Investigation of Energy Mechanism and Acoustic Emission Characteristics of Mudstone with Different Moisture Contents

Characteristics of energy accumulation, evolution, and dissipation in conventional triaxial compression of mudstones with different moisture contents were explored. Stress-strain relations and acoustic emission (AE) characteristics of the deformation and failure of rock specimens were analyzed. (e densities and rates of stored energy, elastic energy, and dissipated energy under different confining pressures were confirmed. (e results demonstrated that the growth rate of absorbed total energy decreases with the increase of moisture content, indicating that the higher the moisture content is, the less the total energy mudstone samples absorb. (e dissipated energy of the soaking sample, by contrast, has the first increase speed, and natural sample comes second at the beginning. When entering the crack stable development stage, the dry sample has the fastest growing rate of dissipated energy, meaning that dissipated energy used for crack propagation gradually decreases with the increase of moisture content. (e AE signals significantly enhance at the initial compression stage and plastic deformation stage with the moisture content decreasing. (e AE location events at the failure moment decrease as the moisture content increasing. (e time that the maximum AE even rate appears is slightly lagged behind the macroscopic failure time, and the AE even rates increase with the decrease of confining pressure. (e above results indicate that the water erosion process on rock reduces the cohesive energy and cohesive force, destroys the micromechanical structure, and minimizes the energy states of rock.


Introduction
e radioactive nuclear waste comes mainly from the waste accompanied with the production of atomic weapons and reactor cores at the nuclear plant.An estimated 440 nuclear power plants worldwide are currently generating more than 20 thousand tons of radioactive waste annually.e harm of low-and intermediate-level radioactive nuclear waste is relatively low, and the disposal measures have run to the period that is relatively mature.e high-level radioactive nuclear waste, by contrast, has a long damping period and does more harm [1][2][3][4].Due to the presence of nuclear radiation from high-level nuclear waste, its disposal site must be well sealed and away from the biosphere to prevent nuclear radiation from harming humans and the natural environment [5][6][7][8][9][10][11].It is recommended to store nuclear waste in deep rock masses in the world, as shown in Figure 1. e excavation, backfilling, and operation of the nuclear waste repository will change the in situ stress field, temperature, and humidity environment in the deep underground.
e occurrence of the excavation damage zone in surrounding rock will greatly enhance the permeability of the surrounding rock and greatly reduce the sealing performance.Generally, rocks (such as granite) usually have disadvantages such as unstable geology, less widespread distribution, and difficulty in recovering damage and permeability after disturbance.Compared with granite, mudstone is a good geological material to seal nuclear radiation.Selecting mudstone as the surrounding rock of the repository has four advantages: (1) Mudstone is a sedimentary rock, which is generally found in structurally stable areas around the earth, and the cost of building a reservoir is relatively low.
(2) e geochemical properties of mudstone are stable.(3) Mudstone has low permeability and diffusivity, which is not conducive to the penetration and di usion of radiation particles.(4) Mudstone has strong deformation ability.e damage can be partially restored during long-term operation, and the permeability will be restored signi cantly with the repair of the damage.
Due to its unique physical and chemical properties, mudstone is extremely sensitive to environmental factors such as temperature, humidity, stress, and groundwater.Especially when the humidity conditions change, the nature and state of the mudstone will change greatly, resulting in hydration expansion and strength reduction, resulting in large deformation or even collapse of the roadway, diverticulum, building matrix, and so on.erefore, studying the physical and mechanical properties of mudstone under di erent water-bearing conditions is a subject with theoretical and practical engineering application value [13][14][15][16][17]. He-hua and Bin [18] carried out creep tests of dry and saturated rock to study the law of rock creep a ected by moisture content.Bidgoli and Jing [19] used the discrete element method to study the e ect of groundwater on rock deformation and strength characteristics.e results show that water has a greater in uence on the deformation parameters of rock and less in uence on the elastic deformation parameters.Gutierrez et al. [20] carried out the experimental study on the mechanisms of chalk-uid interaction, especially for the behavior of petroleum chalk reservoirs during water injection.In order to investigate uid ow and permeability variations, Zhang et al. [21] conducted uniaxial, biaxial, and triaxial tests on large concrete blocks with randomly distributed fractures and rock core samples.e results showed that the permeability around underground openings depended strongly on stress changes and orientations of the natural fractures.Wang et al. [17] proposed a nite element model considering the coupled e ects of seepage, damage, and stress eld in heterogeneous rock, and a series of numerical simulations of the initiation of hydraulic fractures and their subsequent propagation were carried out.
Acoustic emission technology is a good way to study the process of rock deformation and destruction [22].To better understand the factors contributing to the dynamic evolution of rock energy, where fracture deformation and extension may be important, monitoring and imaging via acoustic emission may provide important constraint [23][24][25][26][27][28][29], examining the e ect of a change of loading rate on the evolution of microcracking through acquisition, analysis, and processing the acoustic emission signals.He et al. [30] performed single-face dynamic unloading tests under the true-triaxial condition using the AE monitoring technique and obtained the relationship of frequency-amplitude of AE signals in rock burst stages.e results showed the AE-accumulated energy release rapidly increased from the unloading state of the rock to its failure.In order to detect the symptoms of developing slope instability, Cheon et al. [31] proposed an improved monitoring apparatus and method to evaluate the damage level by means of an AE technique.Zhou et al. [32] carried out di erent shear tests on irregular arti cial sawtooth joints with di erent asperity heights, and the change laws of AE signals were detected and analyzed.e results could promote the application of the AE techniques to warn the dynamic shear failure of in situ joints.
Most recent research has shown that soft rock is easy to soften, swell, and disintegrate in water, and di erent watercontaining states have an important in uence on the mechanical properties and failure mechanism of rock.However, previous studies tend to analyze the deterioration of mechanical properties of rock under saturated or natural conditions and rarely combine energy mechanism to analyze the e ect of water content on rock mechanical properties.erefore, this paper carried out the conventional triaxial compression test of mudstone in dry state, natural state, and saturated state under di erent con ning pressures.According to the energy evolution law and acoustic emission characteristics, the energy 2 Shock and Vibration mechanism of mudstone under different water contents is analyzed.e research results will be important for analyzing the stability of rock mass in engineering.

Experimental Apparatus and Methodology
2.1.Experimental Samples.e mudstone of this paper, which is pure, delicate, grayish black, and brittle, is collected from Huaibei Gubei Coal Mine, located at −575 m to −648 m underground.
e undisturbed samples were sealed in plastic bags immediately after being obtained, shown in Figure 2. Mineral analysis was carried out by the D8 Advance X-ray diffractometer.e mineral composition of mudstone was 19.12% kaolinite, 18.88% illite, 16.47% sodium albite, and 45.53% quartz.e mudstone has a natural density of 2.61 g/cm 3 and a particle density of 2.73 g/cm 3 .According to the requirements of international regulations, the test piece is processed into a standard cylindrical rock sample with a diameter of 50 mm and a height of 100 mm.e nonparallelism of the test piece ends is not more than 0.02 mm. e end face is perpendicular to the axis, and the maximum deviation does not exceed 0.2 °.
In this paper, it is considered that the water content of rock samples is the same.According to the law of water absorption and water loss, the methods of drying and immersing water are adopted to obtain mudstone samples smaller than and larger than the natural water content.e prepared rock samples are divided into three groups.First group is dried.e drying temperature is 105 °C and the drying time is 3 hours.e moisture content of the dried rock samples is expected to be 0.80%.Second group adopts soaking samples in natural environment.e immersion time is 3 hours, and the water content of the immersed rock sample is expected to be about 2.03%.
e third group maintains the natural state, and the moisture content of the natural mudstone is about 1.56%.

Experimental Apparatus and Methodology. Uniaxial and triaxial compression tests were carried out on mudstones
with different moisture contents on the RMT-301 rock mechanical test system (Figure 3).e confining pressure is 0, 15, and 30 MPa. e test procedure is as follows: the confining pressure is applied to the set value at a loading rate of 0.1 MPa/s, and then the axial loading rate is loaded at 0.001 mm/s until the integrated stress-strain curve is obtained.e acoustic emission device PCI-II was used for real-time AE event monitoring during the test, in which the sampling frequency was set to 5 MHz and the threshold was set to 55 dB.In addition, in order to improve the accuracy of three-dimensional positioning of AE events, eight probes are used for data acquisition.Acoustic emissions were monitored with a 16-channel PCI-II system.

Energy Mechanism of Mudstone with
Different Moisture Contents

Energy Principle during Rock Failure
Process. e exterior loads can result in the deformation in the rock unit.e first law of thermodynamics shows that the deformation of the rock unit is usually regarded as a closed-loop system, and there is no heat exchange between the mechanical work and the environment.e total input energy U is equal to the stored energy of a unit volume rock mass, which is the sum of the releasable elastic strain energy U e and dissipative energy U d .e expression is shown in the following equation: Under uniaxial compression, the equations of total stored energy and releasable elastic strain energy are expressed as where σ 1i and ε 1i are the stress and strain values at every point of the stress-strain curve and E 0 is the elastic modulus.Under triaxial compression, σ 2 and σ 3 are equal in value, so the equations of total input energy and elastic strain energy read as follows:

Shock and Vibration 3
where U 0 is regarded as the strain energy stored under static water pressure in the initial loading, and the equation can be expressed as where E 0 and ] are the initial elastic modulus and Poisson's ratio, respectively.

Energy Dissipation under the In uence of Moisture
Content.According to the thermodynamics, the deformation process of the rock is irreversible, which is accompanied by energy dissipation and energy release.Figure 4 gives the variation of stress and energy of the mudstone samples a ected by moisture contents.At the initial compression stage (OA) and line elastic deformation stage (AB), most of the work done by the loads is converted into elastic strain energy stored in the mudstone samples, and a small part turns into dissipated energy.At the stage of the inelastic deformation stage (BC), the growth of elastic strain energy becomes slower with the strain, but the dissipated energy increases gradually.e reason is that the plastic deformation and new cracks begin to gather in the mudstone samples.In addition, the ratio of the dissipated energy to total strain energy has a rapid increase, which could explain that obvious crack propagation and coalescence in rock increases and more damage occurs.When the stress-strain curves enter into the stage of brittle stress drop, the mudstone samples rapidly release the elastic strain energy, and the dissipated energy increases sharply.Almost all of absorbed total energy turns into dissipated energy, which is used for further development of cracks and shear deformation along the slip surface.After failure, the circumferential deformation increases sharply, which results in a rapid increase of negative work done by con ning pressure.erefore, the total energy after failure has a slower growth.
Figure 5 shows the relationship between total energy and strain under di erent con ning pressures.It can be found from the curve that as the water content increases, the total energy absorbed by the mudstone decreases with the increase rate of strain; the higher the water content (saturated sample), the less the total energy absorbed by mudstone, and the lower the water content (dry sample), the more the total energy absorbed by mudstone.is is due to the chemical erosion of water on the rock, which leads to the decrease of the cohesive force of the viscous particles, destroying the internal micromechanical structure and causing the rock to e rock energy storage and energy release are relatively reduced, and the plasticity is enhanced.is is the mechanism that water injection is often used in underground engineering to prevent rock burst.
Figure 6 shows the relationship between dissipation energy and strain under di erent con ning pressures.Dissipated energy is mainly used for the generation and expansion of cracks and the accumulation of damage.It can be seen that, in the compaction phase and the elastic phase, the dissipated energy value is small and the growth is slow.However, the saturated sample grows faster at this stage, and the natural sample grows a little slower.It can be found that the higher the water content is, the more the energy is dissipated. is is mainly due to a series of physical and chemical reactions caused by the interaction of water and rock.So that the water-immersed sample initiates microcrack locally during the loading process, consuming a part of the strain energy.e dry sample is close to the continuous medium after compaction, and the linear elastic characteristic is very obvious.e dissipation energy in the elastic stage is less increased.When entering the elastoplastic stage, the internal cracks of the rock begin to generate and expand, and the proportion of total energy absorbed for dissipation is sharply increased.e growth rate of the dry sample is the fastest, followed by the natural sample, and the saturated sample is relatively lowest.When the rock breaks down, the dissipative energy continues to increase.At this time, the elastic energy accumulated inside the rock is instantaneously Shock and Vibration released, and the total energy absorbed by the rock is basically converted into dissipative energy.After the destruction, the dissipative energy of the dry sample still grows fastest, indicating that after entering the elastoplastic stage of stable development of the fracture, the dissipation energy for crack propagation gradually decreases with the increase of water content.
Figure 7 shows the relationship between elastic energy and strain under di erent con ning pressures.In the compaction phase and the elastic phase, the elastic energy stored in the rock increases rapidly, and the total energy absorbed by the rock is basically converted into elastic energy for storage.
e storage rates of elastic energy of mudstones in di erent water-bearing states are quite different, and the lower the water content, the greater the elastic energy during energy storage.After entering the elastoplastic stage, the rate of increase of elastic energy slows down due to the gradual cracking inside the rock.After instability and destruction, the elastic release is a source of power for rock damage.From the stage of stable development of rupture, the lower the water content, the higher the elastic energy stored in the rock.e release rate of the elastic strain energy of the saturated sample is greater than the release rate of the dry sample.6 Shock and Vibration e elastic energy stored inside the rock always has a maximum value from the generation, accumulation to release process, which is usually called the energy storage limit.Figure 8 shows the relationship between energy storage limit and water content under di erent con ning pressure conditions.It can be seen that under the same con ning pressure condition, the energy storage limit decreases linearly with the increase of water content; that is, the lower the water content is, the higher the maximum elastic strain energy the rock can store.
is is consistent with the analysis results of the previous energy components.In addition, the higher the con ning pressure, the greater the ultimate energy storage, indicating that the increase in con pressure enhances the energy storage capacity of the rock.

Acoustic Emission Characteristics of Mudstone Samples with Different Moisture Contents
AE signal can directly re ect the damage accumulation and crack propagation process in rock under the exterior loads.e AE event, cumulative AE events, and AE event rate are the most common AE parameters, and they can form independent time series.e AE event rate can describe the amount of energy released during rock failure, which means the number of AE events per unit of time.e higher AE event rate usually re ects the more severe internal damage of rock. Figure 9 shows the change process of AE event rate for mudstone samples a ected by moisture contents.As shown in Figure 9, the AE event rates of complete stress-strain curve of rock at each stage have di erent characteristics.At the initial loading stage, the AE event rates of mudstone samples with di erent moisture contents are relatively low, especially soaked samples.e small number of AE events results from the closure of original cracks and porosity, and hardly any new crack generates.When entering into the stage of elastic deformation, the stress-strain curves of mudstone samples are almost linear, so the sti ness of mudstone samples should be nearly constant during loading process.Compared with the initial stage, the AE event rates of mudstone samples at this stage show an increase but still low.When the stress-strain curves step into plastic deformation stage, the AE event rates have a remarkable rise which have increased almost three to ve times in the intensity.e results reveal that new cracks begin to emerge and the damage gradually accumulates.With the increase in load, a sudden enhancement of AE signals is found immediately prior to failure, and the cause might be the emergence of a large or persistent crack inside the samples.When entering into the failure stage, there is a step change step in the AE event rate which corresponds to the irregular in ection points in stress-strain curves.At the same time, the spalling and cracks have come out on the rock surfaces.In conclusion, the AE event rates have a signi cant increase at initial compression stage and plastic deformation stage when the moisture content decreases; the AE signals are more dispersed, and the AE event rates decrease with the moisture content increasing when entering into the failure stage.e above results indicate that the increase of moisture content reduces the brittleness failure characteristics of the mudstone samples.
Nowadays, the AE location method is commonly used in the material crack damage orientation and the investigation of failure process.e failure picture and the corresponding AE location events of the samples are shown in Figure 10.It is found that the AE location events at failure decrease when the moisture content increases.In addition, the failure modes of mudstone samples are closely related to the moisture contents.Speci cally, the single shear damage is more likely to emerge with the moisture content decreasing.It can be seen in the failure pictures that only one main control shearing surface runs through the whole sample, dividing the sample into two triangle cone shaped rocks.In contrast, the combination failure mode of single shear and splitting failure is more likely to emerge with the moisture content increasing.When more water molecules enter the gaps of particles, the cementation action is weakened, and the amount of energy required to fracture the particles decreases.at is why the moisture content can in uence the failure mode of samples.
Figure 11 shows the variation curve of the acoustic emission event rate of dry samples under di erent con ning pressures.Acoustic emission signals are concentrated in the stage of unstable damage and the stage of destruction development.e high acoustic emission event rate indicates that the rock sample has brittle damage.e maximum time of the acoustic emission event rate appears slightly later than the time when the macroscopic failure of the rock sample occurs.Comparing the acoustic emission activities of mudstones under three con ning pressures, it can be found that the acoustic emission event rate generally decreases with the increase of con ning pressure.It shows that the smaller the con ning pressure, the more severe the internal damage of the rock (Figure 12).

Shock and Vibration
In the water-immersed state, the structure of the mudstone is destroyed due to the dissolution of the muddy cement and the water swelling of the clay mineral.When moisture content is decreased, the ne particles are attached to the large particles.en the uidity between the particles is deteriorated, and the structure is enhanced.erefore, the strength of the dry sample is increased, and the strength of the saturated sample is lowered.From the energy point of view, the strain energy of the saturated mudstone is higher than that of the dry mudstone during the loading process,

Shock and Vibration 9
of mudstone must be taken into consideration before the nuclear waste disposal repository is built.

Conclusions
e geological disposal in deep underground engineering is a high-level radioactive waste disposal method adopted extensively in the world.e basic principle of this method is to excavate a single-or multitunnel system situated 500-1000 m below the earth's surface and then to place the final form of radioactive waste into preset position and backfill the tunnel system.Compared with other mother rocks, such as granite, rock salt, and shale, mudstone is usually the optimal option.However, the influence of underground water on mudstone is considerable to the stability of high-level radioactive waste disposal repository, but there are few researches on this topic.Based on conventional triaxial compression expression experiments of mudstone specimens with different moisture contents, the following main points are concluded.
e process from rock deformation, failure to collapse, is an irreversible process of energy dissipation, and moisture content has an important impact on this process.In the dry, natural, and saturated states, the total energy absorbed by the mudstone gradually decreases with the increase of the strain.e higher the water content, the less the total energy absorbed by the mudstone.e reason is that water molecules can easily enter clay minerals such as illite and kaolinite, which will weaken the cohesive force among the particles.
e dissolution of argillaceous cement and the water swelling of clay minerals will destroy the structure of the mudstone and soften the rock.e dissipative energy is small and slow in the compaction phase and the elastic phase.e greater the water content is, the more the energy dissipated is.After entering the elastoplastic phase, the total energy absorbed is used to increase the proportion of dissipative effect.e smaller the moisture content, the faster the growth.After the destruction, the dissipative energy of the dry sample still grows faster, indicating that after entering the elastoplastic stage of stable development of the fracture, the dissipation energy for crack propagation gradually decreases with the increase of water content.
e elastic energy storage rate decreases with the increase of moisture content in the compaction stage and the elastic stage.
e elastic energy growth rate slows down in the elastoplastic stage.e lower the water content in the instability stage, the higher the storage elastic energy.At the same confining pressure condition, the energy storage limit of mudstone decreases linearly with increasing water content.
e time that the maximum AE even rate appears is slightly lagged behind the macroscopic failure time.e maximum AE event rates during the failure process under uniaxial compression lies in the range of 2200-2500, while those under the confining pressure of 15 MPa and 30 MPa, respectively, fall in the range of 1800-2000 and 1500-1700, indicating that the acoustic emission event rate decreases with the increase of water content and decreases with the increase of confining pressure.e lower the water content and the smaller the confining pressure, the more the internal damage of mudstone.

Figure 4 :
Figure 4: Relation curves of stress and energy with strain for mudstone samples under di erent moisture contents and con ning pressures.

Figure 8 :
Figure 8: Relationship between energy storage limit and moisture content.

Figure 9 :
Figure 9: Variation curves of AE event rates for mudstone samples with di erent moisture content under uniaxial compression.(a) Soaked sample.(b) Natural sample.(c) Dry sample.

Figure 10 :Figure 11 :
Figure 10: AE location results and failure pictures of mudstone samples with di erent moisture contents at the moment of rock failure under con ning pressure of 15 MPa.(a) Dry sample.(b) Natural sample.(c) Soaked sample.

Figure 12 :
Figure 12: AE location results and failure pictures of dry samples at the moment of rock failure under di erent con ning pressures.(a) 0 MPa.(b) 15 MPa.(c) 30 MPa.