ISO 12956 Geotextiles and Geotextile Related Products - Determination of Characteristic Opening Size

The ISO 12956 Geotextiles and geotextile-related products - Determination of characteristic opening dimensions standard, developed by the International Organization for Standardization (ISO), describes a method for determining the characteristic size of openings in a single layer of a geotextile or geotextile-like product using the wet screening principle.

In principle, the basis of this test method is to determine the aperture size corresponding to the specified size of the granular material being passed through.

The sample should be handled and folded as little as possible to prevent damage to its structure. The sample is held flat without any load. The sample should be taken in accordance with the ISO 9862 standard. Five test specimens should be cut to the appropriate dimensions for the sieve setup to be used. When preparing samples for woven materials, care should be taken to avoid damaging the structure due to cutting. The use of a thermal cutting device may be beneficial. According to this test method, samples with a width smaller than the sieve diameter should be prepared for testing.

This standard defines a test method for determining the characteristic aperture size of geotextiles and related products. This parameter is crucial in filtration and separation applications as it indicates the size of particles that can pass through the material.

Geotextiles are permeable fabrics used in civil engineering for the following purposes:

• Filtration (allowing water to pass through while retaining soil particles)
• Separation
• Strengthening
• Drainage
• Erosion control

Specific opening size helps engineers select the appropriate geotextile that will retain most soil particles while maintaining adequate permeability. Permeability (ISO 11058 standard) is one of the essential hydraulic properties, along with others.

The aperture size value represents the aperture size at which 90 percent of the apertures are smaller than this diameter (by dry sieving with glass beads or similar methods).

This method uses dry sieving with standardized glass beads or particles. A geotextile sample is placed in a sieving device. Fractions of glass beads with known, narrow size distributions are sieved through the geotextile under controlled conditions. The mass of beads passing through the sieve is measured for different size fractions. A sieving curve is created (bead diameter versus passing percentage). The characteristic aperture size is read from the curve (typically the diameter at which 90 percent of the apertures are smaller).

The ISO 12956 standard specifies the following:

• Sample preparation
• Device (sieves, shaking mechanism)
• Bead properties
• Number of tests (usually more than one sample)
• Calculating and expressing the results.

This standard differs from the “ASTM D4751 Standard test methods for determining the apparent opening size of geotextile materials” standard, which some studies have compared in terms of reliability and correlation.

The ISO 12956 standard is used worldwide in specifications for the following areas:

• Road and railway construction
• landfills
• Coastal and river protection
• Drainage systems
• Filling filters

The ISO 12956 standard is essential because it provides a globally consistent, reliable, and realistic method for determining the characteristic aperture size of geotextiles and related products. This is fundamental to their performance in critical geotechnical applications such as filtration, drainage, and separation. Without standardization, manufacturers, engineers, and testing laboratories may use different methods, leading to inconsistent results, incompatible product specifications, and potential project failures such as soil erosion, clogging, or structural instability.

The main reasons why this standard is considered important are:

• Standardization for consistency and comparability: Geotextiles are used in infrastructure projects worldwide (e.g., roads, dams, landfills, coastal protection). Different testing approaches can yield varying aperture size values; for example, dry screening can produce results 30-100% larger than wet screening due to issues such as static electricity, particle bridging, or glass bead entrapment (ASTM D4751 standard).

The ISO 12956 standard mandates the wet screening (hydrodynamic) principle, where a graded granular mixture is washed through a geotextile on a vibrating table with a controlled flow of water. This reduces discrepancies in results by more accurately simulating real-world hydraulic conditions.

• Ensuring proper filtration and soil compatibility: The aperture size represents the size at which 90% of the particles passing through the geotextile are smaller, and directly determines the retention criteria. This prevents:
• Piping/erosion: If the openings are too large, fine soil particles are washed away, destabilizing the structures.
• Clogging/blinding: If too small, the geotextile will obstruct water flow, leading to accumulation and reduced permeability.

In coastal or road applications, incompatible aperture sizes can lead to premature failure; for example, in silty sands (particle sizes 2 mm to 2 μm), the aperture size must retain most of the fine particles while allowing drainage. The wet method avoids errors in dry testing and is important for design reliability.

• International trade and regulatory compliance: Facilitates global trade by aligning with project specifications and codes. Non-standardized testing can invalidate warranties, fail certifications, or lead to legal disputes in multinational projects.
• Addressing the limitations of alternatives: Compared to the ASTM D4751 (beaded dry sieving) standard, the ISO 12956 standard eliminates significant shortcomings:
• There are no electrostatic problems caused by humidity.
• It uses natural particles for broader validity.
• It provides accurate measurements to approximately 20-45 μm.
• It takes into account the actual particle passage under flow.

Studies show that it correlates better with in-situ behavior and justifies its adoption for sensitive filter design. In high-risk earthquake zones or coastal applications, this standard ensures that geotextiles meet local/international durability requirements.

• Preventing costly failures and supporting regulations: Poor filter design has led to real-world problems, with clogged drains causing pavement failures, pipes under dams/embankments, or pollution in landfills. This standard ensures compliance with design codes and provides third-party verification and quality assurance, reducing risks in high-risk infrastructure.

In conclusion, the ISO 12956 standard recognizes that filtration is a performance-critical function, where small variations in pore size can determine whether a system will last for decades or fail prematurely. This standard provides a reliable, internationally accepted, and field-appropriate method for measuring this characteristic.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services compliant with the ISO 12956 standard.