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Mehdi Benziane, N. Della, Sidali Denine, S. Sert, Said Nouri (2019)
Effect of randomly distributed polypropylene fiber reinforcement on the shear behavior of sandy soilStudia Geotechnica et Mechanica, 41
Ren-peng Chen, Z. Xu, Yunmin Chen, D. Ling, B. Zhu (2010)
Field Tests on Pile-Supported Embankments over Soft GroundJournal of Geotechnical and Geoenvironmental Engineering, 136
D. Gray, H. Ohashi (1983)
Mechanics of Fiber Reinforcement in SandJournal of Geotechnical Engineering, 109
C. Benjamim, B. Bueno, J. Zornberg (2007)
Field monitoring evaluation of geotextile-reinforced soil-retaining wallsGeosynthetics International, 14
J. Zornberg (2002)
Discrete framework for limit equilibrium analysis of fibre-reinforced soilGeotechnique, 52
R. Noorzad, Seyed Zarinkolaei (2015)
Comparison of Mechanical Properties ofFiber-Reinforced Sand under Triaxial Compressionand Direct ShearOpen Geosciences, 7
R. Jewell, C. Wroth (1987)
Direct shear tests on reinforced sandGeotechnique, 37
Zhiwei Gao, Jidong Zhao (2013)
Evaluation on Failure of Fiber-Reinforced SandJournal of Geotechnical and Geoenvironmental Engineering, 139
R. Michalowski (2008)
Limit analysis with anisotropic fibre-reinforced soilGeotechnique, 58
A. Diambra, A. Russell, E. Ibraim, D. Wood (2007)
Determination of fibre orientation distribution in reinforced sandsGeotechnique, 57
R. Michalowski, J. Cermák (2003)
Triaxial Compression of Sand Reinforced with FibersJournal of Geotechnical and Geoenvironmental Engineering, 129
A. Ghaly (1995)
Reinforcing Sand with Strips of Reclaimed High-Density PolyethyleneJournal of Geotechnical Engineering, 121
F. Ahmad, Farshid Bateni, M. Azmi (2010)
Performance evaluation of silty sand reinforced with fibresGeotextiles and Geomembranes, 28
Hong-tao Jiang, Yi-Tao Cai, Jin Liu (2010)
Engineering Properties of Soils Reinforced by Short Discrete Polypropylene FiberJournal of Materials in Civil Engineering, 22
E. Ibraim, A. Diambra, A. Russell, D. Wood (2012)
Assessment of laboratory sample preparation for fibre reinforced sandsGeotextiles and Geomembranes, 34
J. Bray, D. Zekkos, E. Kavazanjian, G. Athanasopoulos, M. Riemer (2009)
Shear Strength of Municipal Solid WasteJournal of Geotechnical and Geoenvironmental Engineering, 135
(2004)
Engineering properties of sand reinforced with strips from waste plastic
N. Consoli, K. Heineck, M. Casagrande, M. Coop (2007)
Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress PathsJournal of Geotechnical and Geoenvironmental Engineering, 133
(2015)
Comparison of mechanical properties of fiber-reinforced sand under triaxial compression and direct shear
applied sciences Article The Influence of Discrete Fibers on Mechanical Responses of Reinforced Sand in Direct Shear Tests 1 1 , 2 1 , 1 , 2 1 Chidochashe Clemency Nhema , Han Ke , Pengcheng Ma *, Yunmin Chen and Shiyu Zhao MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering, Institute of Geotechnical Engineering, Zhejiang University, Yuhangtang Road 866#, Hangzhou 310058, China; chidochashen@hotmail.com (C.C.N.); boske@126.com (H.K.); chenyunmin@zju.edu.cn (Y.C.); elena_zsy@163.com (S.Z.) MOE Key Laboratory of Soft Soils and Geoenvironmental Engineering, Center for Hypergravity Experimental and Interdisciplinary Research, Institute of Geotechnical Engineering, Zhejiang University, Yuhangtang Road 866#, Hangzhou 310058, China * Correspondence: mapengcheng@zju.edu.cn Abstract: To investigate the influences of discrete fiber strips on the mechanical properties of rein- forced sand, a series of direct shear tests were conducted. A method to strictly control the initial orientation of fiber strips in specimen preparation was developed. Under the same normal pres- sure, the peak strength of sand specimens was proportional to the fiber content and was inversely proportional to the fiber initial orientation angle. The influences of initial fiber orientation on peak strength may depend on the stress mobilization in fibers. When the fiber strips distributed at a certain orientation angle were subjected to tensile stress in shearing, they could play an effective role in the peak strength gain of sand and vice versa. Due to the restriction of fibers on the volume dilation of sand specimens, the residual strength of reinforced sand also increased. However, the initial stiffness of reinforced sand was smaller than that of pure sand, which may be related to the precompression of flexible fiber strips and the density inhomogeneity of specimens induced in the specimen preparation process. In addition, the ductility of sand specimens was improved by the Citation: Nhema, C.C.; Ke, H.; introduction of fiber strips, intuitively reflected by the increase in displacement failure. This may Ma, P.; Chen, Y.; Zhao, S. The Influence of Discrete Fibers on also be attributed to the restriction of fiber strips on the deformation of sand specimens. Mechanical Responses of Reinforced Sand in Direct Shear Tests. Appl. Sci. Keywords: reinforced sand; direct shear test; strength; initial stiffness; ductility 2021, 11, 8845. https://doi.org/ 10.3390/app11198845 Academic Editor: Daniel Dias 1. Introduction Reinforced soil is widely used in the field of geotechnical engineering [1–4]. As a type Received: 30 August 2021 of discontinuous granular material, the tensile strength of soil is almost negligible. Through Accepted: 17 September 2021 the introduction of fiber materials, which provide tensile resistance, the mechanical prop- Published: 23 September 2021 erties of soil can be improved significantly. On one hand, fibers can increase the strength of soil; on the other hand, they can decrease the deformation of geotechnical structures. Publisher’s Note: MDPI stays neutral Compared with continuous reinforced materials such as geogrids and geotextiles, discrete with regard to jurisdictional claims in fiber materials such as fiber strips may be more flexible and convenient for application published maps and institutional affil- in some situations. In addition, some fiber strips can be made using reclaimed materials, iations. which is environmentally friendly [5,6]. Fiber content and orientation are important factors that influence the engineering performance of reinforced soil. [7,8] found that the reinforcement effects of fibers increased with fiber content within a certain range. Many researchers conducted experimental and Copyright: © 2021 by the authors. theoretical studies regarding the mechanical properties of reinforced soil with different Licensee MDPI, Basel, Switzerland. main directions of fiber distribution [9–11], generally reporting that the influence of fiber This article is an open access article orientation on the reinforcement effects on soil depends on the stress mobilization in fibers. distributed under the terms and In addition, other factors related to fibers, such as aspect ratio, stiffness, and surface friction conditions of the Creative Commons properties, can also affect the reinforcement efficiency [7,12]. Attribution (CC BY) license (https:// In this study, a series of direct shear tests of sand specimens reinforced with cotton creativecommons.org/licenses/by/ fiber strips were conducted. A method to strictly control the initial orientation of flexible 4.0/). Appl. Sci. 2021, 11, 8845. https://doi.org/10.3390/app11198845 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 8845 2 of 11 fiber strips was developed in the process of specimen preparation. Based on the test results, the influences of fiber content and orientation on the mechanical properties of sand specimens, including strength, initial stiffness, and ductility, were discussed. This study with controlled variables can be helpful to understand the strengthening mechanisms of fibers in reinforced sand, although the situations in practice are much more complicated than the tests. 2. Test Arrangement Extensive series of direct shear tests were performed on sand specimens reinforced with cotton fiber strips to investigate their mechanical characteristics. The sand tested was Fujian standard sand, whose mechanical and physical properties are presented in Table 1. The range of particle size was from 0.1 to 1 mm. The specimens were 30 mm in thickness and 60 mm in diameter. The fibers used in this study were commercially purchased woven strip cotton flexible fibers with a thickness <0.5 mm and width of 10 mm. The strip fibers had a high tensile strength (usually greater than 50 MPa) and negligible compressive strength and flexural stiffness. The length of fiber strips was selected as 30 mm, which was equal to the thickness of the sand specimens. Table 1. Physical properties of Fujian standard sand. Parameter Value Effective size, D (mm) 0.25 Mean particle size, D (mm) 0.37 Uniformity coefficient, C 1.92 Coefficient of curvature, C 1.40 Maximum dry density, r (g/cm ) 1.65 max Minimum dry density, r (g/cm ) 1.33 min Maximum void ratio, e 0.73 max Minimum void ratio, e 0.50 min Grain density, r (kg/m ) Specific gravity, G 2.63 To investigate the influence of initial orientation and content of fiber strips on the mechanical responses of sand specimens, they were selected as controlled variables in the direct shear tests. The initial orientation angel of fiber relative to the shear plane was the angle i shown in Figure 1. In this study, the initial orientation angles of 60 , 90 , and 120 were selected. Note that a new method to place the fiber strips was developed to maintain the flexible fiber strips at a predetermined orientation angle during the process of specimen preparation. As shown in Figure 2a, some plexiglass plates with orientated slots were manufactured, and some needles were inserted into the slots. The fiber strips were placed on the needles; thus, their orientation angles were determined by the slots. During the specimen preparation, plexiglass plates were first placed into the shear box (see Figure 2b). Then, the sand was introduced using a funnel and densified in layers using a steel sharp wire to rearrange the sand particles into a high packing order, along with small hammer blows to further increase the sand density. More attention was paid to the sand around the fibers to avoid the obvious influences of layered densification on the fibers. When the sand was filled close to the top of fiber strips, the plates and needles were slowly lifted. The fibers remained in the sand since the friction along the needle–fiber interface was negligible in comparison to that along the fiber–sand interface (see Figure 2c). In this method, the fibers in the sand specimens were distributed according to the angle of slots, i.e., the predetermined orientations. To prepare the reinforced specimens with initial fiber orientations of 90 and 60 /120 , plexiglass plates with the slots at 90 and 60 relative to the horizontal direction were needed in this study. The average fiber content included in a reinforced sand specimen was defined as a percentage weight of fibers with respect to the dry unit weight of sand. In this study, unreinforced sand (fiber content = 0%) and reinforced sand with fiber contents of 0.04%, 0.2%, and 0.4% were selected for testing. Note Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 11 Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 11 method, the fibers in the sand specimens were distributed according to the angle of slots, method, the fibers in the sand specimens were distributed according to the angle of slots, i.e., the predetermined orientations. To prepare the reinforced specimens with initial fiber i.e., the predetermined orientations. To prepare the reinforced specimens with initial fiber orientations of 90° and 60°/120°, plexiglass plates with the slots at 90° and 60° relative to orientations of 90° and 60°/120°, plexiglass plates with the slots at 90° and 60° relative to the horizontal direction were needed in this study. The average fiber content included in the horizontal direction were needed in this study. The average fiber content included in a reinforced sand specimen was defined as a percentage weight of fibers with respect to a reinforced sand specimen was defined as a percentage weight of fibers with respect to Appl. Sci. 2021, 11, 8845 3 of 11 the dry unit weight of sand. In this study, unreinforced sand (fiber content = 0%) and the dry unit weight of sand. In this study, unreinforced sand (fiber content = 0%) and reinforced sand with fiber contents of 0.04%, 0.2%, and 0.4% were selected for testing. reinforced sand with fiber contents of 0.04%, 0.2%, and 0.4% were selected for testing. Note that, for each combination of controlled variables, three identical sand specimens Note that, for each combination of controlled variables, three identical sand specimens that, for each combination of controlled variables, three identical sand specimens were were tested, and the average results were analyzed to ensure that the conclusions drawn were tested, and the average results were analyzed to ensure that the conclusions drawn tested, and the average results were analyzed to ensure that the conclusions drawn in this in this study had statistical significance. study had statistical significance. in this study had statistical significance. Figure Figure 1. 1. Schematic Schema diagram tic diagof ram dir of dire ect shearct test shear and orientation test and orient angle. ation angle. Figure 1. Schematic diagram of direct shear test and orientation angle. Figure 2. Preparation of reinforced sand specimens: (a,b) angle positioning of fibers in specimens, and (c) layered filling Figure 2. Preparation of reinforced sand specimens: (a,b) angle positioning of fibers in specimens, and (c) layered filling Figure 2. Preparation of reinforced sand specimens: (a,b) angle positioning of fibers in specimens, and (c) layered filling of sand. of sand. of sand. The relative density D of sand could reach 80% in this study, indicating that the tested sand specimens were typical dense sand. The normal pressures of the direct shear tests The relative density Dr of sand could reach 80% in this study, indicating that the The relative density Dr of sand could reach 80% in this study, indicating that the were 100, 200, 400, and 800 kPa. The shearing rate was set as 0.25 mm/min. The direct tested sand specimens were typical dense sand. The normal pressures of the direct shear tested sand specimens were typical dense sand. The normal pressures of the direct shear shear tests were ended when the horizontal shear displacements were larger than or equal tests were 100, 200, 400, and 800 kPa. The shearing rate was set as 0.25 mm/min. The direct tests were 100, 200, 400, and 800 kPa. The shearing rate was set as 0.25 mm/min. The direct to 4 mm. At this strain level, the shear stresses of most specimens reached steady values shear tests were ended when the horizontal shear displacements were larger than or equal (i.e., residual strength of dense sand). shear tests were ended when the horizontal shear displacements were larger than or equal to 4 mm. At this strain level, the shear stresses of most specimens reached steady values to 4 mm. At this strain level, the shear stresses of most specimens reached steady values 3. Results and Discussions (i.e., residual strength of dense sand). (i.e., residual strength of dense sand). Shear stress–displacement responses of sand specimens in the direct shear tests under the normal pressures of 100 and 400 kPa are presented in Figure 3. Because the tested 3. Results and Discussions 3. Results and Discussions sand was typical dense sand, most specimens exhibited softening behaviors after peak points. To intuitively illustrate the influences of fiber strips on the mechanical behaviors of Shear stress–displacement responses of sand specimens in the direct shear tests Shear stress–displacement responses of sand specimens in the direct shear tests sand specimens, the results of direct shear tests were further analyzed from the aspects of under the normal pressures of 100 and 400 kPa are presented in Figure 3. Because the under the normal pressures of 100 and 400 kPa are presented in Figure 3. Because the strength, stiffness, and ductility. tested sand was typical dense sand, most specimens exhibited softening behaviors after tested sand was typical dense sand, most specimens exhibited softening behaviors after peak points. To intuitively illustrate the influences of fiber strips on the mechanical peak points. To intuitively illustrate the influences of fiber strips on the mechanical behaviors of sand specimens, the results of direct shear tests were further analyzed from behaviors of sand specimens, the results of direct shear tests were further analyzed from the aspects of strength, stiffness, and ductility. the aspects of strength, stiffness, and ductility. Appl. Sci. 2021, 11, 8845 4 of 11 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 11 140 (b) (a) 60°, 100 kPa 60°, 400 kPa Fiber Content Fiber Content Unreinforced Unreinforced 0.04% 0.04% 0.20% 0.20% 0.40% 0.40% 0 12345 01 234 Dispalacement (mm) Displacement (mm) (d) 500 (c) 90°, 400 kPa 90°, 100 kPa Fiber Content Fiber Content Unreinforced Unreinforced 0.04% 100 0.04% 0.20% 0.20% 0.40% 0.40% 0123 4 01 234 5 Displacement (mm) Displacement (mm) 120 (f) 500 (e) 120°, 100 kPa 120°, 400 kPa 40 Fiber Content Fiber Content Unreinforced Unreinforced 0.04% 20 0.04% 0.20% 0.20% 0.40% 0.40% 01 23 4 Displacement (mm) Displacement (mm) Figure 3. Shear stress–displacement responses of sand specimens in direct shear tests under normal pressure of 100 kPa Figure 3. Shear stress–displacement responses of sand specimens in direct shear tests under normal pressure of 100 kPa initial initially ly oriente oriented d at (a at ) 60°, (a) 60 (c,) (90°, and ( c) 90 , ande()e 120 ) 120 ° and under normal pressure of and under normal pressure of 400 400 kPa kPa initially initially oriented at oriented at (b) 60 (b,) 60° (d) 90 , (d , ) 90°, and (and f) 120°. (f) 120 . 3.1. Peak Strength 3.1. Peak Strength The peak strength of sand is the maximum shear stress reached during the shearing The peak strength of sand is the maximum shear stress reached during the shearing process, which is an important feature to measure its mechanical performance. To reflect process, which is an important feature to measure its mechanical performance. To reflect the influences of fiber strips on the peak strength of sand, the peak strength ratios, defined the influences of fiber strips on the peak strength of sand, the peak strength ratios, defined as the ratio of peak shear stress of reinforced sand to that of unreinforced sand [13], were as the ratio of peak shear stress of reinforced sand to that of unreinforced sand [13], were calculated and are listed in Table 2. According to its definition, a peak strength ratio larger calculated and are listed in Table 2. According to its definition, a peak strength ratio larger than 1.0 meant that the fiber strips provided an enhancement effect on the peak strength of sand, whereby a larger peak strength ratio led to a larger enhancement effect of fiber strips. Shear Stress (kPa) Shear Stress (kPa) Shear Stress (kPa) Shear Stress (kPa) Shear Stress (kPa) Shear stress (kPa) Appl. Sci. 2021, 11, 8845 5 of 11 than 1.0 meant that the fiber strips provided an enhancement effect on the peak strength of sand, whereby a larger peak strength ratio led to a larger enhancement effect of fiber strips. Table 2. Peak strength ratio of reinforced sand obtained from direct shear tests. Peak Strength Ratio Normal Pressure, Fiber Content, 0.04% Fiber Content, 0.20% Fiber Content, 0.40% (kPa) 60 90 120 60 90 120 60 90 120 100 1.08 1.07 0.92 1.15 1.09 1.00 1.29 1.20 1.00 200 1.06 1.05 0.98 1.23 1.08 0.97 1.36 1.13 0.97 400 1.02 0.98 0.92 1.09 0.99 0.95 1.16 1.04 0.93 800 1.00 1.00 0.95 1.05 1.02 1.00 1.08 1.05 0.98 As can be observed from Table 2, for the specimens sheared at the same normal pressure and fiber content, the peak strength ratios decreased with the initial orientation angle. For most specimens, the fiber strips initially orientated at 60 and 90 increased the peak strength of sand, as indicated by the peak strength ratios greater than 1.0. The effects of fibers initially orientated at 60 were more significant. In contrast, the fiber strips initially orientated at 120 did not improve and, in some cases, even weakened the sand, as indicated by the peak strength ratios smaller than or equal to 1.0. Similar results were also obtained by [12,14]. As mentioned in Section 1, the influence of initial orientation angle of fiber on sand strength may be explained by the stress mobilization in fibers. As measured by [15], the direction of the principle tensile strains in a dense sand specimen was close to 60 relative to the shear direction. Therefore, the tensile stress in fiber strips initially orientated at 60 in this study could be mobilized most effectively; and the most obvious reinforcement effects were observed. For the fibers initially oriented at 90 , because the normal strain in the oblique plane vertical to the fibers was still in the tensile direction, they still provided a reinforcement effect. However, when the fibers were rotated to an angle such that the compressive strain was more dominant on the corresponding oblique plane, such as in the case of the 120 initial orientation, they had a negative effective reinforcement effect because they were in compression. In this situation, the fibers could even reduce the peak strength of sand because the compressed fibers could reduce the density of nearby sand. For the sand specimens with fiber strips of same initial orientation angle, their peak strength ratios increased with the increase in fiber content. This is consistent with the common perception. When the fiber content and initial orientation angle were the same, the peak strength ratios decreased with the increase in normal pressure. According to the Mohr–Coulomb strength criteria of granular materials, the strength of pure sand increases proportionally with the increase in normal pressure. However, the decrease in peak strength ratios indicated that the strength increments induced by the fiber strips did not increase proportionately with normal pressure. This phenomenon is further discussed below by considering the shear strength envelopes of sand specimens obtained from peak strength data. As shown in Figure 4, with the exception of sand specimens with a fiber content of 0.04% and fiber initially orientated at 120 , the strength envelopes of other specimens were all lifted by introducing fiber strips. In the range of 100–200 kPa, the slopes of strength envelopes of reinforced sand were larger than those of unreinforced sand, indicating that the strength increments induced by fibers had a positive correlation with normal pressure. However, in the range of 200–400 kPa, the strength envelopes of reinforced sand were almost parallel to those of unreinforced sand. In other words, the strength increments induced by fibers tended to be constant with the increase in normal pressure. Increasing the peak strength of unreinforced sand with a constant fiber-induced strength increment led to a decreasing peak strength ratio with the increase in normal pressure, as shown in Table 2. Appl. Appl. Sci. Sci. 2021 2021,, 11 11,, x FO 8845 R PEER REVIEW 6 6 of of 11 11 (b) (a) 800 800 700 700 600 600 500 500 400 400 Fiber Orientation 300 300 Fiber Orientation Unreinforced Unreinforced 200 90 200 100 100 100 200 300 400 500 600 700 800 100 200 300 400 500 600 700 800 Normal Pressure (kPa) Normal Pressure (kPa) (c) 800 Fiber Orientation Unreinforced 100 200 300 400 500 600 700 800 Normal Pressure (kPa) Figure 4. Shear strength envelope of reinforced sand with fiber contents of (a) 0.04%, (b) 0.2%, and (c) 0.4%. Figure 4. Shear strength envelope of reinforced sand with fiber contents of (a) 0.04%, (b) 0.2%, and (c) 0.4%. Similar phenomena were observed from different tests by other researchers [4,12,16–18]. Similar phenomena were observed from different tests by other researchers [4,12,16– It is generally believed that these phenomena are related to the different failure modes of 18]. It is generally believed that these phenomena are related to the different failure modes fibers. When the normal pressure was relatively low, the pullout failure was predominant, of fibers. When the normal pressure was relatively low, the pullout failure was and the frictional resistance was proportional to normal pressure. With the increase in predominant, and the frictional resistance was proportional to normal pressure. With the normal pressure, the yield and break failure of fibers gradually played the dominant increase in normal pressure, the yield and break failure of fibers gradually played the role; hence, the strength increments tended to be irrelevant to the normal pressure. The dominant role; hence, the strength increments tended to be irrelevant to the normal threshold of normal pressure that divides the two failure modes is known as the critical pressure. The threshold of normal pressure that divides the two failure modes is known confining stress. Nevertheless, obvious breakage of fiber strips was not observed in the as the critical confining stress. Nevertheless, obvious breakage of fiber strips was not direct shear tests of this study, indicating that the current test results cannot be well observed in the direct shear tests of this study, indicating that the current test results explained using the above analysis. The exact reasons why the fiber-induced strength cannot be well explained using the above analysis. The exact reasons why the fiber- increment did not continuously increase with normal pressure in pullout failure mode is induced strength increment did not continuously increase with normal pressure in not entirely clear due to the lack of more evidence and further research. It is inferred that pullout failure mode is not entirely clear due to the lack of more evidence and further the interactions between fiber strips and sand particles inhibited the continuous increase research. It is inferred that the interactions between fiber strips and sand particles in frictional resistance under high normal pressures. According to the mechanisms of inhibited the continuous increase in frictional resistance under high normal pressures. frictional resistance generation, one possible reason was that the stress orthogonal to the According to the mechanisms of frictional resistance generation, one possible reason was fiber surface, which was transferred from the sand matrix, did not increase proportionately that the stress orthogonal to the fiber surface, which was transferred from the sand matrix, with the increase in normal pressure subjected to the top of sand specimens, influenced by did not increase proportionately with the increase in normal pressure subjected to the top the interactions between fibers and sand particles. of sand specimens, influenced by the interactions between fibers and sand particles. 3.2. Residual Strength 3.2. Residual Strength The shear stress reaches a steady value after the peak strength point in the direct The shear stress reaches a steady value after the peak strength point in the direct shear tests of dense sand, which is known as residual strength. As illustrated in Figure 3, shear tests of dense sand, which is known as residual strength. As illustrated in Figure 3, fiber strips obviously increased the residual strength of dense sand in this study. The fiber residual strips strength obvious of sand ly incr specimens eased the r was esiselected dual strengt as the h of shear dense stress saat nd in the this displacement study. The of res 4 mm idua (i.e., l streng the maximum th of sand speci shearm displacements ens was select of edsome as the she specimens). ar stresThe s at the ratios displacement of residual of 4 mm (i.e., the maximum shear displacements of some specimens). The ratios of to peak strength were calculated and are plotted in Figure 5. The residual strength is Shear Strength (kPa) Shear Strength (kPa) Shear Strength (kPa) Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 11 Appl. Sci. 2021, 11, 8845 7 of 11 residual to peak strength were calculated and are plotted in Figure 5. The residual strength is a valuable parameter for evaluating the engineering performance of soil because high a valuable parameter for evaluating the engineering performance of soil because high residual strength helps to avoid the continuous failure of soil after a short period of high residual strength helps to avoid the continuous failure of soil after a short period of high load. This, in turn, is helpful in the serviceability design of structures. load. This, in turn, is helpful in the serviceability design of structures. (a) 1.0 (b) 1.0 100 kPa 200 kPa 0.9 0.9 0.8 0.8 Unreinforced Fiber Orientation Fiber Orientation Unreinforced 0.7 0.7 60 0 120 0 0.6 0.6 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 Fiber Content (%) Fiber Content (%) (c) 1.0 1.0 (d) 400 kPa 800 kPa 0.9 0.9 0.8 0.8 Fiber Orientation Fiber Orientation 0.7 0 0.7 Unreinforced 90 Unreinforced 0.6 0.6 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 Fiber Content (%) Fiber Content (%) Figure Figure 5. 5. The The r ratio atio of of res residual idual to peak to peak streng strength th of of sand sand spec specimens imens un under der nonormal rmal pressu pressur res of es ( of a) 100, ( (a) 100, b) 200, (b) 200, (c) 400 (c) , an 400, d and (d) 800 kPa (d) 800 kPa. . As shown in Figure 5, the ratios of residual to peak strength had a positive correlation As shown in Figure 5, the ratios of residual to peak strength had a positive correlation with fiber content. Similar to the change rule of peak strength, the fiber strips initially with fiber content. Similar to the change rule of peak strength, the fiber strips initially orientated at 60 had the most obvious effect on the increase in residual strength, and orientated at 60° had the most obvious effect on the increase in residual strength, and the the fibers initially orientated at 90 had relatively weaker but still obvious effects. Note fibers initially orientated at 90° had relatively weaker but still obvious effects. Note that that the fiber strips initially orientated at 120 slightly increased the residual strength of the fiber strips initially orientated at 120° slightly increased the residual strength of dense dense sand, although it did not help to increase the peak strength. This indicated that the sand, although it did not help to increase the peak strength. This indicated that the increase in residual strength may not only be related to stress mobilization in fiber strips, increase in residual strength may not only be related to stress mobilization in fiber strips, as mentioned above. as mentioned above. The softening of dense sand is closely related to its volume dilation characteristic. The softening of dense sand is closely related to its volume dilation characteristic. When the stress ratio reaches a characteristic state stress ratio, the volume of dense sand When the stress ratio reaches a characteristic state stress ratio, the volume of dense sand changes from contraction to dilation during the shearing process, and then softening changes from contraction to dilation during the shearing process, and then softening occurs. The increase in the residual strength led by the introduction of fiber strips may occurs. The increase in the residual strength led by the introduction of fiber strips may be be attributed to the restraint effect of fibers on sand dilation. The volume change of sand attributed to the restraint effect of fibers on sand dilation. The volume change of sand in in direct shear tests can be observed from the vertical displacement of the top surface of direct shear tests can be observed from the vertical displacement of the top surface of the the specimen. The top vertical displacement of the sand specimens with fibers initially specimen. The t op vertical displacement of the sand specimens with fibers initially orientated at 60 under normal pressures of 400 and 800 kPa are presented in Figure 6. orientated at 60° under normal pressures of 400 and 800 kPa are presented in Figure 6. The positive values denote downward displacement and volume contraction, and vice The positive values denote downward displacement and volume contraction, and vice versa. Typical dense sand usually exhibits volume contraction and then dilation during the versa. Typical dense sand usually exhibits volume contraction and then dilation during shearing process. As illustrated in Figure 6, with the increase in fiber content, the volume the shearing process. As illustrated in Figure 6, with the increase in fiber content, the contraction of reinforced sand was enhanced, and the dilation was inhibited. This reflected volume contraction of reinforced sand was enhanced, and the dilation was inhibited. This the restriction of fiber strips on the movement among sand particles, which was caused Residual Strength (kPa)/Peak Strength (kPa) Residual Strength (kPa)/Peak Strength (kPa) Residual Strength (kPa)/Peak Strength (kPa) Residual Strength (kPa)/Peak Strength (kPa) Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2021, 11, 8845 8 of 11 reflected the restriction of fiber strips on the movement among sand particles, which was by the interaction between fibers and the sand matrix. Under the effects of deformation caused by the interaction between fibers and the sand matrix. Under the effects of restriction, the softening behaviors of reinforced sand specimens were limited, and then deformation restriction, the softening behaviors of reinforced sand specimens were the residual strength increased. limited, and then the residual strength increased. (b) 0.8 (a) 0.8 ° 800 kPa, 60 Fiber Content 400 kPa, 60 Fiber Content Unreinforced Unreinforced 0.04% 0.04% 0.4 0.20% 0.2% 0.4 0.40% 0.4% 0.0 0.0 -0.4 -0.4 -0.8 -1.2 -0.8 012 34 0 1234 Shear Displacement (mm) Shear Displacement (mm) Figure 6. Vertical displacement of sand specimens with fibers initially orientated at 60 under normal pressures of (a) 400 Figure 6. Vertical displacement of sand specimens with fibers initially orientated at 60° under normal pressures of (a) 400 and and ( (b b) ) 800 kPa. 800 kPa. 3.3. Initial Stiffness and Ductility 3.3. Initial Stiffness and Ductility Stiffness and ductility are important mechanical properties in the constitutive Stiffness and ductility are important mechanical properties in the constitutive be- behaviors of sand. Stiffness reflects the deformation resistance of soil under external loads. haviors of sand. Stiffness reflects the deformation resistance of soil under external loads. Since Since so soil il is is a type o a type of f disco discontinuous ntinuous granular granular mater material, ial, its its stiffne stiffness ss is is related related to t to the he stre stress ss level. In this section, the tangent stiffness of sand specimens at the displacement of 0.1 mm level. In this section, the tangent stiffness of sand specimens at the displacement of 0.1 mm was calculated, which can be regarded as the initial stiffness. The data points of initial was calculated, which can be regarded as the initial stiffness. The data points of initial stiffness of sand specim stiffness of sand specimens ens under under norm normal al pressu pressur re e of of 2 200 00 kP kPa a ar are e pl plotted otted in in Figur Figure e 7 7. . It It was was foun found d tthat hat a afibe fiber r cont content ent of 0 of .0.04% 04% did not did not obvio obviously usly ininfluence fluence the init the initial ial stif stif fness fness of of sand specimens. For the sand specimens with a higher fiber content (0.2% and 0.4%), sand specimens. For the sand specimens with a higher fiber content (0.2% and 0.4%), the init theia initial l stiffne stifss decre fness decr ase eased d witwith h the increas the increase e in in in initial itial or orientation ientation angl angle, e, w which hich wa wass consistent with the change rule of peak strength mentioned above. However, there was a consistent with the change rule of peak strength mentioned above. However, there was a negative correlation between the initial stiffness and fiber content, and the influences of negative correlation between the initial stiffness and fiber content, and the influences of fiber content on the initial stiffness were obviously larger than those of fiber orientation. fiber content on the initial stiffness were obviously larger than those of fiber orientation. Two possible reasons may have contributed to this. First, the density of the sand close to Two possible reasons may have contributed to this. First, the density of the sand close to fibers may have been slightly lower than that of the sand far away from fibers due to the fibers may have been slightly lower than that of the sand far away from fibers due to the high difficulty of compaction in specimen preparation. The initial stiffness may have been high difficulty of compaction in specimen preparation. The initial stiffness may have been sensitive to the density inhomogeneity of specimens. Ref. [7] proposed another possible sensitive to the density inhomogeneity of specimens. Ref. [7] proposed another possible reason for the stiffness loss. Upon application of the vertical normal pressure, the fiber reason for the stiffness loss. Upon application of the vertical normal pressure, the fiber strips may be pre-compressed because of their low stiffness modulus. In this situation, at strips may be pre-compressed because of their low stiffness modulus. In this situation, at the beginning of the shearing process, sufficient shear distortion has to occur to overcome the beginning of the shearing process, sufficient shear distortion has to occur to overcome the pre-compression of fibers. As a result, the initial stiffness of reinforced sand specimens the pre-compression of fibers. As a result, the initial stiffness of reinforced sand specimens may be lower than that of unreinforced sand specimens of the same density. may be lower than that of unreinforced sand specimens of the same density. Vertical Displacement (mm) Vertical Displacement (mm) Appl. Sci. 2021, 11, 8845 9 of 11 Appl. Sci. Appl. 2021 Sci., 2021 11, x FO , 11, x FO R PEER R P R EER EVIEW REVIEW 9 of 9 of 11 11 Fiber Orientation Fiber Orientation Unreinforced Unreinforced 60° 60° 90° 90° 120° 120° 0.0 0.1 0.2 0.3 0.4 0.0 0.1 Fibe0. r con 2 tent % 0.3 0.4 Fiber content % Figure 7. Initial stiffness of sand specimens under normal pressure of 200 kPa. Figure 7. Initial stiffness of sand specimens under normal pressure of 200 kPa. Figure 7. Initial stiffness of sand specimens under normal pressure of 200 kPa. Ductility reflects the deformation that can be sustained by soil from yield to a failure Ductility reflects the deformation that can be sustained by soil from yield to a failure state. To intuitively illustrate the influences of fiber strips on sand ductility in this study, Ductility reflects the deformation that can be sustained by soil from yield to a failure state. To intuitively illustrate the influences of fiber strips on sand ductility in this study, the failure displacements of the sand specimens, i.e., the displacement of the peak strength state. To intuitively illustrate the influences of fiber strips on sand ductility in this study, the failure displacements of the sand specimens, i.e., the displacement of the peak strength point, are shown in Figure 8 versus the fiber content. It can be observed that the failure the failure displacements of the sand specimens, i.e., the displacement of the peak strength point, are shown in Figure 8 versus the fiber content. It can be observed that the failure displacement increased with fiber content, indicating that the introduction of fiber strips point, are shown in Figure 8 versus the fiber content. It can be observed that the failure displacement increased with fiber content, indicating that the introduction of fiber strips could improve the ductility of dense sand in this study. For the sand specimens with the displacement increased with fiber content, indicating that the introduction of fiber strips could improve the ductility of dense sand in this study. For the sand specimens with same fiber content, the fibers initially orientated at 60° provided the largest ductility could improve the ductility of dense sand in this study. For the sand specimens with the the same fiber content, the fibers initially orientated at 60 provided the largest ductility increments while the fibers initially orientated at 120° provided the smallest. Fiber strips same fiber content, the fibers initially orientated at 60° provided the largest ductility increments while the fibers initially orientated at 120 provided the smallest. Fiber strips may influence the ductility of dense sand via the same mechanism as residual strength. increments while the fibers initially orientated at 120° provided the smallest. Fiber strips As discussed above, fibers can restrain the movement among sand particles and then may influence the ductility of dense sand via the same mechanism as residual strength. As may influence the ductility of dense sand via the same mechanism as residual strength. reduce the volume dilation. Therefore, compared with unreinforced sand, the reinforced discussed above, fibers can restrain the movement among sand particles and then reduce As discussed above, fibers can restrain the movement among sand particles and then sand specimens may need larger macroscopic displacement in the direct shear tests for the volume dilation. Therefore, compared with unreinforced sand, the reinforced sand reduce the volume dilation. Therefore, compared with unreinforced sand, the reinforced the degree of microscopic deformation of their internal structures to reach the failure specimens may need larger macroscopic displacement in the direct shear tests for the sand specimens may need larger macroscopic displacement in the direct shear tests for degree. degree of microscopic deformation of their internal structures to reach the failure degree. the degree of microscopic deformation of their internal structures to reach the failure (a) 2.4 degree. (b) 3.5 100 kPa 200 kPa Fiber Orientation 3.0 Fiber Orientation 2.0 (a) 2.4 (b) 3.5 100 kPa 200 kPa ° ° 2.5 Fiber Orientatio 1 n20 ° 3.0 1.6 Fiber Orientation 2.0 60 60 2.0 2.5 1.2 120 ° 1.6 1.5 2.0 0.8 1.2 1.0 1.5 0.4 Unreinforced 0.5 0.8 Unreinforced Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 11 1.0 0.0 0.0 0.0 0.1 0.2 0.3 0.4 0.00.1 0.20.3 0.4 0.4 Unreinforced 0.5 Fiber Content (%) Unreinforced Fiber Content (%) (c) 3.5 (d) 4.0 0.0 0.0 800 kPa 400 kPa 0.0 0.1 0.2 0.3 0.4 0.00.1 0.20.3 0.4 3.5 3.0 Fiber OrientationFiber Content (%) Fiber Orientation Fiber Content (%) 3.0 2.5 2.5 2.0 2.0 1.5 1.5 1.0 1.0 0.5 Unreinforced Unreinforced 0.5 0.0 0.0 0.0 0.1 0.2 0.3 0.4 0.0 0.1 0.2 0.3 0.4 Fiber Content (%) Fiber Content (%) Figure 8. Failure displacement versus fiber content of sand specimens under normal pressures of (a) 100, (b) 200, (c) 400, Figure 8. Failure displacement versus fiber content of sand specimens under normal pressures of (a) 100, (b) 200, (c) 400, and (d) 800 kPa. and (d) 800 kPa. 4. Conclusions A series of direct shear tests were conducted on dense sand specimens reinforced with cotton fiber strips. The initial orientation angle and content of fibers were controlled variables in the tests to investigate their influences on the mechanical behaviors of sand. The following conclusions could be drawn: (1) The peak strength of sand specimens increased upon the introduction of fiber strips. The increments of peak strength increased with fiber content and decreased with initial orientation angle. The influence of the initial orientation of fiber strips on the peak strength of sand depended on the stress mobilization in fiber strips during the shearing process. (2) The residual strength of sand specimens increased following the introduction of fiber strips, which may be attributed to the restriction of fibers on the volume dilation of sand. The increments of residual strength also increased with fiber content and decreased with initial orientation angle. (3) Fiber strips had negative effects on the initial stiffness of sand specimens, which may have resulted from the precompression of flexible fiber strips and the possible density inhomogeneity of specimens, which was difficult to completely avoid during the specimen preparation process. (4) The ductility of sand specimens was improved by the introduction of fiber strips, as reflected by the increase in failure displacement. This may have also been related to the restriction caused by fibers on the deformation of sand specimens. Author Contributions: C.C.N.—test running and data collection; H.K.—research design, data analysis, and funding acquisition; P.M.—data analysis and writing of the paper; Y.C.—article modification; S.Z.—participation in tests. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R&D Program of China, grant No. 2019YFC1806000. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors would like to acknowledge the financial support from the National Key R&D Program of China, grant No. 2019YFC1806000. Conflicts of Interest: The authors declared no conflicts of interest. Failure Displacement (mm) Failure Displacement (mm) Failure Displacement (mm) shear stress/displacement (kPa/mm) shear stress/displacement (kPa/mm) Failure Displacement (mm) Failure Displacement (mm) Failure Displacement (mm) Appl. Sci. 2021, 11, 8845 10 of 11 4. Conclusions A series of direct shear tests were conducted on dense sand specimens reinforced with cotton fiber strips. The initial orientation angle and content of fibers were controlled variables in the tests to investigate their influences on the mechanical behaviors of sand. The following conclusions could be drawn: (1) The peak strength of sand specimens increased upon the introduction of fiber strips. The increments of peak strength increased with fiber content and decreased with initial orientation angle. The influence of the initial orientation of fiber strips on the peak strength of sand depended on the stress mobilization in fiber strips during the shearing process. (2) The residual strength of sand specimens increased following the introduction of fiber strips, which may be attributed to the restriction of fibers on the volume dilation of sand. The increments of residual strength also increased with fiber content and decreased with initial orientation angle. (3) Fiber strips had negative effects on the initial stiffness of sand specimens, which may have resulted from the precompression of flexible fiber strips and the possible density inhomogeneity of specimens, which was difficult to completely avoid during the specimen preparation process. (4) The ductility of sand specimens was improved by the introduction of fiber strips, as reflected by the increase in failure displacement. This may have also been related to the restriction caused by fibers on the deformation of sand specimens. Author Contributions: C.C.N.—test running and data collection; H.K.—research design, data analy- sis, and funding acquisition; P.M.—data analysis and writing of the paper; Y.C.—article modification; S.Z.—participation in tests. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key R&D Program of China, grant No. 2019YFC1806000. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declared no conflicts of interest. References 1. Ahmad, F.; Bateni, F.; Azmi, M. Performance evaluation of silty sand reinforced with fibres. Geotext. Geomembr. 2010, 28, 93–99. [CrossRef] 2. Benjamim, C.V.S.; Bueno, B.S.; Zornberg, J.G. Field monitoring evaluation of geotextile-reinforced soil-retaining walls. Geosynth. Int. 2007, 14, 100–118. [CrossRef] 3. Chen, R.; Xu, Z.Z.; Chen, Y.M.; Ling, D.S.; Zhu, B. Field Tests on Pile-Supported Embankments over Soft Ground. J. Geotech. Geoenviron. Eng. 2010, 136, 777–785. [CrossRef] 4. Zornberg, J.G. Discrete framework for limit equilibrium analysis of fibre-reinforced soil. Géotechnique 2004, 52, 593–604. [CrossRef] 5. Benson, C.H.; Khire, M.V. Reinforcing Sand with Strips of Reclaimed High-Density Polyethylene. J. Geotech. Eng. 1994, 120, 838–855. [CrossRef] 6. Dutta, R.K.; Venkatappa Rao, G. Engineering properties of sand reinforced with strips from waste plastic. In Proceedings of the International Conference on Geotechnical Engineering, Sharjah, United Arab Emirates, 3–6 October 2004; pp. 186–193. 7. Michalowski, R.L.; Cermák, J. Triaxial compression of sand reinforced with fibers. J. Geotech. Geoenviron. Eng. 2003, 129, 125–136. [CrossRef] 8. Jiang, H.; Cai, Y.; Liu, J. Engineering Properties of Soils Reinforced by Short Discrete Polypropylene Fiber. J. Mater. Civ. Eng. 2010, 22, 1315–1322. [CrossRef] 9. Diambra, A.; Russell, A.; Ibraim, E.; Wood, D.M. Determination of fibre orientation distribution in reinforced sands. Géotechnique 2007, 57, 623–628. [CrossRef] 10. Bray, J.D.; Zekkos, D.; Kavazanjian, E.; Athanasopoulos, G.A.; Riemer, M.F. Shear Strength of Municipal Solid Waste. J. Geotech. Geoenviron. Eng. 2009, 135, 709–722. [CrossRef] 11. Ibraim, E.; Diambra, A.; Russell, A.; Wood, D.M. Assessment of laboratory sample preparation for fibre reinforced sands. Geotext. Geomembr. 2012, 34, 69–79. [CrossRef] 12. Gray, D.H.; Ohashi, H. Mechanics of Fiber Reinforcement in Sand. J. Geotech. Eng. 1983, 109, 335–353. [CrossRef] Appl. Sci. 2021, 11, 8845 11 of 11 13. Noorzad, R.; Zarinkolaei, S.T.G. Comparison of mechanical properties of fiber-reinforced sand under triaxial compression and direct shear. Open Geosci. 2015, 7, 547–558. [CrossRef] 14. Benziane, M.M.; Della, N.; Denine, S.; Sert, S.; Nouri, S. Effect of randomly distributed polypropylene fiber reinforcement on the shear behavior of sandy soil. Stud. Geotech. Mech. 2019, 41, 151–159. [CrossRef] 15. Jewell, R.A.; Wroth, C.P. Direct shear tests on reinforced sand. Géotechnique 1987, 37, 53–68. [CrossRef] 16. Consoli, N.C.; Heineck, K.S.; Casagrande, M.D.T.; Coop, M.R. Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress Paths. J. Geotech. Geoenviron. Eng. 2007, 133, 1466–1469. [CrossRef] 17. Gao, Z.; Zhao, J. Evaluation on Failure of Fiber-Reinforced Sand. J. Geotech. Geoenviron. Eng. 2013, 139, 95–106. [CrossRef] 18. Micha1owski, R.L. Limit analysis with anisotropic fibrereinforced soil. Géotechnique 2008, 58, 489–501. [CrossRef]
Applied Sciences – Multidisciplinary Digital Publishing Institute
Published: Sep 23, 2021
Keywords: reinforced sand; direct shear test; strength; initial stiffness; ductility
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