Overview
ISO 12956:2019 specifies a laboratory test method to determine the characteristic opening size of a single layer of a geotextile or geotextile‑related product using the wet‑sieving principle. The characteristic opening size (O90) is defined as the opening equal to the particle size d90 of the granular material that passes through the geotextile. This international standard is part of the geosynthetics test suite and is used to quantify filtration and drainage performance of geotextiles.
Key topics and technical requirements
- Principle: Determination of O90 by passing a graded granular material through a saturated geotextile specimen under controlled wet‑sieving.
- Apparatus: Sieving unit (50–60 Hz; predominantly vertical motion; 1.5 mm amplitude / 3 mm swing height) with specimen clamping, transparent cover, spray nozzle(s) and collection/filter device.
- Granular material: Cohesion‑less, essentially round particles; coefficient of uniformity Cu between 3 and 20; particle size distribution chosen so d20 ≤ O90 ≤ d80 for accuracy.
- Specimens: Cut five specimens (dimensions per sieve apparatus); handle flat and avoid folding; special preparation guidance for narrow or knitted tubular geotextiles (Annex C).
- Procedure highlights:
- Saturate specimen in water with 0.1% non‑ionic surfactant for ≥12 h.
- Apply granular material at a mass equivalent to (7,0 ± 0,1) kg/m of exposed sieving area.
- Operate sieving for 600 s while spraying water uniformly; collect passed material on filter paper (max opening 10 µm).
- Dry and weigh passed and retained fractions; repeat until three valid specimens measured; use up to five if variability >15%.
- Accuracy & equipment: Balance resolution and allowable errors, drying ovens (≤110 °C), filter paper, and sieving grids are specified to ensure repeatability.
- Annexes: Mesh sizes (A), worked example for O90 determination (B), and knitted tubular preparation (C).
Applications and users
ISO 12956:2019 is essential for:
- Geotechnical engineers assessing filtration compatibility between soils and geotextiles.
- Manufacturers of geotextiles and geosynthetic products for product development and quality control.
- Independent testing laboratories performing standardized performance tests.
- Specifiers and contractors selecting geotextiles for drainage, erosion control, separation and filtration applications.
Practical benefits include consistent measurement of opening size for product comparison, verification of filtration performance in design, and documented test results for procurement and compliance.
Related standards
- ISO 9862 - Sampling and preparation of test specimens for geosynthetics
- ISO 10320 - Identification on site
- ISO 565 and ISO 2591‑1 - Guidance on sieve selection and particle size distribution analysis (referenced for granular material characterization)
Keywords: ISO 12956:2019, geotextiles, characteristic opening size, O90, wet‑sieving, geosynthetics testing, particle size distribution, laboratory method.