An Experimental Study on Mechanical Behavior of Parallel Joint Specimens under Compression Shear

In order to investigate the influence of the joint on the failure mode, peak shear strength, and shear stress-strain curve of rock mass, the compression shear test loading on the parallel jointed specimens was carried out, and the acoustic emission system was used to monitor the loading process.+e joint spacing and joint overlap were varied to alter the relative positions of parallel joints in geometry. Under compression-shear loading, the failure mode of the joint specimen can be classified into four types: coplanar shear failure, shear failure along the joint plane, shear failure along the shear stress plane, and similar integrity shear failure. +e joint dip angle has a decisive effect on the failure mode of the specimen. +e joint overlap affects the crack development of the specimen but does not change the failure mode of the specimen. +e joint spacing can change the failure mode of the specimen.+e shear strength of the specimen firstly increases and then decreases with the increase of the dip angle and reaches the maximum at 45°. +e shear strength decreases with the increase of the joint overlap and increases with the increase of the joint spacing. +e shear stress-displacement curves of different joint inclination samples have differences which mainly reflect in the postrupture stage. From monitoring results of the AE system, the variation regular of the AE count corresponds to the failure mode, and the peak value of the AE count decreases with the increase of joint overlap and increases with the increase of joint spacing.


Introduction
e reliable assessment of rock masses is a common task in rock engineering.e discontinuities such as joints, faults, and bedding planes in the natural rock mass greatly weaken the stability of the rock mass [1][2][3][4].
ese discontinuities control the mechanical properties of rock mass not only because of their interaction with the intact rock but also because of the interaction between themselves [5,6].When the specimen is loaded, new cracks develop near the tips of existing joints and propagate or coalesce with other cracks.e propagation of new cracks and the coalescence of fractures lead to a degradation in the mechanical properties of the rock.e effect law of joints on the failure of rock mass will have important guiding significance for actual engineering.
e mechanical behavior of nonpersistent jointed rock mass has been extensively studied by experiments and numerical simulation.Bobet and Einstein [7] identified five types of failure patterns from uniaxial compressive tests of gypsum specimens with parallel flaws.Wong and Chau [8] carried out uniaxial compression tests on plaster specimens with two flaws and identified nine types of coalescence patterns.Cao et al. [9] investigated the peak uniaxial compressive strength and failure patterns of ubiquitousjoint rock-like specimens by combining similar material testing and numerical simulation.e failure patterns of ubiquitous-joint specimens can be classified into four categories.Bahaaddini et al. [10] studied the effect of joint geometry parameters on the deformation modulus, compressive strength, and failure mode of the rock mass by particle flow modeling.Using photographic monitoring and acoustic emissions monitoring techniques, Yang et al. [11,12] investigated the relationship between the real-time crack coalescence process and axial stress-time behavior for red sandstone containing two unparallel ssures.Chen et al. [13] studied the in uences of joint inclination angle and joint connectivity rate on compression strength and stress-strain curves of rock mass with nonpersistent open joints by conducting uniaxial compression tests on gypsum specimens.Fan et al. [14] performed numerical simulations to study the in uence of multi-nonpersistent joints on mechanical behavior by using PFC3D software package.Yang et al. [15] numerically simulated the mechanical behavior of a jointed rock mass with nonpersistent joints adjacent to a free surface on the wall of an excavation.e aforesaid experiments and numerical simulations illuminated the important in uence of joint geometry on the mechanical behavior of rock masses under compression load.e interaction of rock bridges and the joint has been studied a lot [16,17], whereas the interaction between joints has not been widely studied.Zhang et al. [3,18] have studied the cracking and coalescence behavior in a rectangular rock-like specimen containing two parallel preexisting open aws under uniaxial compression load.e results show that the spacing between two aws and the inclination of a line linking up the inner aw tips have di erent e ects on the coalescence patterns and peak strength of specimens.Nevertheless, the interaction between joint planes in a nonpersistent jointed rock mass has rarely been investigated.And the jointed rock mass is often loaded by the comprehensive shear stress for the actual rock mass engineering, such as high and steep rock slope.
e change of the rock mass with nonpersistent joints depends on joint con guration and loading conditions.
erefore, the research about the interaction between joints under comprehensive shear load will be signi cant.In this paper, the compression shear test loading on the parallel jointed specimens was carried out to look at the interaction of parallel joints and its e ect on the failure mode, strength, and deformation behavior of jointed rock masses.

Specimen Preparation.
e specimens were made of white cement, water, and sand.e volume proportions (V water : V white cement : V silica sand ) in the specimens were 1 : 2 : 1.
e dimensions (height × width × thickness) of each specimen were 100 mm × 100 mm × 30 mm. e existing joints were created by inserting mica sheets (0.6 mm thick; 15 mm long) into the fresh cement mortar paste at the desired location of the joints.e geometric parameters of the specimen and the distribution of joints are shown in Figure 1.Fissure geometry is de ned by three geometrical parameters: joint dip angle, joint overlap, and joint spacing.
e joint inclination of specimens have been de ned as 0 °, 30 °, 45 °, 60 °, 75 °, and 90 °.For each dip angle, the joint overlap varied from 0 mm to 15 mm at 5-unit increments, and the joint spacing varied from 15 mm to 36 mm at 7-unit increments.e in uence of joint orientation and joint overlap on the mechanical behavior of jointed rock mass was investigated by varying α and L 0 , while keeping other geometric parameters constant (e.g., d 15 mm, L r 20 mm, e in uence of joint orientation and joint spacing on the mechanical behavior of jointed rock mass was investigated by varying α and d, while keeping other geometric parameters constant (e.g., L 0 15 mm, L r 20 mm, L j 15 mm).e specimens were not removed from the mold before the modeling material solidi es.Afterwards, the specimens were removed from the mold and soaked in water for 3 days and then were placed into a standard curing box (with the temperature kept at 20 ± 2 °C and humidity kept at 80%) for 25 days before mechanical testing.ree specimens were prepared for each joint distribution.Table 1 shows the ssure geometry information for all the specimens in this study.Each specimen was assigned an ID number using the notation S-a-b-c, where S stands for the sample, a represents joint overlap L 0 , b is the joint spacing, and c is the inclination angle α. e mechanical properties of the intact sample, that is, the tensile strength, uniaxial compressive strength, internal friction angle, cohesion, elastic modulus, Poisson's ratio, are listed in Table 1.e specimen was sandwiched between the two shear boxes.

Experimental Method.
e loading was applied according to a displacement control manner.e loading was controlled by the loading control system (DCS-200).
e loading rate was 0.1 mm/min.During the test, a digital camera was set up to take photographs of the specimen.e crack penetration and coalescence of the sample were monitored by acoustic emission.All specimens are loaded until specimen failure, and the load-displacement curves of the jointed samples are recorded simultaneously via a data acquisition system.

Effect of Parallel Joint on the Failure Mode of Samples
Prefabricated joints play a vital role in the failure of the specimen.Previous experimental and numerical results showed that when a load is applied to specimens with a single ssure, three types of cracks develop from the preexisting ssure (Figure 3): wing cracks, quasi-coplanar secondary cracks, and oblique secondary cracks.e axial load (Figure 4) can be decomposed into the shear load along the middle plane of the specimen and the compressive load along the upper plane of the specimen, and the shear load is equal to the compressive load at the compressive shear angle of 45 °.
For ubiquitous-joint specimens, the mechanical behavior will be more complicated.When compression is applied to the existing aws, tensile or shear cracks will develop from the tips of the existing fractures.As loading continues, these cracks will propagate and join with other cracks, by penetrating through the rock.us, the preexisting ssures will join with neighboring cracks, resulting in various types of failure patterns.According to the observation of the relationship between the failure surface and the

Advances in Civil Engineering
shear stress surface and the joint surface, the failure modes of ubiquitous-joint rock-like specimens can be generally classified into four categories: coplanar shear failure (failure mode I), shear failure along the joint plane (failure mode II), shear failure along the shear stress plane (failure mode III), and similar integrity shear failure (failure mode IV).

e Influence of Joint Inclination on Failure Modes of
Samples.When the joint dip angle is 0 °, wing cracks develop first from the joint tips under the compression shear load.e wing cracks are tensile cracks.And they initiate from the joint tips and propagate in a stable manner towards the direction of maximum compression (Figure 5(a)), which is consistent with the analytical results of the wing crack propagation in the classical fracture mechanics [19].However, as the load increases, the shear stress starts to dominate the crack development.e propagation of wing crack does not continue and closes slowly.e shear stress plane of the specimen is coplanar with the joint surface.erefore, the shear stress along the coplanar plane causes the crack to develop at both ends of the sample and overcome the bridge to expand.Ultimately, the cracks along the coplanar completely are cut through the specimen.e failure phenomenon is defined as a coplanar shear failure (failure mode I).As the shear load is equal to the axial load, the shear failure along the coplanar plane is more likely than compressive failure.erefore, the failure path of the specimen is predictable, and the failure mode is a coplanar shear failure.
At α � 30 °, the shear stress plane intersects the joint plane, and the failure mode of the specimen is complicated.Figure 5(b) shows that the specimen produces a large number of quasi-coplanar secondary cracks under compression shear stress.e quasi-coplanar secondary cracks develop from the tip of the joint at both ends of the shear stress plane and extend along the joint plane to break through the rock bridge.
is failure is defined as shear failure along the joint plane (failure mode II). e failure mode of the 60 °joint specimen is the same as that of the 30 °joint specimen.At α � 45 °, the joint tip of the specimen produces oblique secondary cracks, quasi-coplanar secondary cracks, and wing cracks, as shown in Figure 5(c).
rough the coalescence among the cracks, an irregular shear plane is formed in the middle of the sample.
e failure of the specimen is similar to that of the integrity specimen.is failure is defined as similar integrity shear failure (failure mode IV).
When the joint dip angles are 75 °and 90 °, the shear stress plane is approximately perpendicular to the joint plane.Wing crack developed continuously in the middle of the specimen under compressive shear stress.As shown in Figures 5(e) and 5(f ), with the increasing of the load, the shear stress dominates the crack development, and oblique secondary cracks develop from the tip of the joint near the shear plane and extend along the direction of the shear plane.Oblique secondary cracks constantly connect with the wing crack of the adjacent joints.Finally, a through-crack plane is formed parallel to the shear plane, which is defined as shear failure along the shear stress plane (failure mode III).

3.2.
e Influence of Joint Overlap on Failure Modes of Samples.When the joint dip angle is 0 °, the failure modes of the different joint overlap are failure mode I (Figure 5).With the decreasing joint overlap, more wing cracks are developed in the specimen under axial compression.However, with the increasing load, the shear stress along the shear plane dominates the failure of the specimen and promotes the closure of the wing crack.Finally, the specimens are destroyed by breaking through the rock bridge on the central shear plane.On the whole, the joint overlap only affects the number and distribution of wing cracks and does not change the failure mode of specimens.At α � 30 °, the failure modes of the different joint overlap are failure mode II (Figure 5).As shown in Figure 5, the conclusion can be found that the change of the joint overlap did not change the failure mode under the same joint inclination, but only affected the number of crack growth and the expansion path.Because joint overlap only changes the relative position between joints without changing the joint density and the size of single joint is small, the joint overlap has almost no effect on failure modes.

3.3.
e Influence of Joint Spacing on Failure Modes of Samples.In order to investigate the effect of joint spacing on the mechanical behavior of jointed rock mass, the joint spacing was varied to be 15 mm, 22 mm, 29 mm, and 36 mm.Correspondingly, the jointed rock mass have two, three, four, and four joint planes, respectively.e effect of joint spacing is studied in a range of the joint dip angle.
e other geometric parameters are kept constant, that is, L 0 � 15 mm, L r � 20 mm, and L j � 15 mm.Table 2 displays the failure modes of jointed rock mass having a different joint spacing.It can be seen from Table 2 that, with the increase of the joint spacing, the joint dip plays an important role in the failure mode of the specimen.Under the same joint inclination, the joint specimens have similar failure modes at the different joint spacing, which shows that the joint inclination plays an important role in the failure mode.
When α is 0 °, the failure modes of joint specimens with the different joint spacing are failure mode I.Although the increase of joint spacing changes the joint density of the specimen, it does not change the rock bridge length on the shear plane.erefore, the failure modes of joint samples are the same.When α is 30, with the increases of joint spacing, the failure mode of the specimen changes from failure mode II to failure mode IV, which shows that the failure mode of the specimen changes gradually with the joint spacing increasing.It can be seen from Table 2 that the number of failure mode IV of joint specimens with joint spacing of 15 mm, 22 mm, 29 mm, and 36 mm, respectively, is 1, 2, 4, and 5. On the whole, with the increase of the joint spacing, the number of joints decreases, the weakest effect on the specimen decreases, and the number of specimens in 4 Advances in Civil Engineering failure mode IV increases.erefore, the change of the joint spacing will cause the failure mode of the specimen to change, and the degree of in uence is related to the joint inclination.

Influence of Parallel Joint on Sample Strength
In this experiment, the compression shear angle is 45 °, and the compression stress loading on the specimen is equal to e peak shear strength of the joint specimens is shown in Table 3.

e Effect of Joint Inclination on the Shear Strength of the Specimens.
e joint inclination has a great influence on the peak shear strength which presents a certain law (Figure 6).When the dip angle increases from 0 °to 45 °, the peak shear strength of the specimens shows an increasing trend.From 45 °to 90 °, the peak shear strengths begin to decrease with the increase of the dip angle.All of the specimens for the different joint overlap reach the maximum value of the peak shear strength at 45 °. e joint specimen gets the maximum shear strength at α � 45 °and L 0 � 0 mm which is 9.42 MPa. e peak shear strength is affected by the failure modes of the specimens.
When the joint dip angle α � 0 °, the failure modes of the joint specimens are failure mode I.In the failure mode, joint length on the failure plane is comparatively long and the rock bridge is relatively short.At the same time, the shear stress is concentrated along the joint plane. is is the reason why it is easy to develop quasi-coplanar secondary cracks to break the sample, and the shear strength is relatively small.While the joint dip angle α � 30 °or 60 °, the failure modes of the joint specimens are failure mode II.
e cracks are dominated by the secondary crack in the plane.Owing to the shear stress that cannot be concentrated to the failure surface, the shear strength will increase.At α � 45 °, the failure modes of the joint specimens are failure mode IV. e joint weakening effect is the smallest, and thus, the peak shear strength is the largest.When the joint dip angles are 75 °and 90 °, the failure modes of the joint specimen are dominated by failure mode III.
e shear stress plane is approximately perpendicular to the joint plane.e joints are concentrated in the middle of the specimen.e shear stress-dominated oblique secondary cracks continue to develop and expand, through the sample.e peak shear strength is smaller than 60 °, and the 90 °joint specimen is more susceptible to damage than 75 °.

e Effect of Joint Overlap on the Shear Strength of the
Specimens.Figure 5 shows the effect of joint overlap on the peak shear strength.It can be seen from Figure 7 that the overlap length has a major influence on the peak shear strength of the specimens.In Figure 7, the peak shear strength decreases with the increase of the overlap length; the maximum value is observed at L 0 � 0 mm, and the minimum value is observed at L 0 � 15 mm.At the same joint inclination angle, with the change of the joint overlap, the failure modes are the same and the joint density of the samples is roughly equal, but the uniformity of the joint distribution changes.
e smaller the degree of the joint overlap is, the more uniform the distribution of joints on the specimen is.Under the compressive shear stress, the joints of the specimen are more concentrated, and the damage is easier to occur.us, the peak shear strength decreases with the increase of the overlap length.In the experimental results, as the overlapping length increases from 0 mm to 15 mm, the peak shear strength of α � 0 °, 30 °, and 45 °samples decrease by approximately 47.1%, 39.0%, and 21.2%, respectively.

e Effect of Joint Spacing on the Shear Strength of the Specimens.
e effect of the joint spacing on peak shear strength is shown in Figure 8.At the same joint inclination angle, the peak shear strength of specimens increases with the increase of joint spacing.
e maximum peak shear strength is obtained for a specimen with d � 36 mm, and the minimum peak shear strength is for d � 15 mm.e increase of the joint spacing leads to the increase of the rock bridge length between prefabricated joints, so the joint specimen is more difficult to break.erefore, the larger the joint spacing is, the greater the peak shear strength is.At α � 75 °, the peak shear strength of specimens with the joint spacing of 15 mm, 22 mm, 29 mm, and 36 mm is 5.18, 6.51, 7.12, and 7.17 MPa, respectively, and the relative growth rate of peak shear strength is 25.7%, 9.4%, and 0.7%.e increase of the joint spacing in the real rock mass leads to the decrease of the joint density.However, due to the constant sample size in this experiment, some joint spacing changes did not result in an increase in joint density.In this test, the width of specimens is 100 mm, and the parallel joint specimens of spacing of 29 mm and 36 mm both have 3 rows joints.erefore, the peak shear strength of specimens with 29 mm and 36 mm joint spacing in the same joint inclination is similar.
e peak shear strength of the joint specimen is related to its corresponding failure mode.It can be seen from the failure modes of the joint specimens in Table 2 that, with the increase of joint spacing, the number for failure mode IV of specimens increase.At α � 60 °, the joint spacing increases from 15 mm to 22 mm, and the failure mode transforms from failure mode II to failure mode IV.At α � 75 °, the specimen joint spacing increases from 22 mm to 29 mm, and the failure mode changes from failure mode III to failure mode IV.At α � 90 °, the specimen joint spacing increases from 29 mm to 36 mm, and the failure mode transforms from failure mode III to failure mode IV.At the same joint inclination, the peak shear strength of specimens with failure mode IV is larger than the other samples with the other failure modes, so the peak shear strength of joint specimens increases with the increase of the joint spacing.

Shear Stress-Displacement Curve and AE Analysis
Based on the study of the failure mode and strength of the joint specimen, the shear stress-displacement curve combined with the acoustic emission is used to quantitatively investigate the crack initiation, expansion, and penetration of the specimen.e regular crack development is studied in detail which will help to observe the in uence of joint geometry on the curve and the correlation between shear stress-displacement curve and failure mode and strength.
e study will deepen the understanding of the mechanical behavior of joints under compressive shear stress.
From the start to the failure during testing, the shear stress-displacement curve of samples combined with acoustic emission counts (Figure 9) can be divided into four typical stages: the microfracture closure stage (OA), the elastic stage (AB), the rupture stage (BC), and the postrupture stage (CD).At the microfracture closure stage, the sample is gradually compost under the load due to the closed joints and voids, and the early nonlinear curves are formed.
e acoustic emission activity is active, the counting value is greater, and the range of change is larger.During the elastic stage, the shear stress and displacement of the rock bridge are linearly related to each other.e interior of the rock bridge is lled with a large number of elastic properties under loading.e slope of the elastic stage is obviously higher than that of the microfracture closure stages.As the crack development reduces, the acoustic emission reduces and becomes stable.In the rupture stage, the curve slope gradually decreases, and the  Advances in Civil Engineering curve reaches the peak shear strength.e cracks of the sample joint tip continue to expand and coalescence through overcoming the rock bridge resistance.e active degree of acoustic emission gradually increases.e count of acoustic emission starts to increase from the point B and shows a geometric increase afterwards until the maximum count appears at this stage.In the postrupture stage, as the displacement increases, the shear stress decreases continuously.e rock bridge is broken to form the destructive plane which leads to the sudden drop of stress caused by the local frictional slippage.Finally, the count of acoustic emission begins to decrease.

In uence of Joint Inclination on Shear Stress-Displacement
Curve and AE Analysis.e shear stress-displacement curves (Figure 9(a)) have some di erences especially in the postrupture stage with the changes of joint inclination.Taking the overlap of 15 mm as an example, in the postrupture stage, the sudden drop of stress occurs only at the sample of 45 °joint, and the stress at the other inclinations decrease slowly.
e descending rate of the curve with di erent samples of joint inclination is di erent, and the descending rate of the stress is related to the failure mode of the specimen.e failure mode of the 45 °specimen is similar integrity shear failure.Due to the sudden release of a large amount of energy inside the sample, the stress-displacement curve produces the phenomenon of sudden descent of stress.
e failure modes of 30 °and 60 °specimens are shear failures along the joint plane.Due to the release of aggregated energy, the descending rate of the curve in the postrupture stage is relatively slower than that of 45 °. e failure modes of 75 °and 90 °specimens are shear failures along the shear stress plane.
e peak shear strength of the specimen is small.During the loading process, the cracks develop evenly and the energy accumulation is less.erefore, the descending rate of the curve in the postrupture stage is relatively small.e failure modes of the 0 °specimen are the coplanar shear failure.
e stress-displacement curve is relatively gentle, and the descending rate is the smallest.On the whole, the descending rates of the curve are positively correlated with the peak strength of the sample.e stressdisplacement curves of the specimen with the overlap of 0 mm appear single or multilevel stresses abruptly in the postrupture phase.e curves of specimens at the joint dip angle of 45 °, 30 °, 60 °, and 0 °occur the phenomenon of singlestage stress drop, and the curves of samples at the 75 °, 90 °appear multilevel stresses abruptly.e relationship between the descending rate of the curve and the joint inclination of the sample is similar to that of the sample of 15 mm overlap.Compared with the specimens in the di erent overlap, the curves of samples with 0 mm overlap are prone to sudden stress drop, and the descending rate is greater.On the whole, the descending rates of the curve are positively correlated with the peak strength of the sample.
According to the change of acoustic emission count (Figure 9(b)), the acoustic emission activities can be divided into the active period, the clam period, and the acute period.
e acoustic emissions of samples with di erent inclinations have certain di erences.When the joint dip angle is 0 °, the acoustic emission is relatively active in the stage of microcrack closure and enters the clam period in the elastic stage and then the acute period with geometric growth in the rupture and postrupture stage.Compared with the acoustic emission of the 0 °joint sample, the acoustic emission count of the 30 °joint sample changes greatly in the active period and decreased rapidly due to the sudden stress drop in the postrupture.e clam period of the 45 °specimen is relatively short.
e AE count progressively increases in the acute period which is related to the more development of the crack before the failure of the specimen.For the 75 °joint specimen, due to the multilevel stress drop in the stressdisplacement curve, the acoustic emission counts also show multiple peaks.On the whole, the variation law of acoustic emission is consistent with the joint dip angle, and the joint angle of the specimen plays a key role in the failure mode.erefore, the failure mode is corresponding to the variation of the acoustic emission count.

In uence of Overlap Length on Shear Stress-Displacement
Curve and AE Analysis.As shown in Figure 10(a), the smaller the overlap length is, the larger the peak shear strength and displacement are.Even the curve shapes of the sample for di erent overlap lengths are similar, and the curve slopes change especially in the postrupture stage.For example, the curves of the di erent overlap are gentle when α 0 °. e curves decrease gradually in the postrupture stage.e smaller the overlap is, the faster the descent rate is.At α 45 °, the specimens have a signi cant stress drop in the postrupture stage.At the same time, with the decrease of overlap, the value of stress drop increases.
As shown in Figure 10(b), at α 0 °, the variation rules of acoustic emission counts with the di erent overlap are similar, and the peak value of the AE count decreases with the increase of the joint overlap.e similar variation is due to the same failure mode of the specimens.e peak value of the AE count is related to the shear strength of samples.e Advances in Civil Engineering shear strength of the specimen increases with the decrease of the joint overlap.e larger the peak shear strength is, the greater the energy of the emission is.us, the peak value of the AE count is greater.

In uence of Joint Spacing on Shear Stress-Displacement Curve and AE Analysis.
e shear stress-displacement curve (Figure 11(a)) shows that the larger the joint spacing is, the larger the peak shear strength is.e curves of samples with the di erent joint spacing are similar.However, the slope of the curve varies with the change of the joint spacing.For the specimens with 0 °joint inclination, the curves of the samples with the di erent joint spacing change gently and decrease gradually.e larger the spacing of joints, the faster the rate of descent is.For the specimens with 60 °joint inclination, the curves have a suddenly descent of stress in the postrupture stage, and the larger the joint spacing, the greater the value of the sudden drop in stress.
As shown in Figure 11(b), the variation rules of acoustic emission counts correspond to the shear stressdisplacement curve of samples.For example, the curve of the S-15-22-45 sample produces two sudden drops of stress, and the varied form of the acoustic emission count also has two peaks.e variation rules of acoustic emission counts with the di erent spacing are similar, and the peak value of the AE count increases with the increase of the joint spacing.
e similar variation is due to the same failure mode of the specimens.e peak value of the AE count is related to the shear strength of samples.e shear strength of the specimen increases with the increase of the joint spacing.e larger the peak shear strength is, the greater the energy of the emission is.us, the peak value of the AE count is greater.

Conclusions
A series of jointed rock mass specimens prepared with rocklike materials were utilized to investigate the e ect of the parallel joint on the mechanical behavior of rock mass under compression shear.From the experimental results, the following conclusions can be drawn: (1) According to the relationship between the failure surface and the shear stress plane and the joint surface, four basic failure modes of the specimen are coplanar shear failure, shear failure along the joint plane, shear failure along the shear stress plane, and similar integrity shear failure.e joint dip angle plays a decisive role in the failure mode of the specimen.
e joint overlap a ects the crack development of the specimen but does not change the failure mode of the specimen.e joint spacing can change the failure mode of the specimen, and the degree of in uence is related to the joint inclination.
(2) e peak shear strength increases rstly and then decreases with the increase of joint dip angle and reaches the maximum value at 45 °.With the increase of joint overlap, the peak shear strength decreases.e peak shear strength of specimens increases with the increase of joint spacing.
(3) e shear stress-displacement curve of samples combined with acoustic emission counts can be divided into four typical stages: the microfracture closure stage, the elastic stage, the rupture stage, and the postrupture stage.e shear stress-displacement curve of di erent joint inclination samples is different, and it mainly occurs in the postrupture stage.e shear stress-displacement curves of specimens in the same failure modes are similar.(4) e variation of the AE count for different inclination joint specimens is consistent with the failure mode of the specimen.e variation rules of acoustic emission counts with the different overlap are similar, and with the joint overlap increasing, the peak value of the AE count decreases.e peak value of the AE count increases with the increase of the joint spacing.

Figure 2 :Figure 3 :Figure 4 :
Figure 2: e device of the eld test and schematic diagram.

Figure 5 :
Figure 5: Failure modes of jointed rock mass.

Figure 6 :igure 7 :
Figure 6: E ect of the inclination angle of the joint on the peak shear strength.

igure 8 :Figure 9 :
Figure 9: e e ect of joint inclination on the shear stress-displacement curve and AE count: (a) the shear stress-shear displacement curves of di erent joint inclinations; (b) the variation of acoustic emission count for di erent angle specimens.

Figure 10 :
Figure 10: e e ect of joint overlap on the shear stress-displacement curve and AE count: (a) the shear stress-displacement curve of samples containing di erent joint overlap lengths; (b) the variation of acoustic emission counts for di erent overlapping specimens.

Figure 11 :
Figure11: e e ect of joint spacing on the shear stress-displacement curve and AE count: (a) the shear stress-shear displacement curves with the di erent joint spacing; (b) the variation of acoustic emission count for di erent spacing specimens.

Table 1 :
Mechanics parameters of rock-like material.

Table 2 :
Failure modes for specimens with the different joint inclination and joint spacing.

Table 3 :
e shear strength of joint specimens.