Experimental Study on Reinforcement Properties of Tension-Resistant Reinforced Soil Retaining Wall

Abstract

The tensioned reinforced soil retaining wall, a novel retaining structure, utilizes either anchors or geosynthetic materials as reinforcements that contribute to load-bearing and friction within the structure. This study aims to explore the tension distribution and strain patterns in the reinforcements, and their influence on the reinforced soil retaining walls. To this end, tensile, direct shear, and pullout tests were conducted on GeoStrap@5-50 geotextile strips and TGDG130HDPE geogrids to evaluate the tensile strength and interface strength between the reinforcement and the soil. The characteristics of the reinforcement–soil interface and the deformation behavior under stress were examined, with a comparative analysis of the technical merits of the two types of reinforcements. The results indicate that both the geotextile strips and geogrids enhanced the strength of the reinforced soil, primarily by increasing cohesion. The GeoStrap@5-50 geotextile strips exhibited superior tensile strength compared to the TGDG130HDPE geogrids; the reinforcement with the geotextile and geogrids both enhanced the cohesion of the standard sand, albeit with a slight decrease in the internal friction angle, by 4.6% and 3.1%, respectively, offering enhanced mechanical properties and economic value in reinforced soil retaining wall applications.

1. Introduction

In recent years, the integration of the technical characteristics of tensioned retaining walls and reinforced soil retaining walls has led to the development of a novel type of retaining structure—the tensioned reinforced soil retaining wall. This structure features vertical side panels that are reinforced with continuous tensile elements, enhancing the anchorage between the reinforcements and the panels, reducing the lateral deformation of the panels, and increasing the stability of the reinforced soil retaining wall. Compared to anchored reinforced soil retaining walls, tensioned reinforced soil retaining walls utilize flexible reinforcement materials that can adapt to areas with poor load-bearing capacities, and also exhibit excellent seismic resistance. The unique structural form of tensioned reinforced soil retaining walls significantly reduces the width of the side slopes, providing new solutions for construction conditions with limited space. Currently, the research on tensioned reinforced soil retaining walls is still in its nascent stages, with practice leading and theory lagging. The design methods for tensioned reinforced soil retaining walls need further refinement. The distribution patterns of the tensile forces and strains in the tensioned reinforcements, as well as the deformation of the reinforced soil retaining walls, have not yet been clearly defined, and the working mechanisms of these walls remain under investigation and study.

Yi F. et al. [1] analyzed the effects of normal stress, moisture content, and the pullout rate on the friction parameters of the reinforcement tailings–sand interface through indoor pullout tests. Cheng Hao et al. [2] investigated the shear characteristics of the interface between geogrids and coarse-grained soil under various normal stresses using large-scale direct shear tests. Ren Fei fan et al. [3] synthesized the findings from pullout tests, direct shear tests, and triaxial tests, combined with numerical and theoretical analyses, to summarize the mechanical properties of the reinforcement–soil interface. He identified several influencing factors on the interface properties, confirmed the reliability of using finite element and discrete element methods for model validation, proposed the expansion of the PFO (Particle Flow Code) for mesoscopic studies of the reinforcement–soil interface characteristics, and noted that the existing interface models still fail to balance accuracy and simplicity, necessitating further research. Hou Licheng et al. [4], relying on a high-fill embankment project as a case study, analyzed the stability of embankments with and without geogrids, identifying the reinforcing effect of geogrids on high-fill embankments and studying how the length, tensile modulus, and number of layers of geogrids affect the stability.

The interaction characteristics at the reinforcement–soil interface are crucial for understanding the reinforcement mechanisms of geosynthetic materials, and the parameters at this interface also serve as a fundamental basis for the calculation of structural deformation and stability in the design of reinforced soil structures. [5] Typically, direct shear tests and pullout tests are employed to measure the reinforcement–soil interface parameters, and numerous experiments have demonstrated that the friction coefficient between the reinforcement and soil is related to factors such as the shear velocity, reinforcement properties (including reinforcement type, surface properties, and dimensions), soil properties (particle size distribution, water content, etc.), and overburden loads. A summary of the recent studies on direct shear tests of reinforcement–soil interfaces is presented in

Table 1. Summary of direct shear tests on interface between reinforcement and soil.

References	Reinforcement Type	Fill Soil Type	Study Factors
Gao et al. [6] (2002)	Steel–plastic composite reinforcement strip	Clayey soil	Shear rate, physical and mechanical properties of the fill soil, and reinforcement properties.
Fleming et al. [7] (2006)	Geomembrane	Sandy soil, sand–bentonite mixture, and silt mixture	Soil moisture content and shear rate.
Shi et al. [8] (2009)	Uniaxial and biaxial plastic geogrids	Clayey soil and sandy soi	Soil moisture content.
Basudhar et al. [9] (2010)	Geotextile	Sandy soil	Void ratio of reinforcement ribs.
Yang et al. [10] (2010)	Geogrid	Sandy gravel and clayey soil	Shear rate, lateral wall boundary effect and size effect of the test box, fill thickness, compaction degree, and reinforcement clamping condition.
Vieira et al. [11] (2013)	Geotextile	Silica sand	Number of cyclic direct shear tests and confining pressure.
Hatami et al. [12] (2014)	Geotextile	Clay	Soil moisture content.
Zhao et al. [13] (2014)	Uniaxial and biaxial geogrids	Uniaxial and biaxial geogrids	Type of geogrid.
Xiong et al. [14] (2018)	Geogrid	Standard sand and silty clay	Shear velocity, void ratio of reinforcement, and soil moisture content.

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During direct shear testing, the reinforcement is generally placed horizontally. However, due to the influence of soil filling, the reinforcement may be bent or inclined in the actual soil, potentially affecting the test results. Ma Liao [15] conducted consolidated-drained triaxial direct shear tests with varying reinforcement angles using sisal fibers as the reinforcement material in the reinforced loess, demonstrating that the influence curve of the reinforcement angle on the strength of the linear fiber-reinforced loess exhibits a hump-shaped pattern.

Extensive experimental studies have been conducted on geosynthetic-reinforced soil and fiber-reinforced soil by scholars. The results have shown that factors, such as the number of reinforcement layers, reinforcement type, cross-sectional shape of the reinforcement, and distribution of the reinforcement strips, influence the reinforcement effect in geosynthetic-reinforced soil. Similarly, in fiber-reinforced soil, the type of fiber, the amount and length of fiber added, and the dry density all affect the strength of the reinforced soil. However, these experiments have focused on specific reinforcement materials, and no systematic evaluation method for calculating the reinforcement–soil interface parameters has been established. Therefore, for the Geostrap@5-50 tensile reinforcement strip and TGDG130 HDPE geogrid used in this study, experimental measurements of their reinforcement–soil interface parameters are necessary.

In summary, researchers have conducted systematic studies on reinforced soil retaining walls through indoor model tests and field prototype experiments, analyzing the impact of various factors on tensioned reinforced soil retaining walls and accumulating substantial data for the construction of safe and stable retaining wall projects. However, further research and refinement are needed regarding the mechanical behavior of the reinforcement–soil interface and the selection of reinforcement materials in tensioned reinforced soil retaining walls. This study, in the context of the field conditions tensioned reinforced soil retaining walls are subject to, involves tensile, direct shear, and pullout tests on Geo Strap polyester fiber straps, GeoStrap@5-50 geotextile strips, and TGDG130HDPE geogrids to evaluate the tensile strength of the reinforcements and the strength of the reinforcement–soil interface. It also analyzes the characteristics of the reinforcement–soil interface, deformation behavior under stress, and provides insights for design optimization and review. Further enriching and refining the theoretical framework of counter-tensioned reinforced soil retaining walls, this study provides a solid theoretical basis for the selection of reinforcement materials and the optimization of the structural design of tensioned reinforced soil retaining walls. Consequently, it offers promising mechanical properties and economic benefits in the practical application of reinforced earth-retaining wall engineering.

2. Project Overview

This article is based on Section B of Phase II of the WAE Railway Project. A schematic of the project’s layout is shown in Figure 1 and Figure 2. Section B is the longest segment within Phase II, stretching 222 km in total, with the main line extending 181 km, predominantly within the Abu Dhabi region. It serves as a critical link between Abu Dhabi and Dubai. The UAE Railway Project Phase II is designed as a dual-track, electrified mixed-use high-speed railway for both passenger and freight services. The subgrade work accounts for over 90% of the construction, adhering to a standard gauge of 1435 mm. The design speeds are set at 200 km/h for passenger trains and 120 km/h for freight, with an axle load of 32.4 tons. The standard width of the double-track subgrade is 11.2 m.

3. Experimental Study on Tensile Strength of Reinforcement Materials

3.1. Test Equipment and Materials

In reinforced soil structures, the tensile properties of the geosynthetic materials within the soil directly influence the stability of the reinforced structure. Under the action of soil weight or external loads, the reinforced soil undergoes stress deformation, leading to a relative slip between the reinforcement and the soil. The reinforcement materials are subjected to tensile forces and frictional resistances. When geosynthetic materials with low tensile strength are used, the reinforcement may rupture under tension, compromising the stability of the reinforced soil structure. Therefore, the tensile strength of reinforcement materials holds significant research value. An electronic universal testing machine designed for geosynthetic materials was utilized for the indoor tensile tests, as depicted in Figure 3. The specimens were fixed using a flat compression fixture, and a software control system was employed to stretch the specimens at a pre-set test speed until the termination criteria were met.

For the tensile tests, GeoStrap@5-50 geotextile strips and TGDG130 HDPE geogrids were selected as the reinforcement materials. Figure 4 illustrates the geotextile strips used in the tests, while Figure 5 depicts the geogrids. The tensile test procedure is outlined in Figure 6.

3.2. Processing and Analysis of Test Results

Figure 7 shows the tensile strength–strain relationship curves obtained from the tensile tests of the geotextile strips and geogrids. Distinct differences are observed in the stress–strain curves of both materials. During the initial strain phase, the elastic moduli of both increase. However, as the tensile process progresses, the elastic modulus of the geotextile strips exhibits a complex pattern of initial decrease, followed by an increase, and then a subsequent decrease. In contrast, the elastic modulus of the geogrids gradually decreases. The stress–strain relationships of the geotextile strips and geogrids can be modeled using hyperbolic and polynomial functions, respectively. This variation in the curves is attributed to the differences in material composition, for the geotextile strips consist of polyester fiber bundles encased in polyethylene, while the geogrids are constructed from high-density polyethylene with aligned polymer chains achieved through stretching. These materials differ in terms of their relative molecular mass, distribution, crystalline structure, and orientation.

Based on the test results, the ultimate tensile strength of the Geostrap@5-50 geotextile strips is calculated as 52.50 kN/strip, with an ultimate tensile strain of less than 10%. In contrast, the ultimate tensile strength of the TGDG130 HDPE geogrids is 141.60 kN/m, accompanied by a longitudinal elongation of less than 11.5%. By comparison, the ultimate tensile strength of the Geostrap@5-50 geotextile strips is approximately 37.1% of that of the TGDG130 HDPE geogrids. However, the width of the geotextile strips is only 5% of that of the geogrids, indicating that less than three geotextile strips per unit width can meet the tensile demand of a fully laid geogrid. Considering the tensile strain of the reinforcement materials, the Geostrap@5-50 geotextile strips exhibit higher tensile strength and are more suitable for application in reinforced soil retaining walls with limited deformation requirements.

4. Direct Shear Test Study of Reinforcement–Soil Interface

4.1. Test Equipment and Materials

The friction coefficient is measured using a geosynthetic material pullout–direct shear friction testing apparatus. The shear box comprises an upper model box fixed in a horizontal direction with dimensions of 600 mm × 400 mm, and a lower model box with dimensions of 800 mm × 400 mm. The entire model box is placed on a base to minimize the pressure exerted on the ground by the applied normal load during testing. Rails and a horizontal moving platform are installed between the model box and the base, enabling the model box to slide along the rails for easy replacement. Both the normal and horizontal loading systems utilize inverted hydraulic loading methods to apply normal stress and horizontal stress to the model box. The geosynthetic material pullout–direct shear friction testing apparatus is depicted in Figure 8. For the direct shear tests, Geostrap@5-50 tensile reinforcement strips and TGDG130HDPE geogrids ae selected as the reinforcement materials, and the test fill material is sand soil that meets Chinese ISO standards.

4.2. Test Protocol

For conducting the direct shear tests, standard sand was employed as the filling material, with the geotextile strips and geogrids chosen as the reinforcing elements. The shear rate during the tests was maintained at 1 mm/min, and the relationship between the shear force and shear displacement was investigated under varying normal stresses of 100 kPa, 200 kPa, and 300 kPa, respectively.

During the reinforcement installation, the soil surface was kept flat, and the reinforcement was laid accordingly. The geotextile was laid in a “V” shape with an opening angle of 20° to mimic real-world reinforcement installation methods (as illustrated in Figure 9 and Figure 10). The geotextile was secured to the upper surface of the lower model box using U-shaped steel pins. The geotextile and U-shaped pins are shown in Figure 11 and Figure 12, and the layout of the geotextile is depicted in Figure 13a. The geogrid was cut into specimens with two transverse ribs and laid flat on the surface of the fill material. One end of the geogrid, opposite to the horizontal loading system, was fixed with a clamp on the lower model box to ensure that the geogrid remained fixed to the upper surface of the lower model box during testing, with two transverse ribs maintained throughout. The layout of the geogrid is illustrated in Figure 13b.

4.3. Processing and Analysis of Experimental Results

4.3.1. Analysis of the Shear Stress–Displacement Relationship

Figure 14 and Figure 15 present the shear stress–displacement relationship curves for standard sand, geotextile-reinforced sand, and geogrid-reinforced sand under varying normal loads.

As is evident from these figures, the peak strengths of both the standard sand and the reinforced sand with the geotextile or geogrid increase with increasing normal stress. Furthermore, under the same load, the peak strength of the reinforced soil is higher than that of the standard sand, with a minimal difference in the enhancement provided by the geotextile and geogrid reinforcement.

A strain-softening phenomenon is observed in the standard sand, which is mitigated by the addition of the geotextile or geogrid. At normal stresses of 100 kPa, 200 kPa, and 300 kPa, the decrease in the shear stress after peak strength for the standard sand is 38.49%, 40.27%, and 28.19%, respectively. However, with geotextile reinforcement, the decrease is reduced to 8.94%, 11.33%, and 13.39%, while for the geogrid reinforcement, it is 8.58%, 12.29%, and 9.54%. This is attributed to the tensile resistance generated by the geotextile during shearing, which opposes the soil movement. As the shear displacement increases and the soil enters the residual deformation stage, the reinforcement effect of the geotextile becomes dominant, reducing the post-peak decline in strength. In contrast, the geogrid reinforcement initially relies on the friction between the reinforcement and the soil. Once a certain shear displacement is achieved, the interlocking effect of the ribs comes into play, requiring greater shear force for the same displacement, thus minimizing strain softening. Evidently, the use of reinforcement materials enhances soil ductility and improves the interfacial strength properties of sand.

A comparative analysis of the shear stress–displacement relationship for standard sand and the two reinforced soils under the same vertical stress is presented in Figure 16.

As depicted in Figure 16, both types of reinforced soil and the standard sand reach their peak shear stress rapidly when the normal stress is relatively low, with minor differences in the corresponding shear displacements. However, as the normal stress increases, the shear displacement required for the geotextile–standard sand interface to reach the peak shear stress is slightly greater than that for the geogrid–standard sand interface. This observation underscores the differences in the reinforcement mechanisms between the geotextile and geogrid: the geotextile relies primarily on surface friction resistance, requiring an initial relative displacement for interlocking, and forming a denser reinforcement–soil structure. As the normal stress increases, the relative displacement that needs to be overcome also increases. Conversely, the geogrid, with its ribs, immediately compresses the soil between the ribs upon displacement, generating passive earth pressure, and allowing for the peak shear stress to be achieved with less shear displacement.

4.3.2. Analysis of Shear Strength Parameters

Based on Coulomb’s law (1), the soil’s shear strength parameters, cohesion (c), and internal friction angle (φ) were determined. By performing a linear regression analysis of the relationship between the normal stress and shear stress at the reinforcement–soil interface, Figure 17 was obtained. The constant term of the fitted linear equation represents the interfacial cohesion (c), while the coefficient term represents the tangent value of the interfacial internal friction angle (φ). The calculated results are presented in

Table 2. Shear strength parameters at reinforcement materials and standard sand interface.

Interface Type	Cohesion c/kPa	Friction Angle φ/°
Standard Sand	0	30.66
Geotextile–Standard Sand	13.26	29.25
Geogrid–Standard Sand	20.47	29.70

.

$$\tau =c+\tan \phi$$

(1)

The analysis of the direct shear test results reveals that, compared to standard sand, the reinforcement with the geotextile and geogrid both enhance the cohesion of the standard sand, albeit with a slight decrease in the internal friction angle, by 4.6% and 3.1%, respectively. This mechanism can be attributed to the relatively smooth surface of the geotextile, which reduces the area of interlocking between soil particles when laid on the soil surface, thereby decreasing the frictional interaction at the reinforcement–soil interface. However, the reinforcement effect is not solely dependent upon the frictional interlocking at the reinforcement–soil interface; it also manifests in the restraint provided by the reinforcement to the adjacent soil mass. Under vertical stress, shear stress is generated at the reinforcement–soil interface to accommodate deformations. This shear stress restricts the lateral deformation of the soil, leading to the development of an “apparent cohesion” at the reinforcement–soil interface. Similarly, the geogrid not only restricts the frictional interaction of the soil, but also relies primarily on the embedment of its ribs. Therefore, the geogrid reinforcement results in a greater increase in cohesion compared to the geotextile reinforcement.

4.3.3. Interfacial Friction-like Coefficient

The interfacial friction-like coefficients of the Geostrap@5-50 geotextile and TGDG130HDPE geogrid under different normal stresses were calculated and are presented in

Table 3. Frictional coefficient under different normal stresses during drawing test.

Reinforcement Type	Friction Coefficient under Different Normal Stresses (kPa)
	100	200	300
Geostrap@5-50 Geotextile	0.68	0.64	0.60
TGDG130HDPE Geogrid	0.75	0.70	0.63

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The friction coefficient is obtained by multiplying the maximum shear stress by the contact area between the reinforcement strip and the soil, divided by the normal stress, and multiplied by the contact area between the reinforcement strip and the soil. Since the contact area is the same, the calculation is directly simplified as the maximum shear stress divided by the normal stress. The results indicate that the friction-like coefficient obtained for the geogrid reinforcement is higher than that for the geotextile reinforcement, and both decrease with increasing normal stress. Upon comparison, it is observed that the difference in the friction-like coefficients between the geogrid and geotextile decreases as the normal stress increases, suggesting that under very high normal stress conditions, the friction-like coefficients of the geogrid and geotextile reinforcement are similar. The direct shear tests indicate that the internal friction angles of standard sand, geotextile-reinforced standard sand, and geogrid-reinforced standard sand exhibit minor differences. Primarily, the apparent cohesion at the reinforcement–soil interfaces for the geotextile and geogrid are significantly greater than the soil cohesion, indicating that the presence of the geotextile and geogrid can enhance the shear strength of the soil primarily by improving its cohesive strength.

5. Experimental Study on the Pullout Friction Characteristics of the Reinforcement–Soil Interface

Apart from direct shear tests, pullout tests are commonly used for studying interfacial behavior characteristics. In contrast, direct shear tests measure the friction characteristics of the unilateral shear plane between the reinforcement and the soil, while pullout tests assess the interactive characteristics between the reinforcement and the surrounding soil within a certain range, during the process of the reinforcement being pulled out of the soil. Analyzing the experimental mechanism, the direct shear test involves an overall shear along the reinforcement–soil interface, where the reinforcement and the soil within the lower model box can be considered as a single entity. The interfacial behavior primarily involves shear failure among the soil particles and the friction exerted by the reinforcement. Conversely, in a pullout test, the reinforcement is extracted from the soil, and the shear displacement of the reinforcement–soil interface occurs around the reinforcement, with a transfer process from the pulled-out end to the other end. During this transfer, the interfacial effect gradually manifests, hindering the extraction of the reinforcement, and the frictional resistance between the reinforcement and soil becomes the primary factor.

5.1. Experimental Protocol

The pullout–direct shear friction test apparatus was used to measure the pullout friction coefficient. The apparatus was similar to the one employed in the direct shear tests, requiring only the replacement of the model box. The dimensions of the model box for the pullout tests were 600 mm × 400 mm × 800 mm in length, width, and height. The experimental protocol is presented in

Table 4. Test protocol for reinforcement material.

Interface Type	Pullout Rate (mm/min)	Normal Stress/kPa	Test Results
Geotextile–Standard Sand	1.0	100/200/300	Curve representing the relationship between pullout force and displacement.
Geogrid–Standard Sand

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Reinforcement placement: The surface of the fill material was kept flat for the placement of the geotextile or geogrid. The geotextile was laid in a “V” shape to mimic the reinforcement installation method in real-world applications, with an opening angle of 20°. The folded end of the geotextile extended a certain length outside the pullout box and was clamped, as illustrated in Figure 18a. The geogrid was cut into a specimen with three transverse ribs and the same width as the box, and then evenly laid on the surface of the fill material. One side of the transverse rib of the geogrid extended outside the pullout box and was clamped, as depicted in Figure 18b.

5.2. Experimental Results Processing and Analysis

5.2.1. Analysis of Shear Stress and Shear Displacement Relationship

Figure 19 depicts the pullout shear stress–displacement curves at the interfaces of the geotextile-reinforced and geogrid-reinforced sandy soils under various normal loads.

From the figure, it is evident that with increasing normal stress, both the peak strengths of the geotextile-reinforced and geogrid-reinforced sandy soils increase. When the normal stress is relatively low, the peak strength of the geogrid-reinforced sandy soil is slightly higher than that of the geotextile-reinforced sandy soil. However, as the normal stress increases, the growth rate of the shear strength of the geogrid-reinforced sandy soil slows down, resulting in a peak strength gradually smaller than that of the geotextile-reinforced sandy soil.

Comparing the pullout displacements corresponding to the peak shear stress under the same vertical load in the two figures, it can be observed that as the normal stress increases, the pullout displacements required for both reinforcements to reach the peak shear stress at the reinforcement–soil interface gradually increase. Moreover, the displacement required for the geogrid reinforcement is greater than that for geotextile reinforcement. This observation is contrary to the results of the direct shear tests, as the pullout test involves a gradual transfer of the pullout force to the reinforcement, unlike the overall shear in the direct shear tests. The smooth surface of the geotextile facilitates easier extraction, requiring a smaller pullout displacement to reach the peak pullout force. Conversely, the transverse ribs of the geogrid accumulate soil particles during pullout, resulting in a gradual increase in the pullout force as the displacement increases, necessitating a larger pullout displacement to achieve the peak pullout force.

5.2.2. Analysis of Shear Strength Parameters

The soil’s shear strength parameters, cohesion (c), and internal friction angle (φ) were determined using Coulomb’s law (1). A linear regression analysis was performed on the relationship between the normal stress and shear stress at the reinforcement–soil interface, resulting in Figure 20. The constant term of the fitted linear equation represents the interfacial cohesion (c), while the coefficient term represents the tangent value of the interfacial internal friction angle (φ). The calculated results are presented in

Table 5. Shear strength parameters at reinforcement materials and standard sand interface.

Interface Type	Cohesion c /kPa	Friction Angle φ/°
Geotextile–Standard Sand	16.04	26.77
Geogrid–Standard Sand	31.84	21.18

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Based on the pullout test results, it is found that, compared to standard sand, the reinforcements with the geotextile and geogrid increase the cohesion but decrease the internal friction angle. The internal friction angles of the geotextile-reinforced and geogrid-reinforced soils, measured by pullout tests, are reduced by 12.7% and 30.9%, respectively. However, the cohesion of the geogrid-reinforced sandy soil increases significantly.

5.2.3. Interfacial Friction-like Coefficient

The interfacial friction-like coefficients of the Geostrap@5-50 geotextile and TGDG130HDPE geogrid under different normal stresses were calculated and are presented in

Table 6. Friction-like coefficients under different normal stresses in drawing friction characteristic experiment.

Reinforcement Type	Friction Coefficient under Different Normal Stresses (kPa)
	100	200	300
Geostrap@5-50 Geotextile	0.64	0.61	0.55
TGDG130HDPE Geogrid	0.72	0.53	0.50

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As indicated in the table, the friction-like coefficient of the geogrid reinforcement is higher than that of the geotextile reinforcement under a relatively low normal stress. However, as the normal stress increases, the friction-like coefficient of the geogrid reinforcement decreases rapidly. At a normal stress of 300 kPa, the friction-like coefficient of the geogrid reinforcement becomes lower than that of the geotextile reinforcement.

5.2.4. Comparison with Direct Shear Test Results

For the same reinforcement material, the pullout test yields a higher interfacial cohesion but a lower internal friction angle than the direct shear test. For the geotextile reinforcement, the difference in the friction-like coefficients measured by the direct shear and pullout tests is minimal. However, for the geogrid reinforcement, the friction-like coefficient measured by the direct shear test is greater than that measured by the pullout test, and the discrepancy becomes more significant with increasing normal stress.

Under the same normal stress, the shear displacement required to reach the peak shear strength in the pullout test is greater than in the direct shear test. For the geotextile reinforcement, the change in displacement is not significant; however, for the geogrid reinforcement, the displacement increases by 120.3% to 165.7%, verifying that the reinforcement–soil interface undergoes overall shear in the direct shear test, while the shear in the pullout test gradually propagates from the pullout end to the other end.

6. Results Analysis

(1)

The ultimate tensile strengths of the Geostrap@5-50 geobelt and the TGDG130HDPE geogrid, as determined through tensile tests, are 52.5 kN/strip and 141.6 kN/m, respectively. From a force-bearing perspective, the ultimate tensile strength of the geogrid significantly exceeds that of the geobelt. Considering both the tensile elongation and material usage, the ultimate tensile strength of the Geostrap@5-50 geobelt is approximately 37.1% of the TGDG130HDPE geogrid’s, yet its reinforcing material width comprises merely 5% of the geogrid’s width. This implies that, within a unit width, less than three Geostrap@5-50 geobelts are sufficient to meet the tensile requirements of a fully laid geogrid. Upon analyzing the tensile elongation properties of these reinforcing materials, it is deemed that the Geostrap@5-50 geobelt exhibits superior tensile performance, offering favorable mechanical characteristics and economic value for applications to reinforced soil retaining walls.

(2)

The direct shear tests reveal that both the geotextile and geogrid reinforcements enhance the shear strength of the soil and mitigate strain softening. An analysis of the soil’s shear strength parameters indicates that while the internal friction angle decreases compared to that of standard sand, the cohesion increases significantly. This is attributed to the tensile resistance generated by the geotextile during shearing, which opposes the soil movement. As the shear displacement increases and the soil enters the residual deformation stage, the reinforcement effect of the geotextile becomes dominant, reducing the post-peak decline in strength. In contrast, the geogrid reinforcement initially relies on the friction between the reinforcement and the soil. Once a certain shear displacement is achieved, the interlocking effect of the ribs comes into play, requiring a greater shear force for the same displacement, thus minimizing strain softening. The improvement in the strength of the reinforced soil by the geotextile and geogrid is primarily achieved through the enhancement of cohesion strength.

(3)

The pullout tests show that reinforcement with the geotextile and geogrid increases the cohesion of standard sand, while reducing the internal friction angle. The decrease in the internal friction angle of the soil after reinforcement with the geogrid is more significant than that with the geotextile, while the increase in the cohesion is approximately twice that of the geotextile. This suggests that the interaction mechanisms between the geogrid and the geotextile and the soil differ. This observation is contrary to the results of the direct shear tests, as the pullout test involves a gradual transfer of the pullout force to the reinforcement, unlike the overall shear in the direct shear tests.

(4)

A comparison of the results from the direct shear and pullout tests reveal that the friction strength obtained from the direct shear tests is slightly higher than that from the pullout tests. This discrepancy is attributed to the different interfacial stress modes, where the shear stress distribution at the reinforcement–soil interface in the direct shear tests is more uniform. Not only does the interfacial friction strength differ, but the displacement required to reach the friction strength in the direct shear tests is significantly smaller than that in the pullout tests. Therefore, in practical engineering applications, direct shear tests are more suitable for scenarios involving unilateral displacement and small displacements, while pullout tests are applicable to scenarios with bilateral displacement and larger displacements.

7. Conclusions

This study focuses on tensioned reinforced soil retaining walls used in high-speed, heavy-load railway systems. Building upon the foundations of anchored reinforced soil retaining walls, the tensile strength of the reinforcement materials and the interface strength between the reinforcements and soil are evaluated through indoor tensile tests, direct shear tests, and pullout tests. This research enriches and refines the theoretical framework for tensioned reinforced soil retaining walls. It provides a solid theoretical basis for analyzing the tensile forces, strain distribution patterns, mechanical properties, and for optimizing the structural parameters of tensioned reinforced soil retaining walls. This study further examines the characteristics of the reinforcement–soil interface and the stress–deformation behaviors, and it compares the technical advantages of two different types of reinforcement materials. The findings offer theoretical guidance for the selection and structural optimization design of reinforcement materials for tensioned reinforced soil retaining walls, highlighting their superior mechanical properties and economic value in geotechnical engineering applications.

8. Recommendations

The tension-resistant reinforced soil retaining wall embodies the dual advantages of both anchored retaining walls and reinforced soil retaining walls. It achieves interconnection and anchoring between the reinforcement materials and the facing panels at both ends through continuous reinforcement placement, fostering a mutual anchoring effect. The flexible reinforcement further reduces the self-weight of the retaining wall, diminishing the requirements on the foundation-bearing capacity. This design exhibits characteristics such as reduced deformation, land conservation, wide geographical adaptability, superior seismic performance, and economic safety. In practical engineering applications, design and construction must consider not only stability and durability, but also the tensile properties of the reinforcing materials, the frictional behavior at the reinforcement–soil interface, their impact on the reinforced soil retaining wall, and their economic value.