Shear Strength Behavior of Sand Reinforced by Kraft Paper Using the Ring Shear Apparatus

To explore the reinforcement effects of different reinforcement methods, kraft paper was used as reinforcement material, and shear tests were carried out in sand to study the reinforcement effects of kraft paper perpendicular and parallel to the shear plane. *e test results show that the two reinforcement methods can effectively improve the strength of sand and the orthogonal reinforcement form is more superior. *e existence of reinforced materials greatly improves the cohesion of sand, but does not significantly improve the internal friction angle. *e width of reinforcement material has little effect on the reinforcement effect and shows different variation laws under different reinforcement forms.

With the development of geosynthetics technology, an increasing number of new materials are being used [9][10][11][12], with satisfactory results. In recent years, reinforcement technology has been widely used in construction in China. Reinforced structures such as reinforced retaining walls and reinforced subgrades effectively improve the tensile and shear strength of the earth fill and reduce the engineering cost to some extent. In collapsible loess or cohesive force sandy soil areas, reinforcement materials enhance the overall construction stability. Research on reinforcement materials, reinforced soil, and reinforcement methods has been advancing rapidly. e interaction between reinforcement material and soil is complex due to the diversity of physical and mechanical properties [13,14]. To some extent, the interaction affects the strength and deformation characteristics of reinforced soil.
Research on strength deformation characteristics of reinforced soil traditionally uses a three-axis test [15][16][17]. In [18], horizontal and vertical three-axis shear tests were conducted on unconsolidated reinforced saturated sand and its strength characteristics and failure modes were studied. e results showed that the shear strength of the reinforced sand increased significantly. In [19], the shear strength of reinforced soil under different reinforcement conditions was studied through three-axis UU shear tests. e results showed that reinforcement effectively increased cohesion, but had little effect on the internal friction angle. In [20], three-axis creep tests were conducted on plain soil and reinforced soil with different confining pressures and reinforced layers to explore the long-term degeneration characteristics of reinforced soil and the reinforcement mechanism involved. To study the strength deformation characteristics of coarse-grained soil and reinforced coarse-grained soil, three-axis shear tests were conducted in [21,22]. In [23], a GDS dynamic threeaxis system was used to study the deformation characteristics of reinforced loess under different dynamic loads. e results showed that the antivibration performance of soil is improved with an increase in the number of reinforced layers. Tests have been conducted to explore the interaction between geosynthetics and soil [24][25][26][27][28][29], [30] and mathematical models have been proposed [31][32][33][34][35][36][37]. Although model tests can help in understanding the dynamic response characteristics of reinforced soil, numerical simulation results are more intuitive [38][39][40][41][42][43][44][45][46]. In [47], [48], [49,50], and [51], the two-dimensional barrier was broken to conduct three-dimensional discrete element analysis. Numerical analysis has been regarded as an effective and convenient means of studying the behavior of different types of structures under self-weight and loading conditions in geotechnical engineering.
Under external load, the interaction between the reinforcement material and soil is reflected in the frictional resistance produced by their surfaces coming into contact. e mechanical properties of the soil are changed, and the deformation characteristics of the reinforced soil are influenced. Moreover, different methods of reinforcement, stress level, and stress-strain state greatly influence the interaction between the reinforcement material and the soil; the stress-strain relationship becomes complex, which further affects the shear strength of the reinforced soil. us, accurate determination and selection of the shear strength parameters of reinforcement materials must be considered during the construction process [52]. e ring shear apparatus has a uniform stress distribution owing to its special structure. It has certain advantages when analyzing the stress, strain, and shear strength properties of each contact surface under large displacement. In [53][54][55], the interfacial shear properties of different reinforcement materials and different soils were studied using the ring shear test and the differences in shear strength of different reinforcement materials were verified. To explore the dynamic response characteristics of reinforced sand, a series of load tests, tensile tests, and shear tests have been conducted. e results showed that different reinforcement and test methods have different effects on the strength of reinforced sand [56][57][58][59]. Currently, there are few studies on using kraft paper to increase the strength of sand. In [60], kraft paper was added in sand and clay at 5%, 10%, and 15% of dry soil mass. e results show that the content and humidity have significant effects on the improvement of soil strength. Fiber makes the soil ductile and can maintain the shear strength at high strain level. However, further research is necessary due to the complexity of the mechanical behavior of reinforced sand.
On the basis of previous studies, the shear strength of the sand reinforced with kraft paper was measured by using the ring shear apparatus. To explore the reinforcement effect of different reinforcement methods, kraft paper was used as the sand reinforcement material to study the stress-strain relationship and residual strength characteristics of the reinforced soil under large displacements. Controlling the relative density of the sand, the ring shear tests were conducted to assess the strength and deformation characteristics of kraft-reinforced sandy soil with different widths and different reinforcing methods (parallel shear planes and vertical shear planes). Different widths, two reinforcement methods under different external load stress and strain characteristics, and variations in the shear strength were used to explore the reinforcement effects on the strength of the reinforced soil.

Test Materials.
Dry and clean medium coarse sand was selected for the experiments. e particle size was concentrated from 0.5-2.0 mm. e grain gradation curve is shown in Figure 1. e maximum dry density and minimum dry density of the sand used in the test were 2.26 g/cm 3 and 1.62 g/cm 3 , respectively. e reinforcement material used in the test was kraft paper meeting the 100 g specification of the nonstripe envelopes specified. e kraft paper is flexible, strong, environmentally friendly, and durable as a food packaging material. It has some antiwet deformation ability and can withstand low temperature as well as thermal expansion and contraction; these properties prevent its deformation, distortion, and wrinkling. Kraft paper made of wood fiber and resin binder has high tensile strength, rupture strength, and dynamic power. e tensile strength of the kraft paper per unit width measured by using a digital material testing machine was 80 N/cm, with the strain of 0.67 at failure.

Test Instrument.
Ring shear apparatus plays an important role in the research of the landslide disaster mechanism [61][62][63]. e ring shear apparatus used in this test was a dynamic ring shear apparatus of SRS-150 unsaturated soil produced by GCTS Testing Systems in the United States. e inner ring diameter of the shear box was 100 mm, outer ring diameter was 150 mm, effective height was 25 mm, shear area was 98 cm 2 , shear plane was constant, and shear displacement was continuously maintained. e ring shear apparatus can be used to determine the shear strength and residual strength parameters under large displacements. e maximum axial pressure of the ring shear device was 10 kN, and the shear rate range was 0.001-1 rev/ min. e operating principle of the ring shear instrument is shown in Figure 2.

Test Scheme.
In this experiment, the ring shear instrument was used to study the reinforcement effect of kraft paper as the reinforcement material for a sandretaining wall. Different from the linear failure formed by the unreinforced sand-retaining wall, the failure surface of the reinforced sand-retaining wall is an approximate circular shear surface, as shown in Figure 3. At the top of the slope, the reinforced material is in orthogonal contact with the failure surface, and at the bottom of the slope, the reinforced material is in nearly parallel contact with the failure surface. erefore, two reinforcement forms of kraft paper and shear plane are designed, which are orthogonal and parallel. e reinforcement of the sample is shown in Figure 4. e experiment takes two forms of reinforcement and carries out the four widths of kraft paper under four vertical stresses. A total of 32 sets of tests are carried out (Table 1).

Analysis of Stress-Strain Characteristics.
According to the test scheme, the ring shear test was conducted on the vertical pressure of the reinforced sand and the nonreinforced sand under conditions of 15 kPa, 30 kPa, 40 kPa, and 50 kPa. e relationship between the shear stress and shear displacement of nonreinforced sand under different pressure conditions is shown in Figure 5. Under different vertical pressures, the shear time is a constant concussion of the wrong curve. e shear stress-shear displacement curve [64] for cohesive soils such as loess is very different; the sand particles differ in size, and the cohesive force is almost zero. When the constant vertical pressure is subjected to torsion shear, the reciprocal extrusion between the particles on the shear surface is turned over and friction dislocation occurs. e shear strength of the sand is in the range of the lower boundary of the shear stress-shear displacement curve. In this study, the shear strength of the sand is defined by the peak point of the upper boundary curve.
As shown in Figure 5, under different vertical pressures, the shear deformation of nonreinforced sandy soil is fast; the shear stress reaches the peak value. A brief strain softening occurs later, and the shear force decreases. With an increase in shear displacement, shear stress continues to increase in the later stage, which is characterized by strain hardening. e hardening property is more obvious when the vertical pressure is low; the vertical constraint during sand shear is smaller between particles over the required resistance, dislocation, and flip. e dilatancy effect between particles is relatively obvious in the shear process of the nonreinforced sand.
e compression displacementshear displacement curve is shown in Figure 6. Compared with the trend of the holistic shear shrinkage stress under a vertical pressure of 50 kPa, when the vertical pressure is 15 kPa and 30 kPa, the compression curves indicate dilatancy, shear contraction, and an oscillatory trend. us, to overcome the sand friction, dislocation resistance, and dilatancy, the sand is turned over and cut. Under a low vertical pressure, the shear stress increases with an increase in shear displacement. When the shear stress reaches the second peak with an increase in shear displacement, the nonreinforced sand enters the strainsoftening region. e shear stress-shear displacement curves for two reinforcement methods under different vertical pressures and with different reinforcement material widths are shown in  e shear stress-shear displacement curve of the reinforced soil is the same as that of the nonreinforced sandy soil. e shear stressshear displacement curve of kraft paperreinforced sand is also a shock dislocation curve. In the shear stress-shear displacement curve of kraft paper-reinforced sand with a vertical pressure of 50 kPa and a kraft paper width between 0.2 cm and 0.6 cm, the region of peak shear strength is reached. e vibration characteristic curve of shear stress in the 6-8 mm region of shear displacement is shown in Figure 15. e shear stress vibration amplitude of the vertical-reinforced sand is larger than that of the horizontal-reinforced sand and the nonreinforced sand, indicating that vertical reinforcement is a spatial constraint in the process of sand friction reversal that effectively increases the frictional resistance between sand particles and promotes the "enhancement effect" of shear stress in the process of sand particle turning.
is pattern of reinforcement is parallel to the horizontal ring fabric of a shear plane due to a reduced friction fault on the shear plane under the soil particles. e contact surface between the kraft paper and sand particles is smoother; the shear stress-shear displacement curve of the relative volatility is smaller compared with the vertical-reinforced sand, especially at 0.6 cm. e horizontal annular kraft paper-   Advances in Civil Engineering reinforced sand shear stress-shear displacement curves for the shear stress range of shocks are much less than those for the nonreinforced sand shear stress range of shocks. Comparing Figures 5, 8, 10, 12, and 14, it can be seen that (1) the characteristics of the softened area of the verticalreinforced sand are not obviously different from those of the nonreinforced sand. e shear stress and shear displacement curves enter the strain-hardening area after shear initiation.
(2) e strain-hardening characteristics of vertical kraft-reinforced sand are more obvious than those of nonreinforced sand. Under higher pressure, vertical reinforcement can help sand continuously resist greater shear stress during shearing.
Comparing Figures 5,7,9,11, and 13, it can be seen that (1) the early strain-softening characteristics of horizontal-reinforced sandy soil parallel to the shear plane are different from those at different vertical pressures. At high pressure, the early strain-softening characteristics are less than those of nonreinforced sand. Under the restraining effect of 40 kPa and 50 kPa, the reinforcement effect is evident at the beginning of shear, and the frictional resistance of the sand particles on the shear surface continuously increases. (2) e shear stress-shear displacement curve strain-hardening characteristic of horizontal-reinforced sand is influenced by the width of the reinforcement material. When 0.6 cm kraft paper is used as the reinforcement material, the strain-hardening characteristics of the shear stress-shear displacement curve are lower than those of nonreinforced sand, due to the reduction in the friction force of the sand particles from placing the reinforcement material parallel to the shear surface. When the vertical pressure is relatively high, the strain-hardening phenomenon is more obvious when narrow vertical kraft paper is used as the reinforcement material. When the vertical pressure is relatively high, the strain-hardening characteristics decrease, and overall stiffness is observed. When a relatively narrow reinforcement material is placed parallel to the shear plane, the frictional resistance between the sand particles and the reinforcement material can be enhanced to some extent by higher pressure.
From the analysis, the shear strength of the nonreinforced sand is defined as the first peak point in the strain-softening zone. e shear strength of the verticalreinforced sand and the horizontal-reinforced sand is defined as the peak point of the strain-hardening zone.

Analysis of Strength Characteristics of Sands Reinforced by
Kraft Paper. Figure 16 shows the shear strength curve of vertical kraft paper-reinforced sand with different widths. e shear strength curve of vertical-reinforced sand is higher than that of nonreinforced sand, indicating that the vertical reinforcement method enhances the shear strength of sand. However, the effect of reinforcement on the shear strength of sand is more obvious under low pressure; 0.2 cm stiffened kraft paper exhibits the best effect. e shear strength is higher under different vertical pressures; when the 0.8 cm kraft paper is used as the reinforcing material, the reinforcement effect is most stable under different vertical pressures. e difference between the shear strength and the nonreinforced sand shear strength (approximately 11 kPa) is generally stable with increasing vertical pressure. Moreover, except for the optimum bar width of 0.2 cm, the    Figure 12. Figure 17 shows the fitting curve of the shear strength and the vertical pressure of the horizontal annular kraft paper with different widths. When the kraft paper is placed in a ring parallel to the shear surface, the shear strength of sand is enhanced to some extent. e kraft paper with 0.4 cm width is the best for enhancing the shear strength of sand. e shear strength is greatest under vertical pressure. e reinforcement effect of 0.2 cm width kraft-reinforced sand is more stable. e shear strength of the kraft-reinforced sand under different vertical pressures is essentially the same as that of the nonreinforced sand (approximately 9 kPa). When the kraft paper is parallel to the shear plane, except for the optimum 0.4 cm bar width, the reinforcement effect on the shear strength (with bar widths of 0.2 cm, 0.6 cm, and 0.8 cm) is more obvious with decreasing kraft paper width.
To evaluate the influence of different reinforcement widths and reinforcement methods on the shear strength of sands, the reinforcement effect of the kraft paper on the shear strength of sand is evaluated. C is the cohesiveness of the nonreinforced sand, Cr is the cohesive force of the reinforced sand, φ is the internal friction angle of the nonreinforced sand, and φr is the internal friction angle of the reinforced sand. e cohesion growth rate of the reinforced sand is defined as εc � (cr−c)/c, and the relative error rate of the internal friction angle of the reinforced sand is φr � (φr−φ)/φ, as shown in Table 2. Figures 14 and 15 show the relation curves of the cohesive force and the internal friction angle with the reinforcement width of the reinforced sand for the two reinforcement methods. From Table 2, we know that, in this test pressure range, using kraft paper as the reinforcement material can significantly enhance the cohesive force of sand; its cohesive force growth rate reaches 2025%-4950%, which has some effect on the internal friction angle. e relative error range of the internal friction angle is −16.2%-−5.41%.
From Table 2 and Figure 18, we can see that (1) the cohesive force of the reinforced sand with 0.2 cm width is greatest when perpendicular to the vertical reinforcement of the shear surface. At this time, the cohesion growth rate reaches 4950%. When the width of the bar was 0.4 cm, the cohesive force of the reinforced soil was the lowest; in the later stage, the cohesion showed a    Advances in Civil Engineering gradual increasing trend with increasing reinforcement width.
(2) When placed in parallel, reinforced soil cohesion increases with an increase in reinforcement width from 0 cm to 0.4 cm. e maximum cohesion occurs when the reinforcement width is 0.4 cm; the cohesive strength growth rate is 4250%. (3) e cohesion growth rate of the vertical reinforcement is higher than that of the horizontal ring reinforcement under equal reinforcement width. Under the optimal reinforcement condition, the cohesion growth rate of the vertical reinforcement is higher than that of the horizontal ring reinforcement, indicating that vertical reinforcement is more effective than horizontal ring reinforcement in improving the cohesive force of sand.
From Table 2 and Figure 19, we can see that although the internal friction angle is less influenced by the width of the reinforcement material, the change trend is different with different reinforcement methods. When the kraft paper is parallel to the shear surface, the internal friction angle decreases with decreasing reinforcement width. When the kraft paper is perpendicular to the shear plane of the vertical reinforcement and the reinforcement is 0.2 cm-0.8 cm, the internal friction angle increases with increasing reinforcement width, indicating that, with reinforcement parallel to the horizontal shear plane ring, the friction between the sand particles continuously decreases with increasing width in this test pressure range.

Conclusions
In this paper, a comprehensive set of ring shear tests were carried out on samples of sand reinforced with parallel and vertical elements. e following conclusions are drawn from the results: (1) e nonreinforced sand in the ring shear process curves of shear stress-shear displacement exhibits a shock slip curve; the increase in the shear displacement curve is divided into strain-softening and strain-hardening zones. Strain-softening and strain-hardening characteristics decrease with vertical stress. e curve enters the strain-hardening area after shear start up, and the strain-hardening property is enhanced compared with nonreinforced soil. (2) e amplitude of the shear stress-shear displacement curve for kraft paper-reinforced sand is different for different reinforcement methods. e amplitude for reinforced sand with vertical reinforcement on the shear plane is significantly larger than for horizontalring-reinforced sand. (3) e two reinforcement methods enhance the shear strength of sand. However, the optimum width of the horizontal shaft parallel to the shear plane was 0.4 cm; the optimum width of the vertical bar perpendicular to the shear plane was 0.2 cm. (4) In this test pressure range, using kraft paper as the reinforcement material significantly enhances the cohesive force of sand; the internal friction angle has little influence on the width of the bar, and the trend of the change is different with different reinforcement methods.

Data Availability
Research data can be obtained from the corresponding author at any time.

Conflicts of Interest
e authors declare no conflicts of interest regarding the publication of this paper.

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