The project examined the suitability of the current German guideline for geotextile filters in road construction, particularly the lower bound of the characteristic opening width (O90). The guideline, published in 2016, distinguishes three safety cases and specifies allowable opening widths. While the upper limit was considered technically feasible, the lower limit was largely a concession to the products available on the market. In practice, most geotextiles sold for road construction have opening widths between 0.06 mm and 0.08 mm, the lower bound of the guideline. Field experience has shown that such narrow openings lead to a time‑dependent loss of permeability and to clogging (colmatation) when the filter is exposed to overlying soil. This reduces the hydraulic effectiveness of the filter and can compromise slope stability by increasing pore‑water pressure.

To investigate these issues, the research team conducted a comprehensive market survey and a field survey of geotextile use in road construction. Both surveys confirmed that only products with the lower opening‑width range and thin filter layers are available and used. The core of the project was the development and application of laboratory test methods to evaluate filter performance under realistic conditions. A test rig was designed and built, and a series of preliminary tests were carried out before the main system tests. The system tests comprised long‑term permeability tests (lateral flow tests, LFT), cyclic loading tests, and suspension tests with kaolin to simulate fine‑particle transport. Four representative soils were selected: two erosion‑ and suffosion‑prone soils (wide‑graded sand and a light plastic silty loam) and two relatively stable soils (fine sand and a sand‑clay mix). Seven geotextiles with varying opening widths (0.06–0.3 mm) and filter thicknesses (1 mm to 3.3 mm) were chosen from the market. For the upper opening‑width limit, a woven monofilament fabric was used because no woven products with the upper limit exist.

The experimental results showed that the narrow opening widths of the available geotextiles lead to a pronounced reduction in permeability over time, especially under cyclic loading and when fine particles are present. In the suspension tests, the amount of kaolin transported into the soil pores increased with higher solid passage and higher water flow rates, confirming the risk of clogging. The data also revealed that the lower opening‑width limit of 0.06–0.08 mm is insufficient for soils prone to erosion or suffosion, as the filters fail to maintain adequate hydraulic conductivity. Consequently, the project recommends raising the lower bound of the opening width in the guideline to reduce clogging risk and improve the reliability of geotextile filters in road construction.

The project was funded by the German Federal Office for Civil Engineering (BASt) under the grant S 204. It involved collaboration between research institutions, which designed the test methods and conducted the experiments, and industry partners that supplied the geotextiles and provided technical expertise on product characteristics. The research spanned two years, during which the team coordinated data collection, analysis, and the preparation of recommendations for the guideline. The findings provide a scientific basis for revising the filter rules, thereby enhancing the safety and cost‑effectiveness of road construction projects that rely on geotextile filtration.