Mechanical Properties and Acoustic Emission Characteristics of Karst Limestone under Uniaxial Compression

Firstly, I-RPT ultrasonic detector was used to test the wave velocity of karst limestone with different initial microstructure and water content. 0en, RMT-150B rock testing machine and DS2-16B acoustic emission system were used to test the acoustic emission (AE) under uniaxial compression. Mechanical properties and AE characteristics were obtained during rock failure. 0e detailed relationship between stress-strain and AE characteristics was studied in this paper. Research results indicated the following: (1) For samples with many primary fissures and defects, wave velocity in dry state was larger than that in its natural state. From natural state to saturated state, the wave velocity tended to increase. For samples with good integrity, wave velocity increased with increasing of water content. (2) In the dry state, the samples presented tension failure. In saturated state, the samples presented tension-shear failure. For samples with cracks and good integrity, samples showed brittle failure. For samples with many corrosion pores which showed ductile damage under natural and saturated state, the spalling phenomenon was enhanced under saturated state. (3) With increasing of water content, the peak stress and AE peak reduced dramatically. In brittle failure, AE peak could be considered a sign of failure. In ductile failure, AE activity decreased gradually with the decrease of stress. (4) 0e mechanical properties and AE characteristics corresponding to four main fracture propagation types were also discussed.


Introduction
China has the most widely distributed karst area in the world.
e distribution area of soluble rock in China is mainly located in the southwest, total area is about 3,400,000 km 2 (Figure 1), among which the exposed carbonate area reaches 910,000 km 2 [1,2].e development of Western China is an important strategy for China's economic development, in which transportation construction is one of Western China's most important economic policies: many highway and railway tunnels have been built in recent years [3][4][5][6].However, due to the concealment of tunnel engineering, an increasing number of geological disasters have occurred during the tunnel construction [7][8][9][10].Karst disasters, such as water and mud inrush, have become the most serious geological hazards during tunnel constructions in Southwest China (Figure 2) [11][12][13][14][15][16][17][18].e hazards are closely related to rock properties.Study on the mechanical properties of rocks provides the parameters for engineering design, which helps prevent and decrease geologic hazards [19][20][21][22][23].
e failure of brittle rock is a process of internal microcrack generation, expansion, and coalescence, which can cause stress relaxation and energy release, resulting in the acoustic emission (AE) phenomenon [24][25][26].AE technique can collect and analyze acoustic waves during rock failure.AE characteristic can well reflect the compressive deformation and damage evolution process of rocks, which can thus help scholars understand rock failure mechanism and the inherent condition changes [27][28][29].Commonly used AE parameters include event rate, ring-down counts, energy rate, amplitude, average of signal level (ASL), and frequency.
Rock generally contains various microcracks, ssures, pores, joints, and other defects.It always contains water in nature.e defects and water have obvious e ects on the mechanical properties and AE characteristics of rocks [30][31][32][33].Numerous studies have been conducted on rocks or rock-like materials with preexisting or native ssures under uniaxial compression.Results of these studies indicate that the geometry of preexisting aws a ect fractures initiation and coalescence behavior, whose quantity and angle signi cantly in uence the mechanical properties and AE characteristics of a specimen [34][35][36][37].Water content also has an important impact on the AE emission characteristics in the sample failure process.Water has an obvious softening e ect on the primary ssures in rock samples.Moreover, pore pressure is inferred to take part in the sample deformation and promotes fracture network generation [38][39][40].ese cause the promoting plastic behaviour and the dissipation of internal energy.From the macroscopic view, the strength and AE activity will decrease [41][42][43].
Few studies have investigated the mechanical properties and AE characteristics of rock by considering the coupling e ect of initial microstructure and water content.In the present research, AE monitoring tests under uniaxial compression were conducted on karst limestone specimens

Rock Sample and Preparation.
Karst limestone was from three sections of the Qiyue-shan Tunnel of Lichuan-Wanzhou Highway construction sites in Hubei Province, China.e tunnel area lies in the southwest fold mountain zone of Hubei Province and is a typical karst area.e cover layer mainly consists of Quaternary silty clay, which is from the accumulation of ancient landslide.e bedrock is thicklayered limestone of Jialing River formation, Lower Triassic.
Fresh rocks without obvious cracks were selected in the construction site and wrapped with multilayer preservative films.e rocks were then prepared as Φ 50 mm × 100 mm cylindrical samples.As shown in Figure 3, group A samples contained obvious cracks and group B samples contained numerous corrosion pores.Group C samples were relatively complete in structure, without obvious defects.Each group is divided into three sets according to their water content.
e samples of the first set were sufficiently dried, the second set was in its natural state, and the third set consisted of water-saturated specimens.
e first and third sets of samples were placed in an electric thermostatic drying oven to maintain constant-temperature drying for 24 h at 110 °C.For the third set, after drying, the samples were saturated by vacuum pumping immersion method.e vacuum pressure was kept at 100 kPa, and the pumping time was 6 h.Each test was repeated three times to reduce test errors.erefore, 9 sets of 27 experiments were conducted in this study.e nonmetallic I-RPT ultrasonic detector was used to record the speed of pressure wave (Pwave) and shear wave (S-wave).

Equipment and Loading Condition
e P-wave transducer frequency was 50 KHz, the S-wave transducer frequency was 150 KHz, the sampling length was 1024, and the sampling period was 0.4 μs.
e loading equipment used in this study was the RMT-150B computer-controlled triaxial testing machine, which can provide 1000 kN maximum axial pressure and conduct uniaxial or triaxial loading tests.is study used the displacement control mode for the test process, which was 0.002 mm/s.e DS2-16B AE detection system was used for real-time monitoring of AE.To eliminate the effect of noise on the AE test, the threshold was set to 50 mV and the preamplifier gain was set 40 dB.Two AE sensors were arranged symmetrically on the sides of the sample, and the contact face was coupled with Vaseline.

Physical Parameters of the Samples.
e physical parameters of the rock samples are shown in Table 1.As the  water content of the natural state cannot be obtained directly, the average values of the same group samples in the natural state are presented.According to Figure 5, for samples with many primary fissures and defects (group A and B), the wave velocity in dry state was larger than that in its natural state.From natural state to saturated state, the wave velocity tended to increase.
is outcome was attributed to the various defects on the rock samples.After water absorption, the water molecule could weaken the connecting capacity between particles, part of the water was adsorbed by the defects, and water membranes were formed.e water membranes could cause the refraction and reflection of elastic wave, which caused decrease of wave velocity.With increasing of water content, the defects were saturated and the rock matrix and defects were connected well by the water.is effect caused the wave velocity to go straight through the rock without refraction and reflection, which caused the increase of wave velocity.For group C, because the cracks were not obvious, the water membranes reflection had almost no effect on the wave, and the wave velocity increased with the increase of water.

Mechanical Properties of Karst Limestone under Uniaxial Compression.
According to previous studies, the failure process of brittle rock can be generally divided into five stages: crack closure, elastic deformation, fracture initiation and stable growth, fracture damage and unstable growth, and failure and post peak [44][45][46].According to the experimental results, the stress-strain curves of karst limestone samples under uniaxial compression were showed in Figures 6-8. 4 Advances in Materials Science and Engineering e samples under the dry state showed obvious elasticbrittle failure.No plastic deformation occurred before peak strength and after the compaction stage and linear elastic deformation stage, and the stress dropped suddenly to zero with ringing sound of rupture.For the group A samples under natural and saturated state, a small part of plastic deformation appeared near peak strength and then damaged rapidly.For the group B samples under natural and saturated state, an obvious plastic deformation existed before peak strength.After peak strength, the stress decreased gradually and showed ductility damage.For the group C samples under natural and saturated state, the stress dropped sharply after compaction stage, linear elastic deformation stage, and plastic deformation stage.
Tables 2-4 show the peak strength ratio of the limestone samples with different water content.For Group A, the ratio was 1 : 0.93 : 0.83.For Group C, it was 1 : 0.88 : 0.70.e dry limestone sample had the highest strength, its peak strength decreased gradually with the increase in water content.Meanwhile, the influence of water content on the axial strain  Advances in Materials Science and Engineering and total strain at peak stress was not obvious.For Group B, the ratio was 1 : 0.65 : 0.29, and the water-softening e ect was reinforced because of the poor initial microstructure.showed the fracture modes of rock samples with di erent initial microstructure and water content.For group A, the fracture modes showed tension failure with many vertical fractures and sheet debris.In saturated condition, the samples presented tension failure with local shear failure.For group B, both tension and spalling failure existed, as many vertical and horizontal fractures were observed near the corrosion pores.Under vertical loading, stress concentration appeared at the horizontal sides of corrosion pores, which caused fractures to develop and coalescence at the horizontal direction.e horizontal fractures were also irregular.With increasing of water content, the spalling of blocks likewise increased.For group C, the fracture modes under dry and natural state mainly belonged to tension failure, and local shear failure likely

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Advances in Materials Science and Engineering existed for the natural state.Under the saturated condition, the X-shape or Y-shape fractures, which showed tensionshear mixed failure, were clearly observed.is count includes rate number and cumulative number.e selected AE parameters depend on the sensitivity, signal-to-noise ratio, and frequency response of the monitoring system.us, the AE monitoring system should be kept synchronous with the load system.

Analysis of the AE Characteristics of Samples.
e stress curve and AE activity in the failure process of samples are shown in Figure 12. e AE activity of the sample during the loading process was consistent with its stress variation, which could be classified into two categories: brittle failure, which meant the rock samples were damaged immediately upon reaching peak stress, as shown in Figure 12(a); ductile failure, the rock samples damaged gradually after peak stress, as shown in Figures 12(e) and 12(f ).Advances in Materials Science and Engineering AE activity decreased dramatically with the increase of water content and decrease of peak strength.When water molecules reached the rock granular space, the contact bond strength of particles reduced, which caused weakening of rock strength at the macro level; meanwhile, pore water pressure can promote the generation of fracture networks during hydraulic fracturing [38][39][40][47][48][49].For group A, AE peak decreased more than 70% from dry state to saturated state.For group B, AE peak decreased more than 90% from dry state to saturated state and decreased about 50% from natural state to saturated state.For group C, AE peak decreased more than 80% from dry state to saturated state.
Water content had a significant influence on AE activity.High strength and less water content meant more intense AE activity, but it was not a linear relationship because primary cracks and defects also had major impacts.e coupling effect of water and native defects had a major influence on the fracture initiation and expansion, which caused the macroscopical differences in the evolution characteristics, mechanical properties, and failure modes of AE.Xu et al. [50] thought that the AE activity was seriously affected by the rock type, the structure, and the level of uniformity, and the load mode and load control mode could directly affect the rock failure process, in turn, affect the AE activity.
From the macro point of view, with the increase of water content, the sample strength and AE peak were reduced significantly.From the microscopic point of view, water affected the formation and expansion of fractures, which caused the change in the mechanical properties and AE characteristics in the loading process.Figure 13 shows the four main fractures propagation types of karst limestone failure under uniaxial compression in this study.
(1) Vertical tension fractures: ese fractures corresponded to elastic-brittle failure, where the AE peak was obvious, indicating the sample failure.
(2) Vertical tension fractures with local shear plane: ese fractures mainly belonged to elastic-plasticbrittle failure, where an obvious plastic deformation stage was observed, in which the AE signal was very active.e AE peak in this rupture type was lower than that in elastic-brittle failure.
(3) Shear-tension mixed fractures: ese fractures had obvious shear surfaces, which is elastic-plastic-brittle failure.e AE activity increased with the loading, and the AE peak was not obvious.(4) Spalling of corrosion pores: With the stress centralized around the corrosion pores, the stress-strain curve presented the features of elastic-plastic-ductile failure.e AE peak was not obvious near the peak stress.In the post-peak failure stage, the AE activity reduced gradually with the decrease of stress.

Discussions
e elastic wave velocity of rock is mainly affected by rock type, density, fissure or interlayer, and water content.Kahraman [51] studied the relationship between the elastic wave velocity and the saturation or porosity of rock based on Gassmann equation and Wood theory.However the study cannot explain the rapid rise of the wave velocity after the saturation threshold.For this reason, Yang et al. [47] considered the influence factors such as gravity and other factors to modify the relation between the wave velocity and the saturation or porosity in the whole process of full water satiety, and the law obtained was consistent with the laboratory test results.For karst limestone with many primary fissures and defects, the wave velocity initially decreased and then increased with the increase of water content, this was similar with Yang et al. [47].For group C samples with good initial microstructure, the wave velocity increased with the increase of water content; this was similar with Guo et al. [52].erefore, the initial microstructure and water content have an important impact on wave velocity.It is noteworthy that the result is static, while elastic wave velocity of rock is changing and shows obvious stage characteristics in the failure process [53].erefore, further research on the wave velocity during the whole failure process is needed to study.e acoustic emission (AE) of rock materials is a phenomenon where rock elastic strain energy is released in the  Advances in Materials Science and Engineering failure process, and it occurs upon the expansion of internal primary cracks and defects as well as upon the generation or breakage of microcracks under the stress action [54][55][56].e results indicated that different initial microstructure and water content cause the difference of mechanical properties and AE characteristics.On the microscopic level, initial microstructure and water content have an important impact on the cohesion between mineral particles; meanwhile, the pore water pressure can promote fracture initiation and growth.at causes the different number and direction of macroscopic fractures.erefore, different initial microstructure and water content result in different failure modes, which are corresponding to different AE characteristic.
Chen et al. [57] showed that the AE characteristics of marble under uniaxial and triaxial compression are different.
e confining pressure had a major influence on the mechanical properties and AE characteristics of the rock.Su et al. [58] investigated the AE characteristics of sandstone under different loads by conducting uniaxial compression, conventional triaxial, and triaxial unloading tests.eir results showed that the mechanical properties and AE characteristics of sandstone were significantly different in different loading modes.e buried depth of a karst tunnel is often large, the ground stress is high.us, further research on the AE characteristics of limestone under triaxial compression is required.

Conclusions
I-RPT ultrasonic detector was first used to test the wave velocity of karst limestone with different initial microstructure and water content.
en RMT-150B rock testing machine and DS2-16B acoustic emission system were used to test the acoustic emission (AE) under uniaxial compression.Mechanical properties and AE characteristics were obtained during rock failure.e detailed relationship between stress-strain and AE characteristics was studied in this paper.Research results indicated the following: (1) For samples with many primary fissures and defects, wave velocity in dry state was faster than that in its natural state.From natural state to saturated state, the wave velocity tended to increase.For samples with good integrity, wave velocity increased with increasing of water content.(2) e peak strength of samples decreased with the increase of water content, while the influence of water content on the axial strain and total strain at peak stress was not obvious.In the dry state, the samples presented tension failure with numerous vertical cracks and sheet debris, and the failure degree was serious.In saturated state, the samples presented tension-shear failure.For samples with cracks and good integrity, samples showed brittle failure.For samples with many corrosion pores showed ductile damage under natural and saturated state, and the spalling phenomenon was enhanced under the saturated state.(3) In brittle failure, the sample failed immediately after the AE peak, which could be used as a sign of sample brittle failure.e AE peak was considerably lower in ductile failure than in brittle failure.For group A, the  peak count rate decreased more than 70% from dry state to saturated state.For group B, the peak count rate decreased more than 80% from dry state to saturated state and reduced about 50% from natural state to saturated state.For group C, AE peak decreased more than 80% from dry state to saturated state.
e duration of the AE active stage under natural or saturated state was longer and more obvious; this is caused by the water-softening effect that increased the plastic deformation.(4) Four main crack propagation types of karst limestone failure were observed under uniaxial compression: vertical tension cracks, vertical tension cracks with local shear plane, shear-tension mixed cracks, and spalling of corrosion pores.Four types correspond to different AE characteristics under uniaxial compression.

Figure 1 :
Figure 1: e karst region distribution in China.

Figure 4
shows the testing equipment.

Figure 6 :Figure 7 :Figure 8 :
Figure 6: e stress-strain curves of group A samples with di erent water content.(a) Group A, dry state.(b) Group A, natural state.(c) Group A, saturated state.

Figure 9 :
Figure 9: Failure modes of group A. (a) Dry state.(b) Natural state.(c) Saturated state.

Table 1 :
List of rock samples physical parameters.

Table 2 :
Strength of group A samples.

Table 3 :
Strength of group B samples.

Table 4 :
Strength of group C samples.