A Comparative Study of the Pullout Strength of Geostraps and Geogrids in Reinforced Soil

Abstract

The sustainable development of geotechnical infrastructure necessitates using durable, efficient, and environmentally resilient reinforcement materials. This study investigates the pullout performance of geostraps to assess their potential as a sustainable alternative to conventional geosynthetics. This study focuses on the pullout performance of geostraps, flexible, polymeric reinforcement materials. There has not been a thorough study of their pullout resistance, which directly affects the stability and durability of reinforced soil structures. Pullout tests were conducted on sandy soil in a controlled environment. The experimental findings from the pullout test were then validated in a numerical model. The model was used to determine the pullout resistance of different grades of geostraps for comparative analysis. This helped to identify the possible application areas based on the pullout capacity of various grades. The results obtained for the geostraps were then compared with those in the established literature on geogrids. Initially, the pullout resistance of the M65 geostrap was up to 20% higher than that of a biaxial geogrid. This makes it a suitable option for reinforced earth applications. However, the maximum pullout resistance of geogrids was up to 8% higher than that of geostraps when subjected to a surcharge of 17 kN m⁻² in poorly graded sand. This study highlights the potential of geostraps as reinforcement materials, particularly in challenging environments where conventional geosynthetics may underperform. Future research may explore their behaviour with different soil types and other controlled environmental factors to establish their broader applicability and design charts.

1. Introduction

Soil reinforcement is a common geotechnical technique used to improve the strength, stability, and overall performance of soil, especially in weak or challenging ground conditions. The mechanical behaviour of soil is improved by introducing reinforcing materials, including geosynthetics, natural fibres, and cementitious agents [1,2,3]. The application of this technique is particularly critical in the construction of embankments, retaining structures, subgrade enhancement, and slope stabilisation. Contemporary advancements in soil reinforcement have emphasised integrating sustainable methods and eco-friendly materials. For instance, natural fibres such as bamboo, coir, and jute have been investigated as sustainable alternatives to synthetic reinforcements [4]. Exploring novel soil blends, including frozen soil mixtures, also provides temporary reinforcement during excavation or tunnelling operations by temporarily increasing the stiffness and bearing capacity in cold regions or artificial ground freezing applications [5,6]. Geosynthetics, such as geogrids and geotextiles, are also widely utilised to offer soil confinement and tensile resistance, increasing load-bearing capacity and minimising deformation [7], leading to better ride comfort [8]. The interaction between the soil matrix and the reinforcing material significantly influences the overall system performance, with parameters such as interface friction, bond strength, and pullout resistance as critical design considerations [9].

Using geosynthetics in civil engineering has significantly advanced the design and construction of reinforced soil structures. Materials such as geogrids, geotextiles, and geostraps have been pivotal in improving the mechanical behaviour of soil, enabling applications in retaining walls, embankments, and other geotechnical projects [10,11,12]. Geosynthetics serve as reinforcements by providing tensile strength, enhancing soil stability, and ensuring durability under various loading conditions [13]. Among these materials, due to their unique characteristics, geostraps have emerged as a promising alternative to traditional geogrids. This study evaluates the pullout behaviour of the geostrap, comparing its pullout performance with that of the biaxial geogrid [14].

The pullout test is a widely recognised method for assessing the interaction between soil and reinforcement materials. It simulates the conditions experienced in reinforced soil structures, where the bond strength between the soil and the reinforcement plays a crucial role in overall stability [15]. Existing studies have extensively analysed geogrids under different soil and stress conditions. Geogrids, with their open grid structure, rely heavily on interlocking mechanisms, while geotextiles use surface friction and adhesion to achieve pullout resistance [16]. The particle size is pivotal in determining the geogrid and the soil interface friction. Geostraps, polymeric units with a flexible and ribbed profile, combine the benefits of both interlocking and friction, making them an innovative choice for geotechnical applications.

Geostraps offer unique advantages over conventional geosynthetics due to their design and material properties. Their flexible nature allows for greater adaptability to soil deformations, reducing the risk of rupture or overstress in dynamic conditions. An anchored geostrap reinforcement installation in an unpaved road is shown in Figure 1. The testing site was situated on NH148B in Haryana, India. The ribbed surface of the geostrap enhances frictional interaction with the surrounding soil, potentially improving pullout resistance [17].

While geogrids have demonstrated excellent performance in various applications [18,19], certain limitations, such as deformation under high loads or inefficiency in fine-grained soils, highlight the need for alternative reinforcement solutions. Geostraps address these limitations with their hybrid characteristics, making them a viable option for challenging environments and load scenarios [20]. Geostraps offer a unique blend of these properties, combining them to provide a beam effect. The tensioned membrane or beam effect is the tension developed in a geosynthetic-reinforced strap to resist a vertical load [21,22,23]. However, to mobilise the tensioned membrane effect, the pavement structure must deform significantly [24]. As the geostrap has a high stiffness compared to the surrounding soil, the bent state of the geostrap offers an upward normal reaction to reduce the net vertical stress. Also, the ribbed surface and polymeric composition allow for high frictional resistance and tensile strength, while its flexibility ensures adaptability under varying soil conditions [25]. Despite its potential, limited studies exist on the pullout behaviour of the geostrap, necessitating further investigation to fully understand its capabilities and optimise its design for field applications. Furthermore, the reinforcement–soil interface and the displacement under stress were examined [26], with a comparative analysis of the technical merits of the two reinforcements. The results indicate that both the geotextile strips and geogrids enhanced the strength of the reinforced soil, primarily by increasing cohesion. In addition to their well-documented static performance, geosynthetics also exhibit important dynamic properties that influence their effectiveness in transportation and infrastructure applications subjected to cyclic or transient loading, such as those caused by traffic or seismic events. Under dynamic loads, geosynthetics typically display time-dependent deformation, characterised by viscoelastic or viscoplastic behaviour, depending on the polymer type and loading frequency [27].

In this study, the pullout strengths of different grades of geostraps were compared based on simulation results obtained using ABAQUS software. A numerical model was validated based on the experimental findings for the M60 grade of geostrap. Further, the performance of the M65 grade of geostrap was compared with the available published data on geogrids. The comparison of both types of geosynthetics plays a pivotal role for a field engineer and practitioner, enabling them to identify the suitability, applicability, and effectiveness of geostraps and geogrids for sand.

This study demonstrates the potential of the geostrap as a reinforcing material, especially in challenging or problematic soils where traditional geosynthetics might underperform. Furthermore, more research should be carried out on the durability and pullout performance of geostraps under various loading and confinement conditions. Future studies could examine the effects of environmental degradation, long-term creep behaviour, and cyclic loading responses. Empirical correlations and generalised design frameworks will be established, with investigations conducted across various soil types and controlled environmental parameters. Ultimately, this will facilitate broader adoption in routine and crucial soil reinforcement situations.

2. Materials and Methods

This study involved a pullout test conducted on site to evaluate the interaction of geostraps with sandy soil. The test was performed for a geostrap with an M65 strength grade on locally available soil. The soil gradation had 98.39% sand, with C_u and C_c values of 2.47 and 1.07, respectively, and was classified as poorly graded sand (SP). The lab-tested physical properties of the geomaterials used in the study are mentioned in

Table 1. Physical properties of geomaterials.

Geomaterial	Properties	Testing	Value
Soil	Classification	IS 2720 (Part 4) a	SP
	OMC (%)	IS 2720 (Part 8) b	11.6
	MDD (g cc−1)	IS 2720 (Part 8) b	1.853
	CBR (%)	IS 2720 (Part 16) c	16.28
	Angle of internal friction, Φ (°)	IS 2720 (Part 13) d	33
Geostrap (grade M65)	Width (mm)	Vernier Caliper	91
	Thickness (mm)	Micrometer	1.8
	Ultimate tensile strength, UTS (kN)	ASTMD-6637 e (Method A)	64.14
	Elongation at UTS (%)	ASTMD-6637 e (Method A)	9.45

^a [28], ^b [29], ^c [30], ^d [31], and ^e [32].

.

A specially designed pullout test apparatus was employed, consisting of a soil–geostrap assembly, pullout loading system, and monitoring equipment to measure displacement, as shown in Figure 2. The geostrap was anchored and embedded in the soil with free ends connected to the loading mechanism. The pullout resistance for the M65 grade of geostrap was determined based on the effective area method [33] using the following expression:

$${\tau }_{av}=\frac{F_{max}-F_T}{2B{L}_T}$$

(1)

where

• τ_av = the average pullout resistance;
• F_max = the maximum pullout force;
• B = the width of the reinforcement;
• L = the length of the reinforcement.

Figure 2. (a) Site images of anchored geostrap, (b) pullout test assembly, and (c) schematic diagram of the pullout test.

Figure 2. (a) Site images of anchored geostrap, (b) pullout test assembly, and (c) schematic diagram of the pullout test.

According to the guidelines, a cover of at least 150 mm of soil should be provided above the reinforcement [34]. Therefore, the geostrap was placed 150 mm below the top surface of the subgrade. The tests were conducted with varying normal stresses to simulate field conditions for different vehicular loadings. The pullout force and displacement of the geostrap were noted for each cycle of the pullout test. A pressure gauge utilising strain gauge sensors was used to determine the hydraulic pressure applied by the jack, while an LVDT (Linear Variable Differential Transformer) displacement transducer was used to measure the displacement of the geostrap. After that, the setup was prepared again for a different surcharge value. Once the experimental data for the M65 grade of geostrap was obtained, they were reproduced in numerical analysis. After successfully validating the model, the results for different grades (M50, M37.5, and M25) were found numerically. The results were then compared to determine the effectiveness of each grade in providing possible areas of application based on the pullout strength. After that, the M65 grade of geostrap was compared with biaxial geogrids to establish a comparative analysis and their possible areas of applications based on the findings.

2.1. Design Tensile Strength of Geostraps

The geostraps were made of high-tenacity polyester yarn encased in a polymer sheath. Different types or grades of geostraps are available globally based on different tensile strengths. These were simulated, and their properties are tabulated in

Table 2. Different grades of geostraps along with their properties.

Geostrap Grade	Mass per Unit Length (g/m)	Characteristic Short-Term Tensile Strength (kN/m)	Partial Material Factor, γm	Partial Consequence Factor γn
M25	103	25	2.50	1.0
M37.5	111	37.5	2.43	1.0
M50	138	50	2.38	1.0
M65	179	65	2.36	1.0

. Geostraps of different grades will impart different tensile strengths to the soil subgrade, which further impacts their pullout resistance.

According to Geoguide 6 [35], the design tensile strength, T_D, of a geostrap is expressed as

$$T_D=\frac{T_{ult}}{{\gamma }_m{\gamma }_n}$$

(2)

where

• T_ult = the characteristic short-term tensile strength;
• γ_m = the partial material factor on the tensile strength of a geostrap;
• γ_n = the partial consequence factor to account for failure.

The partial material factor (γ_m) was considered for the environmental effects on material durability, construction damage, creep, hydrolysis, and stress rupture for a design life of 120 years at 30 degrees Celsius. The value of γ_m varies from 2.36 to 2.61 for different grades of geostraps. The partial consequence factor (γ_n) accounts for the potential negative impact of various factors such as installation, damage, creep, weathering, and degradation due to chemical or biological reaction. γ_n varies from 1.0 to 1.1 for different consequence factors.

Assumptions:

The following assumptions were made in the present study:

• The friction between the front end of the geostrap and the connecting edge of the pullout box is negligible.
• The parameters obtained from the lab are independent of the scale effect.
• The effective area method was used to determine the pulling force, and, accordingly, the displacement was measured at the front of the geostrap.

The first assumption was made as proper greasing between the front end of the geostrap and the connecting edge of the pullout box assembly was performed. The second assumption implies that the characterisation and properties of the geostrap and soil used for the lab and field testing were the same. The third assumption suggests that out of the three ways for evaluating the average pullout resistance, total area, effective area, and maximum slope area, the effective area method was used as it related more to the actual test conditions [15].

2.2. Numerical Simulation

The stabilisation of unpaved roads using geogrid reinforcement has been studied [36]. On the same line of action, a physical problem of geostrap-reinforced unpaved road is solved using the Finite Element Method (FEM) in ABAQUS software. An elastoplastic constitutive model for the subgrade soil was made. In addition to elastoplastic models, Mohr–Coulomb, viscoplastic, and hypoplastic models can be used for different types of soil. Finite Element Analysis (FEA) theories state that local variation in cell sizes should be minimal, as large aspect ratios can result in the magnification of errors and can cause low degrees of accuracy. Therefore, the aspect ratio in this study was maintained to be 1:1 or 1. The unpaved test section was 500 mm long, 1000 mm wide, and 500 mm deep. An additional surcharge of 17 kN m⁻² was added to compare it with the geogrid-reinforced subgrade. The material parameters used in the modelling are mentioned in

Table 3. Properties of materials used in FEM modelling.

Material	Element Type	Thickness (m)	Placement Depth (m)	E (MPa)	ν
Subgrade	Solid	0.5	-	E = 40 a	0.3
Geostrap	Solid	0.01	0.150	Eg = 65 b	0.42

^a E = modulus of elasticity of sand, ν = Poisson’s ratio [37]; ^b E_g = tensile stiffness of geostrap [38].

. A C3D8R element type was used for the geostrap and subgrade, an 8-node linear brick.

3. Results

3.1. Convergence Test and Validation

Various mesh sizes were compared to analyse the scale effect of mesh size. Different numbers of subgrade elements were considered. The subgrade model showed convergence when the mesh seed size was 20 mm. Therefore, the meshing was performed with a 50 × 25 × 25 number of elements for a 1000 mm × 500 mm × 500 mm subgrade soil mass. The subgrade was then subjected to various axial loads to obtain deformation values at the centre-top of the subgrade. The strain-controlled mode causes deformations of fixed magnitude to be determined. The convergence test for stresses at a point, A, at the centre-top of the subgrade is shown in

Table 4. Mesh convergence test for stress values at the centre-top point, ‘A’, of the subgrade with simulation time.

Mesh Size	Meshed Subgrade	Stress at Point A	Simulation Time
0.015		7.8119	3 h 40 m
0.020		7.8117	2 h 38 m
0.025		7.8105	1 h 20 m
0.030		7.7941	40 m

. The time taken for the simulation performed on a computer with 32 GB RAM and a 6 GB graphics card with Core i5 is also mentioned.

The results obtained from the experimental analysis were used to validate the numerical model for the same test setup. The model was subjected to the same conditions, and a pullout force per unit length vs. deformation plot was obtained for the geostrap, which was compared with the experimental findings for validation. The validation results are shown in Figure 3.

A couple of reasons can be given for the slight deviation (around 5%) of the numerical results from the experimental findings. These deviations were due to the fully controlled testing in numerical models and the calculations for each element/mesh, the homogeneity of the subgrade and the geosynthetic material, and the consistency of the density and other mechanical properties in the numerical model.

The Von Mises stresses in the subgrade and geostrap (enlarged) with the top half removed from the viewport for visualisation are shown in Figure 4. The strain-controlled model uses the following expression for pullout:

$$t_s=K_s{∆}_s$$

(3)

where

• t_s = the traction/pullout force;
• K_s = the stiffness in the shear plane;
• ∆_s = the displacement.

Figure 4. Stress contours in the subgrade due to pullout of the geostraps of (a) M25 and (b) M65 grades (top half removed and rotated for visualisation).

Figure 4. Stress contours in the subgrade due to pullout of the geostraps of (a) M25 and (b) M65 grades (top half removed and rotated for visualisation).

3.2. Comparison of Different Grades of Geostraps

The pullout test for different grades of geostraps was conducted using ABAQUS software. Figure 5 shows a plot of the pullout force and the displacement of various grades of geostraps. As per the results obtained, the M65-grade geostrap exhibits 15.25 kN m⁻¹ of pullout resistance per unit length, followed by 14.5 kN m⁻¹, 13.4 kN m⁻¹, and 12.1 kN m⁻¹ for the M50, M37.5, and M25 grades, respectively. The judicial use of the desired grade of geostrap, considering the soil type and load, may cut project costs and achieve sustainability goals. The failure envelope can be effectively used to determine the pullout force per unit length for intermediate grades of geostraps.

3.3. Pullout Capacity: Geostrap vs. Geogrid

The pullout capacity obtained for the M65-grade geostrap was compared with that of a biaxial geogrid reinforcing the same type of subgrade soil (SP) by Ochiai for the same surcharge value of 17 kN m⁻². The comparative plot of the pullout strengths of the geostrap and geogrid is shown in Figure 6.

The geostrap shows high initial pullout resistance (up to 20% more) compared to the geogrid. Before any displacement, the geostrap withstands 18 kNm⁻¹ of pullout force per unit length compared to 15 kNm⁻¹ for the geogrid. However, the maximum pullout resistance of the geogrid (53 kNm⁻¹) is more than that of the geostrap (49 kNm⁻¹). The frictional and interlocking effect of the geostrap dominates at the beginning due to the high contact area. As soon as the displacement started, the effective contact area reduced simultaneously. This constantly reduced the reinforcement effect of the geostrap. Furthermore, this led to the interlocking effect of the geogrid dominating the limited frictional resistance of the geostrap. Therefore, it can be deduced from the findings that geostraps may prove highly effective as reinforcements for low-volume roads and backfill materials. However, geogrids may provide more promising results for poorly graded sand in high-volume roads.

4. Discussion

The findings of this study highlight the importance of assessing the pullout performance of reinforcement materials to support sustainable geotechnical infrastructure development. The experimental pullout tests conducted on sandy soil provided valuable insights into the interface behaviour between the soil and the geostraps under controlled boundary conditions. A significant contribution of this study is the integration of experimental data into a numerical model for validation and extended analysis. The model demonstrated high consistency with the test results, with deviations well within acceptable limits. This confirms the reliability of the numerical approach in predicting geostrap behaviour under various conditions, enabling a more comprehensive evaluation beyond the limitations of physical testing. Based on the results, the applications of different grades of geostraps based on the maximum pullout resistance are shown in

Table 5. Applications of different grades of geostraps based on their pullout capacity.

Geostrap Grade	Maximum Pullout Strength (kN/m2)	Application Area	Soil Conditions	Remarks
M25	12.1	Landscaping fill, shallow slope stabilisation	Well-compacted, non-cohesive to moderately cohesive soils	Suitable for non-critical zones with low shear stress and minimal surcharge
M37.5	13.4	Light traffic embankments, shallow foundation beds	Moderately firm soils with moderate moisture	Improves bearing and settlement resistance in lightly loaded zones
M50	14.5	Reinforced slopes, pavement subgrade improvement	Soft to firm soils with variable moisture	Common for road-base stabilisation and slopes under moderate traffic loads
M65	15.25	Embankments over soft soils, seismic reinforcement	Weak, compressible, or aggressive soils	Preferred in critical areas with high loads; long-term performance is required

.

Higher-grade geostraps could be suitable for critical infrastructure requiring a higher tensile capacity, whereas lower grades may suffice for temporary or less critical applications. A comparison of the M65 grade of geostrap with the biaxial geogrid for sandy soil was also performed. Based on the findings, the area of application for both types of reinforcement is shown in

Table 6. Application of different geosynthetics based on their initial and final pullout strengths.

Type of Geosynthetics	Initial Pullout Strength (kN/m)	Initial Strength Difference (%)	Final Pullout Strength (kN/m)	Final Strength Difference (%)	Final Pullout Displacement (mm)	Application Area	Remarks
Geostrap M65	18	20	49	8.16	40	Embankments over soft soils, seismic reinforcement, etc.	High initial pullout strength suitable for low-volume roads
Geogrid	15		53		60	Pavements on weak subgrade, erosion control, etc.	High total pullout strength suitable for high-volume roads

.

Both geostraps and geogrids are polymeric compositions that may lead to longer service life, corrosion resistance, and potentially lower environmental impact if manufactured from recycled or low-energy input materials. The flexibility of the materials also suggests easier handling, transportation, and installation, which can contribute to reducing the overall carbon footprint of infrastructure projects.

5. Conclusions

This study involves an experimental analysis to validate a numerical model for finite element analysis. The deviation of the experimental results from those of the numerical analysis is around 5%. The numerical model was then tested to determine the pullout strength of different grades of geostraps. The pullout performance of the M65 grade of geostrap was compared with that of a biaxial geogrid. The analysis highlights the potential of geostraps as efficient, durable, and environmentally resilient alternatives for reinforced soil structures. Based on the findings of this study, the following may be concluded:

The M65 grade of geostrap exhibits a higher pullout resistance (15.25 kN m⁻¹), followed by the M50 (14.5 kN m⁻¹), M37.5 (13.4 kN m⁻¹), and M25 (12.1 kN m⁻¹) grades of geostraps. This wide range of pullout resistance offers ample options for different geotechnical challenges. Considering the soil type and load, the judicious use of the desired grade of geosynthetics may cut project costs and achieve sustainability goals. Based on the pullout resistance offered in sand, their applications can be listed as shown in Table 5.

Geostraps initially exhibited greater pullout resistance in sandy soil than geogrids due to their combined reliance on friction and partial interlocking. However, the maximum pullout resistance of the geogrid (53 kNm⁻¹) was higher than that of the geostrap (49 kN m⁻¹). Based on the test results, the application area of both geostraps and geogrids can be determined as shown in Table 6. Therefore, it can be deduced from the findings that geostraps may prove highly effective as reinforcements for low-volume roads and backfill materials. However, in the case of high-volume roads, geogrids show more promising results for poorly graded sand.

The pullout behaviour of geostraps represents a significant advancement in geosynthetic reinforcement materials. By comparing the performance of geogrids and geostraps, a versatile and efficient solution for reinforced soil structures is provided. The findings of this study underscore the potential of geostraps to address the limitations of conventional geosynthetics, paving the way for more robust and adaptable geotechnical designs.

Scope for Future Research

While this study highlights the advantages of geostraps in sandy soil conditions, further research may expand their applicability. Investigations into their performance in fine-grained soils, such as clay and silt, would provide a more comprehensive understanding of their capabilities. Additionally, long-term studies addressing the effects of environmental factors, such as temperature fluctuations, moisture, and chemical exposure, would enhance their reliability in real-world conditions.