Biaxial Geogrid Pull-Out Testing: Standards, Methods, and Interface Performance Evaluation

The biaxial geogrid pull-out test is a critical laboratory method for evaluating the anchorage capacity and soil–geogrid interaction of biaxial geogrids used in reinforced soil structures, roadway stabilization, and embankment engineering. This article provides a comprehensive technical overview of biaxial geogrid pull-out testing, covering applicable standards such as ASTM D6706, EN 13738, and GB/T 17637, sample requirements, testing procedures, key performance parameters, and practical engineering applications. Whether you are a geotechnical engineer, laboratory technician, or product manufacturer, this guide will help you understand how pull-out resistance data supports safe and economical reinforced soil design.

Technical Overview

A biaxial geogrid is a polymeric reinforcement product—typically manufactured from punched-and-drawn polypropylene, coated polyester yarns, or welded polymer strips—characterized by a grid structure with apertures that provide tensile strength in both the machine direction (MD) and cross-machine direction (CMD). When embedded in soil, the transverse ribs act as bearing members that resist lateral displacement, making the quality of the soil–geogrid interface a decisive factor in reinforced soil performance.

The biaxial geogrid pull-out test (also known as the pullout resistance test or soil–geogrid interface friction test) measures the force required to extract a geogrid specimen from a confined soil mass under controlled normal stress. The test quantifies the interface shear strength, the pull-out resistance per unit width, and the interaction (bond) coefficient between the geogrid and the surrounding soil. These parameters are essential inputs for limit equilibrium analyses and internal stability design of mechanically stabilized earth (MSE) walls, reinforced soil slopes, and basal reinforcement layers.

The test is governed by internationally recognized standards, including ASTM D6706 (Standard Test Method for Measuring Geosynthetic Pullout Resistance in Soil), EN 13738 (Determination of Pullout Resistance), and GB/T 17637 (Determination of pull-out friction characteristics of geosynthetics). Testing is typically performed by accredited third-party laboratories or in-house geotechnical laboratories using large-scale pull-out boxes equipped with servo-controlled loading and data acquisition systems.

Testing Samples

Extruded Biaxial Polypropylene Geogrids

These are integrally formed geogrids produced by punching a polymer sheet and biaxially stretching it to orient the molecules in both directions. Specimens are cut with representative apertures and a sufficient number of transverse ribs (typically a minimum of three bearing ribs within the confined soil zone) to ensure valid pull-out behavior rather than localized single-rib extraction.

Coated Polyester (PET) Biaxial Geogrids

Woven or knitted high-tenacity polyester geogrids with PVC or polymer coating are tested to evaluate how the coating, junction geometry, and rib stiffness influence interface friction and pull-out capacity under sustained and short-term loading conditions.

Welded and Composite Biaxial Geogrids

Geogrids assembled from welded polypropylene or polyethylene strips, as well as fiberglass and basalt-fiber composite grids used in pavement applications, are tested to verify junction integrity under pull-out loading and to confirm that the bearing members do not debond or fail prematurely at the welded or adhesive junctions.

Specimen Preparation Requirements

• Specimen width sufficient to minimize side boundary effects, commonly 300 mm to 600 mm depending on box geometry and aperture size.
• Embedded (anchorage) length inside the soil, typically 300 mm to 1,000 mm, with the clamped length extending outside the box through a low-friction sleeve.
• Cutting aligned with MD and CMD ribs so that transverse bearing ribs are perpendicular to the pull direction.
• Conditioning at standard laboratory temperature (23 ± 2 °C) and humidity prior to testing.
• A minimum of three replicate specimens per normal stress level for statistical reliability.

Testing Items

The biaxial geogrid pull-out test evaluates the following key performance parameters:

• Peak pull-out resistance (kN/m): the maximum extraction force per unit width of geogrid recorded during the test.
• Residual pull-out resistance (kN/m): the post-peak resistance at large displacements, important for ductility assessment.
• Displacement at peak resistance (mm): indicates the interface deformation required to mobilize full anchorage.
• Interface shear strength parameters: apparent adhesion (ca) and interface friction angle (δ) derived from multi-level normal stress testing using Mohr–Coulomb regression.
• Pull-out interaction coefficient (Ci or μ): the ratio of the soil–geogrid interface shear strength to the internal shear strength of the soil alone, used directly in reinforced soil design.
• Secant pull-out stiffness (kN/m per mm): the initial slope of the force–displacement curve, reflecting serviceability performance.
• Failure mode classification: pull-out (sliding) failure along the interface versus tensile rupture of longitudinal ribs or junction failure.
• Effect of confinement: variation of resistance with normal stress levels, verifying linear or near-linear normal stress dependency.

Testing Methods

Test Apparatus and Setup

Testing is conducted in a rigid pull-out box, typically 300 mm to 1,500 mm in internal length, 300 mm to 1,000 mm in width, and 200 mm to 600 mm in depth. The apparatus includes a normal pressure application system (pneumatic air bag or rigid pressure plate), a serrated or wedge-type clamping head attached to a servo-controlled actuator, a calibrated load cell, linear variable displacement transducers (LVDTs) to capture both clamp and in-soil geogrid displacement, and a computerized data acquisition system. A low-friction sleeve at the box front wall eliminates erroneous boundary friction at the geogrid exit point, as required by ASTM D6706.

Sample Preparation and Soil Placement

The selected backfill—commonly a well-graded sand, gravel, or crushed stone representative of the project soil—is placed and compacted in layers to the target dry density at the specified moisture content. The lower soil layer is placed and leveled, the geogrid specimen is positioned on the surface with the clamped end passing through the front sleeve, and the upper soil layer is compacted with care to avoid damage or pre-stressing of the specimen. Density and moisture are verified for each test to ensure repeatability.

Test Procedure

• Apply the target normal stress (commonly 20, 50, 100, and 150 kPa) and allow the soil–geogrid system to consolidate under load.
• Initialize the actuator and apply pull-out displacement at a constant rate of 1.0 ± 0.1 mm/min in accordance with ASTM D6706.
• Continuously record pull-out force, clamp displacement, and in-soil displacement until either peak resistance followed by a defined post-peak drop is observed, or 15% cumulative strain is reached, or tensile rupture occurs.
• Repeat the test at each normal stress level with fresh specimens to construct a complete dataset for interface parameter regression.
• Exhum the specimen after testing and document the failure mode, rib deformation, and any junction damage photographically.

Data Reduction and Analysis

Pull-out force per unit width is plotted against displacement for each confining stress. The average interface shear stress is computed as the pull-out resistance divided by twice the total embedded plan area, accounting for both the top and bottom geogrid surfaces. Peak interface shear stresses from multiple normal stress levels are regressed using the Mohr–Coulomb failure criterion to obtain the apparent adhesion intercept and the interface friction angle. The pull-out interaction coefficient is then calculated as the ratio of the interface shear strength at a given normal stress to the corresponding soil–soil shear strength measured in a large-scale direct shear test, in accordance with ASTM D6706 and EN 13738 methodology.

Quality Assurance and Standards Compliance

All testing should be performed under documented quality management systems, with calibrated load cells, displacement transducers, and pressure gauges traceable to national metrology standards. Reports issued by accredited third-party laboratories typically include specimen dimensions, soil classification and compaction data, normal stress levels, complete force–displacement curves, calculated interface parameters, and failure mode documentation, providing the objective evidence required for design acceptance, product certification, and construction quality verification.

Applications

Biaxial geogrid pull-out test data directly support the design, certification, and quality control of reinforced soil systems across multiple engineering sectors:

• Mechanically stabilized earth (MSE) walls: determining reinforcement anchorage length and internal stability against pull-out failure.
• Reinforced soil slopes (RSS): verifying bond coefficients for staged construction on soft or steep sites.
• Basal reinforcement of embankments: confirming that geogrid layers over soft subgrade can mobilize sufficient interface resistance for load transfer.
• Roadway and pavement base stabilization: validating that biaxial geogrids interlock with aggregate and provide lateral confinement, enabling reduced base thickness and longer service life.
• Working platforms and haul roads: ensuring safe bearing capacity over weak ground for heavy construction equipment.
• Railway subgrade improvement and ballast reinforcement: assessing long-term interface performance under cyclic loading conditions.
• Product development and certification: benchmarking aperture geometry, rib thickness, and junction efficiency of new biaxial geogrid products against established performance criteria.
• Construction disputes and forensic engineering: providing objective laboratory evidence of whether supplied materials meet specified pull-out performance requirements.

FAQ

What is the difference between a pull-out test and a direct shear test for geogrids?

A pull-out test extracts the geogrid from within the soil mass, evaluating anchorage resistance along both faces of the reinforcement and the progressive mobilization of bearing members along the embedded length. A direct shear test (such as ASTM D6638 or ISO 12957-1) shears the soil across a fixed plane at the geogrid interface, measuring shear resistance at a constant interface area. Pull-out parameters govern reinforcement anchorage design, while direct shear parameters govern soil–reinforcement sliding assessments.

What normal stress levels are used in biaxial geogrid pull-out testing?

Typical laboratory programs apply at least three normal stress levels—commonly 20, 50, and 100 kPa, sometimes extended to 150 or 200 kPa—to bracket the overburden stresses expected in the field. Multiple stress levels are necessary to derive the interface friction angle and apparent adhesion through Mohr–Coulomb regression and to compute design interaction coefficients.

How is the interaction coefficient calculated?

The pull-out interaction coefficient is defined as the ratio of the maximum soil–geogrid interface shear strength to the internal shear strength of the same soil at the same normal stress. Values between 0.6 and 1.2 are commonly reported for biaxial geogrids in granular backfill, depending on aperture size, rib geometry, soil gradation, and compaction quality. Design codes may impose minimum bond coefficient requirements for specific applications.

Which failure mode indicates a valid test result?

A valid pull-out test produces a clear peak followed by softening, indicating that the geogrid slid along the soil interface rather than rupturing. If the longitudinal ribs or junctions fail in tension before the interface is mobilized, the test measures the in-soil tensile capacity rather than anchorage resistance, and the specimen geometry, embedment length, or confining stress must be revised and the test repeated.

Why must a sleeve be installed at the front wall of the pull-out box?

Without a low-friction sleeve, friction between the moving geogrid and the rigid front wall of the box is falsely recorded as interface resistance, inflating the measured pull-out force. ASTM D6706 and comparable standards require an extended sleeve inside the box so that the reported force reflects only the soil–geogrid interaction within the confined test zone.

How long does a biaxial geogrid pull-out test take?

At the standard displacement rate of 1 mm/min, a single test typically runs 30 minutes to several hours depending on the displacement required to reach peak and post-peak criteria. Including specimen preparation, soil compaction, consolidation, and three normal stress levels with replicates, a complete test program generally requires one to two weeks in the laboratory.

Can field pull-out tests replace laboratory testing?

Field pull-out tests, performed by extracting installed geogrids with hydraulic jacks and reaction frames, provide valuable verification of actual construction quality and in-situ soil conditions. However, laboratory tests under ASTM D6706, EN 13738, or GB/T 17637 deliver controlled, repeatable, and standards-compliant parameter determination for design, and the two methods are best used in a complementary manner on major projects.