ISO 12958-1:2020

Overview - ISO 12958-1:2020 (Geotextiles in‑plane water flow index test)

ISO 12958-1:2020 specifies an index test method to determine the constant‑head in‑plane water flow capacity of geotextiles and geotextile‑related products (factory‑assembled products only). The standard measures volumetric flow per unit width under defined normal compressive stresses and hydraulic gradients, providing a repeatable indication of how water flows parallel to the plane of the product. This part of the series complements ISO 12958-2, which covers the full in‑plane water flow performance test.

Key topics and technical requirements

• Scope: Index test for in‑plane water flow capacity of factory‑assembled geotextile products.
• Measured quantities: in‑plane flow Q, in‑plane water flow capacity q_p_index(σ/i), hydraulic gradient i, and normal compressive stress σ.
• Test principle: constant‑head apparatus measuring flow along the specimen under varying σ and typical hydraulic gradients.
• Apparatus requirements:
  • Maintain constant head for hydraulic gradients at least 0.1 and 1.0.
  • Discharge head ≤ 100 mm (corrections required if exceeded).
  • Loading mechanism to apply constant normal stress within ±5%.
  • Contact surfaces: closed‑cell foam rubber (F) or smooth rigid membrane (R); report boundary used.
  • Minimum apparatus dimensions: width ≥ 0.2 m, effective flow length ≥ 0.3 m; handle specimen thicknesses up to 50 mm.
  • Apparatus leakage ≤ 5% of actual test flow.
• Water quality & conditions:
  • Temperature typically 18–22 °C (preferably near 20 °C).
  • Dissolved oxygen ≤ 10 mg/kg (to avoid air‑clogging).
  • Water filtered; recirculation permitted with temperature control.
• Specimens:
  • Cut three specimens in machine direction and three in cross‑machine direction.
  • Typical minimum dimensions: length ≥ 0.3 m (flow direction), width ≥ 0.2 m.
  • Handle specimens flat and avoid folding.
• Reporting & calculations: procedures for calculating q_p_index, corrections for temperature, graphical representation, and required test report content.

Applications

• Assessing drainage capacity and lateral transmissivity of geotextiles and geocomposites.
• Quality control and comparability of factory‑assembled drainage layers used in:
  • Landfill and subbase drainage systems
  • Road and railway subdrainage
  • Retaining wall and slope drainage
  • Stormwater management and erosion control systems
• Providing index values for design selection, product datasheets, and procurement specifications.

Who should use this standard

• Geosynthetics manufacturers and product developers
• Independent testing laboratories and QA/QC teams
• Civil and geotechnical engineers specifying drainage materials
• Specifiers, contractors and regulators requiring standardized test data

Related standards

• ISO 12958-2 - In‑plane water flow performance test
• ISO 9862 - Sampling and specimen preparation
• ISO 9863-1 - Thickness determination (single layers)
• ISO 5813 - Dissolved‑oxygen determination
• ISO 2854 - Statistical interpretation of data
• ISO 10320 - Geosynthetics identification

Keywords: ISO 12958-1:2020, geotextiles, in‑plane water flow, water flow capacity, index test, geosynthetics, hydraulic gradient, constant‑head apparatus.