figure6(a), the pores are represented as micropores, intermediate pores, and microcracks, respectively, from left to right, and the corresponding peaks are marked as P1, P2, and P3. The corresponding data of the natural core without confining pressure was defined as the initial state and injection time defined as “Ini,” represented with the discontinuous double peaks. It was revealed that the water was trapped mainly in the micropores, a small amount in the microcracks, and the connectivity was weak. When the confining pressure (σ2=σ3=0.5MPa) was applied, the deformation was different due to the directionality of the layered structure. The bedding planes were compressed under stress in the x-axis direction, and the water was extruded in the interparticle micropores; thus, P1 declined when the injection time was equal to 0. The stress in the y-axis direction caused the thin layers to easily bend and deform due to the interparticle platy fissures generated by the shrinkage of clay minerals. The strength was significantly lower in the direction of schistosity than in any other direction. Along the schistosity planes, the rock’s tensile strength and shear strength were at their weakest [36]. The intermediate pores and the microcracks started to appear or expand parallel to the weaker layers, and their connectivity improved, as showed by the continuous trimodal with distinct peaks. As the injected pressure (σw=0.5MPa) was applied, the water tended to flow into the microcracks and intermediate pores quickly and filled them. With the extension of the water injection time, the water infiltrated into the micropores gradually until the specimen was saturated. Hence, P1 increased, and the T2 curves of 565 min and 679 min were nearly overlapped, indicating that the specimen had reached saturation. In these processes, the internal water pressure in the narrow space increased, and the water shared the platy skeleton stresses under the injected pressure of 0.5 MPa. Because the injected and confining pressures were equal, the elastically deformed skeletons recovered, and the mesopores and some microcracks disappeared in the saturated state; the overlapping cures clarified it with the discontinuous double peaks. It can be seen that when the lamina surface of the carbonaceous slate rock sample was parallel to the axis, the low surrounding pressure caused the lamina particles to bend elastically, which made its connectivity better and improved the permeability of the rock sample. The pore water pressure changed the effective stress of the rock sample, and the microparticle elastic deformation was restored.Figure 6
T2 curves of the sample. (a) Water injection process. (b) Triaxial compression process.
(a)(b)T2 curves of the saturated specimen with confining pressure of 0.5 MPa in the process of increasing axial pressure per 2 MPa within the range of 0 to 20 MPa were presented in Figure 6(b). Significant reduction of P1 is under the axial pressures of 4 MPa to 8 MPa, while P2 and P3 were almost unchanged. However, as the axial pressure increases, the peak value of P1 no longer drops, and the peak values of P2 and P3 rise while the value of T2 gradually decreases. These variation patterns showed that under the pressure of the vertical laminar surface, the platy skeletons were slightly bent under axial stress due to the presence of void water and strengthened resistance to the deformation by pore pressures, some particles broken on the weakest positions, and more microcracks formed.


### 3.3. The Changes in the Pore Size and Amount

Based on the combination of Equations (1) and (2), the change in transverse relaxation time is positively correlated with the change in pore size, and the change in T2 value corresponding to each peak can reflect the change in the pore size. Figure 7 shows the curve of the transverse relaxation time of each peak during the water injection and triaxial compression processes. The water injection was the process of continuously filling the inside of the sample with water.Figure 7
T2 crest values of each peak. (a) Water injection process. (b) Triaxial compression process.
(a)(b)The structures of the specimen changed in the confining pressure, the intermediate pores appeared, and the connection apparently developed. During the water-injected processes, interior pores were filled with water constantly. As demonstrated in Figure7(a), T2 of P1 increased from 1.02 ms to 1.36 ms, and T2 of P2 raised from 18.15 ms to 51.05 ms and then dropped to 31.20 ms, while that of P3 showed a slight decrease from 1145.4 ms to 1073.96 ms and disappeared eventually. The size of the micropores and intermediate pores magnified, while the microcracks diminished slightly. In Figure 7(b), the T2 of P1 remained nearly constant during the compression process, except that it increased from 1.226 ms to 1.362 ms when the axial pressure increased between 4 MPa and 8 MPa. The T2 values of P2 and P3 in the axial pressure range of 2–14 MPa were just offset from each other and then dropped with the increase of axial pressure. Based on those phenomena, reasonable conclusions were that the confining pressure enlarged the pore size and intermediate pores appeared, creating a good condition for the water to migrate between the micropore and microcrack. The voids of the specimen were nearly full of water after 90 minutes, and pore water pressure increased due to the injected pressure; intermediate pores increased significantly and integrated with the microcracks. Those behaviors inferred that some clay minerals in particles were dissolved into the water, moved to the microcracks with injected water, and then deposited [12]. Those changes in the size of the micropores may be attributed to the interaction of the hydraulic action with stress and the clay particles expanding to absorb water. The intermediate pores were closed with the balance of internal injected pressure and external confining pressure, and water-moving tunnels were blocked. The lower pressure perpendicular to the bedding planes enlarged the micropores while squeezing the intermediate pores and microcracks.TheT2 peak area is the integral area of the spectrum curve with the coordinate axis, representing the number of pores. In this context, each peak area can be estimated using the T2 spectrum curve to examine how the number of pores changes under different conditions. As shown in Figure 8(a), the total area and area of P1 increased gradually while that of P3 decreased, and the variation range of P2 was small, and it disappeared after 90 minutes. It was reasonable that the total peak area gradually magnified and eventually stabilized as the amount of injected water increased during the water injection process. During the triaxial compression test, the total area of the peaks changed synchronously with that of P1, which decreased at first and dropped sharply between 4 and 6 MPa and then stabilized. The P2 converted to P3 between 2 and 14 MPa, and the sum of P2 and P3 remained at 250 a.u. Since the micropores were dominant in quantity, the part micropores were closed under the axial pressure of 4-6 MPa. At the same time, the total amount of mesopores and macropores remained unchanged, as seen in Figure 8(b). The results indicated that water quickly entered the microcracks, while the micropores were abundant and poorly connected, requiring a long time to fill. The variation trend of each peak was different. The amount of water in microcracks was reduced due to the elastic deformation of the particle skeleton recovering, and some water in the micropores was squeezed out due to the interaction of pore water pressure and axial pressure.Figure 8
T2 spectrum area of each peak. (a) Water injection process. (b) Triaxial compression process.
(a)(b)The relative proportions of each peak area in the saturation process were shown in Figure9(a). The proportions of micropores were 88.11% and 93.06% in the initial and saturation state, respectively. Under the confining pressure of 0.5 MPa, the intermediate pore formed, acting as the tunnel between the micropores and microcracks. The water in microcracks declined from 60.69% to 24.86%, and that in micropores increased from 29.66% to 69%, while the proportion of water in the intermediate pores remained at about 10% during the injection time between 0 to 60 min and disappeared at last. In the triaxial compression process, the water in micropores dropped from 92% to about 87%, but water in the intermediate pore and microcrack increased from about 8.01% to about 12%, as seen in Figure 9(b). It can be concluded that the intermediate pores emerged intermittently under the confining pressure, the water volume increased, and more water filled the pores. As the amount of water increased, the stress in pore water and particles redistributed, elastic deformation of particles recovered, and the intermediate pores were finally closed or expanded into microcracks.Figure 9
The changes of the relative peak area ratio. (a) Water injection process. (b) Axial compression process.
(a)(b)In summary, new intermediate pores were generated in the specimen under confining pressure and became a water channel between the micropores and microcracks. Besides, some clay minerals in the particles were dissolved by water and transferred between the micropores and microcracks. During the saturated process, the water also shared the particle stress; the internal pore water pressure and the external confining pressure were balanced at saturation. Moreover, the flake particles underwent a recoverable elastic deformation, and the sample’s PSD recovered similarly to the initial state. In the triaxial compression, the number of micropores reduced in a low-stress state, and some intermediate pores formed and merged into the macropores as the axial pressure increased.


### 3.4. The Porosity and Distribution of the Pore Size

In the test, four standard porosity samples of 1%-35 ml, 5%-35 ml, 10%-35 ml, and 20%-35 ml were selected to calibrate theT2 spectrum data of the specimen, and the relationship between signal amplitude per unit volume and porosity was obtained. The fitting curve expression was Y=117.78X−51.716, where Y is T2 spectrum amplitude and X is porosity. The water absorption per volume during saturation and the porosity of the specimen is shown in Figure 10.Figure 10
The changes of the water content and porosity. (a) Water content during water injection. (b) Porosity during the compression process.
(a)(b)The pores were continuously filled with water before reaching the saturation stage, and the change in measured signal quantity represented the variation of water quantity in the pore. As seen in Figure10(a), the volumetric water content of the sample increased from 0.86% to 1.13% during the first 93 minutes of water injection and remained stable. After that, the water content remained constant, at a maximum of 1.13%. The pores were full of water, the volume of water equaled the volume of pores, and the porosity converted into the saturated rock specimen. In Figure 10(b), the porosity of the rock sample showed a linear downward trend when axial pressure was less than 4 MPa. From 4 MPa to 6 MPa, a sharp decrease from 1.09% to 0.94% was noticed, indicating that microporous particles were crushed and compacted. The porosity remained unchanged at 0.93% between 8 and 20 MPa. It can be seen that the confining pressure changed the PSD, the connectivity of the pore, and the permeability, and the specimen gradually reached saturation under a certain injected pressure. The porosity of the saturated rock sample almost ceased to change after a sudden decrease at a lower stress state and no longer increased with the increasing stress, indicating that only some compression occurred under the action of axial pressure.


### 3.5. The Variation Characteristics of PSD in the Specimen

As shown in Equation (1), the T2 value of the NMR is directly proportional to the ratio of surface area to volume of pores, and the surface relaxation coefficient determines PSD. However, research on the surface relaxivity of carbonaceous slate is limited. The NMR and MIP were usually applied to determine the PSD of samples and get the surface relaxivity [37, 38]. The sample morphology from SEM observation indicated that the slit pore types dominate the samples (Figure 4), and the pore shape factor in Equation (2) was equal to 1. When the surface relaxation coefficient was 175, the pore size distribution (PSD) curves from the MIP and NMR experiments complemented perfectly in the overlapped pore size range. The PSD of carbonaceous slate determined by MIP and NMR confirmed that the slate had an extensive range of pore sizes from about 20 nm to more than 200 μm and that the porosity of the slate was mainly dependent on the pore volume of micropores less than 1 μm. The microcracks, with a pore size of over 10 μm, were usually less abundant. The rock core’s pore structure consisted of discontinuous micropores and microcracks without intermediate pores ranging in size from less than 1 μm to approximately 10 μm, as shown in Figure 11.Figure 11
The PSD from the NMR and MIP.The signal amplitude in theT2 spectrum of the sample was converted into the porosity component, and the relaxation time was converted into the pore size in the saturation and triaxial compression process. The quantitative relationship between pore size and pore throat corresponding water content and porosity is demonstrated in Figure 12.Figure 12
The PSD variation of sample. (a) Water amount during water injection. (b) Porosity during the compression process.
(a)(b)Figure12(a) shows the distribution of the volumetric water content of the rock sample containing pores of various sizes during the water injection process. The pore structure varied greatly when the confining pressure was applied, the intermediate pores appeared, and the connectivity improved significantly. The maximum diameter of the micropores increased from 0.5 μm to 1 μm, while the maximum diameter of the microcracks increased from 300 μm to 600 μm. Under the interaction of the pore water pressure and confining pressure, the voids’ size expanded and then shrank. It was worth noting that the diameter of the microcracks was nearly unchanged in initial and saturated states, while that of micropores enlarged slightly. The peak value of micropore water content decreased from 0.041% to 0.018% and then increased as injection time progressed. On the contrary, the peak value of microcrack water content increased from 0.008% to 0.04% and then descended to 0.01%. With the axial pressure increasing perpendicular to the bedding plane, the pore structure of the rock sample was consistent with that in the saturated state, lacking the intermediate pore. As shown in Figure 12(b), when the axial pressure increased from 4 MPa to 6 MPa, the micropores’ minimum diameter magnified while the microcrack’s maximum diameter diminished, and the porosity peak decreased significantly from 0.08% to 0.065% and then tended to remain stable. It was revealed that the particles around the microcrack experienced elastic deformation, and some clay minerals dissolved into water and were transferred. The above analysis showed that the lower confining pressure changed the pore size distribution of the specimen and improved the connectivity, which improved the permeability of the dense slate. With the gradual increase of axial pressure, the micropores compressed, and the porosity sharply decreased in the early stage and then maintained at a smaller value. While the size of microcracks diminished in the later stage, the porosity showed an increasing trend, indicating that microfracture began to sprout gradually.The pore throat distribution diagrams were obtained according to the porosity component and pore diameter graphs, as shown in Figures13(a) and 13(b). It was shown that the pore size of the micropore was less than 0.63 μm, and the microcrack was about 10 μm. During the water injection process, the volumetric water content component of the micropore varied the most, from about 0.05% to 0.26%, and that of the microcrack changed under 0.05%. There was a small amount of water that emerged in the intermediate pores of pores under the confining pressure of 0.5 MPa. As the saturation increased, the inner stress borne by the pore particles and the pore water changed, and the intermediate pores were finally closed. When the axial pressure was less than 6 MPa, the porosity component of the microstructure changed considerably. While it was almost constant between 8 MPa and 20 MPa. It inferred that the particles were crushed and the porosity was minimum, and the porosity component of the macropores increased slightly for larger axial pressure, which probably inducing the fracture expansion.Figure 13
The pore throat distribution of the sample. (a) Water injection process. (b) Triaxial compression process.
(a)(b)


### 3.6. The Deformation Mechanism of the Slate

Carbonaceous slate is a low-grade metamorphic rock that forms a dense structure with low permeability under oriented high-pressure stress levels, leading to slaty cleavage. The deformation of the slate is controlled by the layered structure, which contains numerous micropores and intersecting microcracks or fractures. The deformation mechanisms of the carbonaceous slate in the coupling of the hydromechanical actions can be explained as follows: under a certain stress condition, mesopores were formed, and the pore connectivity was significantly improved due to particle bending. Thus, the number and diameter of the interparticle pores and microcracks in the samples both increased. The good connectivity and the deformation of the throat will substantially affect the porosity and permeability [39]. As interparticle voids expanded in the slate, the area of the rock in contact with water increased, and more mineral particles enlarged after absorbing water. The cementation between the mineral particles eventually disintegrated, accelerating the hydraulic process and washing away the microparticles presented inside the cracks. Therefore, the preferred pathways were formed for groundwater; the slate became increasingly loose and fractured. In addition, the change of external stress disturbed the water flow and altered the pore water pressure, which affected the effective stress of the rock, the porosity, the permeability, and the slate’s PSD [2, 40]. High pore pressure reduced the effective stress that favors short-term stability but can lead to longer-term deformations and damage as the pore pressure dissipates; it can explain the large deformation of tunnel surrounding rock in the seepage section. The softening effect of groundwater on the slate decreased the internal cohesion and stiffness and reduced the strength with the immersion time increasing, which favored the carbonaceous slate’s deformation [13, 41]. Finally, the swelling due to some clay materials such as kaolinite was another factor affecting the large deformation. To summarize, groundwater and stress redistribution remarkably influence slate deformation; the rock masses easily slide along structural planes or detach, flex, and break. It can be revealed that the rock mass deformation increased rapidly with the appearance of groundwater and stress disturbance during the tunnel construction, such as in the Zhegu mountain tunnel [42] and the Minxian tunnel [43].


## 3.1. Surface Morphology and Composition of the Slate

The surface morphology and structural composition of carbonaceous slate were observed through SEM at the magnification of 1600–5000 times. At lower magnification (see Figures4(a) and 4(d)), small amounts of granular quartz and flaky mica are visible in the section, which are surrounded by a large number of carbonaceous and clay minerals that are filled in and cemented into groups of flaky particles of varying sizes. The mica-enriched areas form laminated surfaces with a silky luster. Multiple particles are aligned in a directional stack and create clusters. The thickness of the cluster is considerably smaller than the other dimensions. As seen in Figures 4(b) and 4(c), the microcrack that forms when the particles are snapped by tectonic stress can be seen clearly. The intergranular micropores are narrow slit-type pores that are too small to be seen clearly. It inferred that they were created by the shrinkage of clay minerals due to water loss in the process of diagenesis. Analyzing the energy spectrum curves from the surface energy-dispersive X-ray spectrum in the experiment indicated that the carbonaceous slate elements mainly contained carbon, oxygen, silicon, calcium, aluminum, and other elements (in Figure 4(e)). There was no hydrogen element and almost no iron element. The element distribution of samples was uneven, and the amount was various.Figure 4
SEM-EDS images in samples. (a) The grains and particles. (c) Micropores in the fracture section. (b) The microcracks in the surface. (d) The particles and clusters. (e) The energy spectrum of elements and its location.
(a)(b)(c)(d)(e)The results revealed that the carbonaceous slate was the finest-grained foliated metamorphic rock and presented a dense foliated texture leading to low permeability. Its slaty cleavages were orientated in planes perpendicular to the direction of metamorphic compression. The presence of well-developed slaty structure cleavage, lamination, or bedding planes in carbonaceous slate makes its physical, mechanical, and hydraulic properties highly anisotropic or transversely isotropic [35].


## 3.2. The Interaction of the Hydraulic Action and Stress on the Slate

NMR technology detects the hydrogen-containing fluid in the internal pores of the slate to analyze the structural pore size distribution, porosity, and so on. NMR transverse relaxation time (T2) distribution is consistent with the pore size distribution, and the T2 peak position on the relaxation time axis is proportional to the pore size. The test data was accurate and dependable since the EDS results revealed that carbonaceous slate contained no hydrogen and little iron, the hydrogen signal found in the T2 spectrum was entirely derived from water, and the iron had little impact on the magnetization throughout the test.The water injection test was a process in which the natural sample reached the saturation state under the constant injection pressure of 0.5 MPa; the infiltration amount and distribution position in the core during the water injection process were characterized by collecting the various NMR signals. The fluid and pore volumes were equal when the rock was saturated, and the signals from two adjacent times were nearly constant. Then, from 0 to 20 MPa, axial forces every 2 MPa were applied while being contained by pressures of 0.5 MPa. The microstructural characteristics of the core, such as the pore size distribution, porosity, and other parameters in those processes, were measured by collecting theT2 curves of the water by NMR technology. The relationship of the layered planes with the stress and water pressure in the 3D coordinate is shown in Figure 5, the schistosity planes were orthogonal to the x-axis, and the z-axis was parallel to the height of the core. The injected water flowed along the cleavage between the bedding planes from the top to the bottom of the core.Figure 5
The core with the pressures in orthogonal coordinates.
(a)(b)(c)As shown in Figure6(a), the pores are represented as micropores, intermediate pores, and microcracks, respectively, from left to right, and the corresponding peaks are marked as P1, P2, and P3. The corresponding data of the natural core without confining pressure was defined as the initial state and injection time defined as “Ini,” represented with the discontinuous double peaks. It was revealed that the water was trapped mainly in the micropores, a small amount in the microcracks, and the connectivity was weak. When the confining pressure (σ2=σ3=0.5MPa) was applied, the deformation was different due to the directionality of the layered structure. The bedding planes were compressed under stress in the x-axis direction, and the water was extruded in the interparticle micropores; thus, P1 declined when the injection time was equal to 0. The stress in the y-axis direction caused the thin layers to easily bend and deform due to the interparticle platy fissures generated by the shrinkage of clay minerals. The strength was significantly lower in the direction of schistosity than in any other direction. Along the schistosity planes, the rock’s tensile strength and shear strength were at their weakest [36]. The intermediate pores and the microcracks started to appear or expand parallel to the weaker layers, and their connectivity improved, as showed by the continuous trimodal with distinct peaks. As the injected pressure (σw=0.5MPa) was applied, the water tended to flow into the microcracks and intermediate pores quickly and filled them. With the extension of the water injection time, the water infiltrated into the micropores gradually until the specimen was saturated. Hence, P1 increased, and the T2 curves of 565 min and 679 min were nearly overlapped, indicating that the specimen had reached saturation. In these processes, the internal water pressure in the narrow space increased, and the water shared the platy skeleton stresses under the injected pressure of 0.5 MPa. Because the injected and confining pressures were equal, the elastically deformed skeletons recovered, and the mesopores and some microcracks disappeared in the saturated state; the overlapping cures clarified it with the discontinuous double peaks. It can be seen that when the lamina surface of the carbonaceous slate rock sample was parallel to the axis, the low surrounding pressure caused the lamina particles to bend elastically, which made its connectivity better and improved the permeability of the rock sample. The pore water pressure changed the effective stress of the rock sample, and the microparticle elastic deformation was restored.Figure 6
T2 curves of the sample. (a) Water injection process. (b) Triaxial compression process.
(a)(b)T2 curves of the saturated specimen with confining pressure of 0.5 MPa in the process of increasing axial pressure per 2 MPa within the range of 0 to 20 MPa were presented in Figure 6(b). Significant reduction of P1 is under the axial pressures of 4 MPa to 8 MPa, while P2 and P3 were almost unchanged. However, as the axial pressure increases, the peak value of P1 no longer drops, and the peak values of P2 and P3 rise while the value of T2 gradually decreases. These variation patterns showed that under the pressure of the vertical laminar surface, the platy skeletons were slightly bent under axial stress due to the presence of void water and strengthened resistance to the deformation by pore pressures, some particles broken on the weakest positions, and more microcracks formed.


## 3.3. The Changes in the Pore Size and Amount

Based on the combination of Equations (1) and (2), the change in transverse relaxation time is positively correlated with the change in pore size, and the change in T2 value corresponding to each peak can reflect the change in the pore size. Figure 7 shows the curve of the transverse relaxation time of each peak during the water injection and triaxial compression processes. The water injection was the process of continuously filling the inside of the sample with water.Figure 7
T2 crest values of each peak. (a) Water injection process. (b) Triaxial compression process.
(a)(b)The structures of the specimen changed in the confining pressure, the intermediate pores appeared, and the connection apparently developed. During the water-injected processes, interior pores were filled with water constantly. As demonstrated in Figure7(a), T2 of P1 increased from 1.02 ms to 1.36 ms, and T2 of P2 raised from 18.15 ms to 51.05 ms and then dropped to 31.20 ms, while that of P3 showed a slight decrease from 1145.4 ms to 1073.96 ms and disappeared eventually. The size of the micropores and intermediate pores magnified, while the microcracks diminished slightly. In Figure 7(b), the T2 of P1 remained nearly constant during the compression process, except that it increased from 1.226 ms to 1.362 ms when the axial pressure increased between 4 MPa and 8 MPa. The T2 values of P2 and P3 in the axial pressure range of 2–14 MPa were just offset from each other and then dropped with the increase of axial pressure. Based on those phenomena, reasonable conclusions were that the confining pressure enlarged the pore size and intermediate pores appeared, creating a good condition for the water to migrate between the micropore and microcrack. The voids of the specimen were nearly full of water after 90 minutes, and pore water pressure increased due to the injected pressure; intermediate pores increased significantly and integrated with the microcracks. Those behaviors inferred that some clay minerals in particles were dissolved into the water, moved to the microcracks with injected water, and then deposited [12]. Those changes in the size of the micropores may be attributed to the interaction of the hydraulic action with stress and the clay particles expanding to absorb water. The intermediate pores were closed with the balance of internal injected pressure and external confining pressure, and water-moving tunnels were blocked. The lower pressure perpendicular to the bedding planes enlarged the micropores while squeezing the intermediate pores and microcracks.TheT2 peak area is the integral area of the spectrum curve with the coordinate axis, representing the number of pores. In this context, each peak area can be estimated using the T2 spectrum curve to examine how the number of pores changes under different conditions. As shown in Figure 8(a), the total area and area of P1 increased gradually while that of P3 decreased, and the variation range of P2 was small, and it disappeared after 90 minutes. It was reasonable that the total peak area gradually magnified and eventually stabilized as the amount of injected water increased during the water injection process. During the triaxial compression test, the total area of the peaks changed synchronously with that of P1, which decreased at first and dropped sharply between 4 and 6 MPa and then stabilized. The P2 converted to P3 between 2 and 14 MPa, and the sum of P2 and P3 remained at 250 a.u. Since the micropores were dominant in quantity, the part micropores were closed under the axial pressure of 4-6 MPa. At the same time, the total amount of mesopores and macropores remained unchanged, as seen in Figure 8(b). The results indicated that water quickly entered the microcracks, while the micropores were abundant and poorly connected, requiring a long time to fill. The variation trend of each peak was different. The amount of water in microcracks was reduced due to the elastic deformation of the particle skeleton recovering, and some water in the micropores was squeezed out due to the interaction of pore water pressure and axial pressure.Figure 8
T2 spectrum area of each peak. (a) Water injection process. (b) Triaxial compression process.
(a)(b)The relative proportions of each peak area in the saturation process were shown in Figure9(a). The proportions of micropores were 88.11% and 93.06% in the initial and saturation state, respectively. Under the confining pressure of 0.5 MPa, the intermediate pore formed, acting as the tunnel between the micropores and microcracks. The water in microcracks declined from 60.69% to 24.86%, and that in micropores increased from 29.66% to 69%, while the proportion of water in the intermediate pores remained at about 10% during the injection time between 0 to 60 min and disappeared at last. In the triaxial compression process, the water in micropores dropped from 92% to about 87%, but water in the intermediate pore and microcrack increased from about 8.01% to about 12%, as seen in Figure 9(b). It can be concluded that the intermediate pores emerged intermittently under the confining pressure, the water volume increased, and more water filled the pores. As the amount of water increased, the stress in pore water and particles redistributed, elastic deformation of particles recovered, and the intermediate pores were finally closed or expanded into microcracks.Figure 9
The changes of the relative peak area ratio. (a) Water injection process. (b) Axial compression process.
(a)(b)In summary, new intermediate pores were generated in the specimen under confining pressure and became a water channel between the micropores and microcracks. Besides, some clay minerals in the particles were dissolved by water and transferred between the micropores and microcracks. During the saturated process, the water also shared the particle stress; the internal pore water pressure and the external confining pressure were balanced at saturation. Moreover, the flake particles underwent a recoverable elastic deformation, and the sample’s PSD recovered similarly to the initial state. In the triaxial compression, the number of micropores reduced in a low-stress state, and some intermediate pores formed and merged into the macropores as the axial pressure increased.


## 3.4. The Porosity and Distribution of the Pore Size

In the test, four standard porosity samples of 1%-35 ml, 5%-35 ml, 10%-35 ml, and 20%-35 ml were selected to calibrate theT2 spectrum data of the specimen, and the relationship between signal amplitude per unit volume and porosity was obtained. The fitting curve expression was Y=117.78X−51.716, where Y is T2 spectrum amplitude and X is porosity. The water absorption per volume during saturation and the porosity of the specimen is shown in Figure 10.Figure 10
The changes of the water content and porosity. (a) Water content during water injection. (b) Porosity during the compression process.
(a)(b)The pores were continuously filled with water before reaching the saturation stage, and the change in measured signal quantity represented the variation of water quantity in the pore. As seen in Figure10(a), the volumetric water content of the sample increased from 0.86% to 1.13% during the first 93 minutes of water injection and remained stable. After that, the water content remained constant, at a maximum of 1.13%. The pores were full of water, the volume of water equaled the volume of pores, and the porosity converted into the saturated rock specimen. In Figure 10(b), the porosity of the rock sample showed a linear downward trend when axial pressure was less than 4 MPa. From 4 MPa to 6 MPa, a sharp decrease from 1.09% to 0.94% was noticed, indicating that microporous particles were crushed and compacted. The porosity remained unchanged at 0.93% between 8 and 20 MPa. It can be seen that the confining pressure changed the PSD, the connectivity of the pore, and the permeability, and the specimen gradually reached saturation under a certain injected pressure. The porosity of the saturated rock sample almost ceased to change after a sudden decrease at a lower stress state and no longer increased with the increasing stress, indicating that only some compression occurred under the action of axial pressure.


## 3.5. The Variation Characteristics of PSD in the Specimen

As shown in Equation (1), the T2 value of the NMR is directly proportional to the ratio of surface area to volume of pores, and the surface relaxation coefficient determines PSD. However, research on the surface relaxivity of carbonaceous slate is limited. The NMR and MIP were usually applied to determine the PSD of samples and get the surface relaxivity [37, 38]. The sample morphology from SEM observation indicated that the slit pore types dominate the samples (Figure 4), and the pore shape factor in Equation (2) was equal to 1. When the surface relaxation coefficient was 175, the pore size distribution (PSD) curves from the MIP and NMR experiments complemented perfectly in the overlapped pore size range. The PSD of carbonaceous slate determined by MIP and NMR confirmed that the slate had an extensive range of pore sizes from about 20 nm to more than 200 μm and that the porosity of the slate was mainly dependent on the pore volume of micropores less than 1 μm. The microcracks, with a pore size of over 10 μm, were usually less abundant. The rock core’s pore structure consisted of discontinuous micropores and microcracks without intermediate pores ranging in size from less than 1 μm to approximately 10 μm, as shown in Figure 11.Figure 11
The PSD from the NMR and MIP.The signal amplitude in theT2 spectrum of the sample was converted into the porosity component, and the relaxation time was converted into the pore size in the saturation and triaxial compression process. The quantitative relationship between pore size and pore throat corresponding water content and porosity is demonstrated in Figure 12.Figure 12
The PSD variation of sample. (a) Water amount during water injection. (b) Porosity during the compression process.
(a)(b)Figure12(a) shows the distribution of the volumetric water content of the rock sample containing pores of various sizes during the water injection process. The pore structure varied greatly when the confining pressure was applied, the intermediate pores appeared, and the connectivity improved significantly. The maximum diameter of the micropores increased from 0.5 μm to 1 μm, while the maximum diameter of the microcracks increased from 300 μm to 600 μm. Under the interaction of the pore water pressure and confining pressure, the voids’ size expanded and then shrank. It was worth noting that the diameter of the microcracks was nearly unchanged in initial and saturated states, while that of micropores enlarged slightly. The peak value of micropore water content decreased from 0.041% to 0.018% and then increased as injection time progressed. On the contrary, the peak value of microcrack water content increased from 0.008% to 0.04% and then descended to 0.01%. With the axial pressure increasing perpendicular to the bedding plane, the pore structure of the rock sample was consistent with that in the saturated state, lacking the intermediate pore. As shown in Figure 12(b), when the axial pressure increased from 4 MPa to 6 MPa, the micropores’ minimum diameter magnified while the microcrack’s maximum diameter diminished, and the porosity peak decreased significantly from 0.08% to 0.065% and then tended to remain stable. It was revealed that the particles around the microcrack experienced elastic deformation, and some clay minerals dissolved into water and were transferred. The above analysis showed that the lower confining pressure changed the pore size distribution of the specimen and improved the connectivity, which improved the permeability of the dense slate. With the gradual increase of axial pressure, the micropores compressed, and the porosity sharply decreased in the early stage and then maintained at a smaller value. While the size of microcracks diminished in the later stage, the porosity showed an increasing trend, indicating that microfracture began to sprout gradually.The pore throat distribution diagrams were obtained according to the porosity component and pore diameter graphs, as shown in Figures13(a) and 13(b). It was shown that the pore size of the micropore was less than 0.63 μm, and the microcrack was about 10 μm. During the water injection process, the volumetric water content component of the micropore varied the most, from about 0.05% to 0.26%, and that of the microcrack changed under 0.05%. There was a small amount of water that emerged in the intermediate pores of pores under the confining pressure of 0.5 MPa. As the saturation increased, the inner stress borne by the pore particles and the pore water changed, and the intermediate pores were finally closed. When the axial pressure was less than 6 MPa, the porosity component of the microstructure changed considerably. While it was almost constant between 8 MPa and 20 MPa. It inferred that the particles were crushed and the porosity was minimum, and the porosity component of the macropores increased slightly for larger axial pressure, which probably inducing the fracture expansion.Figure 13
The pore throat distribution of the sample. (a) Water injection process. (b) Triaxial compression process.
(a)(b)


## 3.6. The Deformation Mechanism of the Slate

Carbonaceous slate is a low-grade metamorphic rock that forms a dense structure with low permeability under oriented high-pressure stress levels, leading to slaty cleavage. The deformation of the slate is controlled by the layered structure, which contains numerous micropores and intersecting microcracks or fractures. The deformation mechanisms of the carbonaceous slate in the coupling of the hydromechanical actions can be explained as follows: under a certain stress condition, mesopores were formed, and the pore connectivity was significantly improved due to particle bending. Thus, the number and diameter of the interparticle pores and microcracks in the samples both increased. The good connectivity and the deformation of the throat will substantially affect the porosity and permeability [39]. As interparticle voids expanded in the slate, the area of the rock in contact with water increased, and more mineral particles enlarged after absorbing water. The cementation between the mineral particles eventually disintegrated, accelerating the hydraulic process and washing away the microparticles presented inside the cracks. Therefore, the preferred pathways were formed for groundwater; the slate became increasingly loose and fractured. In addition, the change of external stress disturbed the water flow and altered the pore water pressure, which affected the effective stress of the rock, the porosity, the permeability, and the slate’s PSD [2, 40]. High pore pressure reduced the effective stress that favors short-term stability but can lead to longer-term deformations and damage as the pore pressure dissipates; it can explain the large deformation of tunnel surrounding rock in the seepage section. The softening effect of groundwater on the slate decreased the internal cohesion and stiffness and reduced the strength with the immersion time increasing, which favored the carbonaceous slate’s deformation [13, 41]. Finally, the swelling due to some clay materials such as kaolinite was another factor affecting the large deformation. To summarize, groundwater and stress redistribution remarkably influence slate deformation; the rock masses easily slide along structural planes or detach, flex, and break. It can be revealed that the rock mass deformation increased rapidly with the appearance of groundwater and stress disturbance during the tunnel construction, such as in the Zhegu mountain tunnel [42] and the Minxian tunnel [43].


## 4. Conclusions

In order to reveal the microscopic deformation mechanism of surrounding rock caused by ground stress and groundwater disturbance during tunnel excavation, the carbonaceous slate samples containing fractures were taken as the research object, and water injection and triaxial compression tests were performed by NMR that could real time applied the load to the specimen. Combined with SEM-EDS and MIP experiments, the mineral component, surface morphology, and pore characteristics were analyzed, and the slate deformation mechanisms were revealed under hydromechanical coupling. According to the obtained findings, the following conclusions were drawn:(1)
Carbonaceous slate is formed by the layered structure of the flaky particles, containing very fine silk mica, quartz, and clay minerals. The flake particles are arranged in a directional manner in the diagenetic process to form a dense structure with poor permeability. There are a large number of micropores and a small number of microcracks intersecting with them. The connections between micropores and microcracks are weak due to a lack of intermediate pores. The stratified structure, intergranular micropores, and microcracks affect the slate’s macroscopic anisotropy and deformation characteristics(2)
The deformation mechanism of carbonaceous slate under the hydromechanical coupling is complex. Changes in the geologic environment cause strain/strain variations in the rock mass, resulting in the enlargement or closure of micropores and microfractures in the layered rock, altering the connectivity and permeability of the rock mass. The changes in pore surface and volume further alter the physical interaction between water and rocks. This physical interaction specifically swells and weakens the cohesive strength when some of the clay mineral particles in the rock mass absorb water, and some particles flake off and dissolve into water and are deposited at micropores and the tips of microfissures or lost along through channels, changing the porosity and matrix hardness and deformation modulus of the rock. In addition, the increase or decrease of pore water pressure makes the change the effective stress applied to the slate framework, and the disturbed pore pressure field needs enough time to reach reequilibrium, interfering that this was the root cause of ductile damage in saturated rock samples(3)
The microstructure of carbonaceous slate determines its macroscopic deformation characteristics to a certain extent. In the diagenetic process, the directional arrangement of the lamellar grains produced a layered structure, showing the lateral isotropic attributes of the rock mass. The disturbance of the Muzhailing tunnel excavation caused large deformation of the surrounding rocks in some areas. The stress field was intrinsically coupled with groundwater seepage, which was an important causal factor of the significant deformation and severe seepage occurring along the structural plane in the water-rich area. It was related to the pore space being quite sensitive to mechanical and hydraulic alterations. Therefore, the leakage during construction should be controlled by technical measures to reduce the stress changes that parallel the layered structural plane



---

*Source: 5490136-2023-02-21.xml*