Filtration and separation in railway earthworks cover three functions defined in DBS 918 039: filter elements in drainage installations, separation and filter elements beneath base courses, and nonwovens for formation improvement directly under the ballast on existing lines. In each case, fines from the subgrade must be kept out of the drain gravel, the protective layer or the ballast, while water can still pass. Naue supplies a DB HPQ-qualified Secutex® nonwoven for each application case: Secutex® 151 GRK 3, Secutex® 251 GRK 4 and Secutex® R 901 DB.
Secutex® nonwovens are used wherever water must drain from the track structure while the soil stays in place.
Nonwoven placed directly under the ballast during track renewal on existing lines.
Filter trench at the toe of the cutting slope that lowers the water table beneath the formation.
Nonwoven-wrapped drain gravel around perforated pipes and in pipeless drainage slots.
Separation and filter layer between subgrade and protective layer on new and upgraded lines.
Partial replacement of the protective layer on low-speed existing lines with favourable ground conditions.
Where the nonwoven sits in the track structure decides its function, the application case under DBS 918 039 and the Secutex® product. The requirements for opening size and robustness differ accordingly.
Railway nonwovens are installed under drain gravel, protective layer material or ballast, and must survive placement and compaction without losing their filter function. Secutex® nonwovens are mechanically bonded (needle-punched) from virgin polypropylene. They adapt to uneven subgrades and are matched to the installation stresses through their robustness class (GRK).
Practical advantages for site teams:
Step-by-step installation of a filter trench:
DBS 918 039 defines three application cases for nonwovens with filter and separation functions. They differ in their position within the track structure and in what the nonwoven may replace.
Key principle: The filter element keeps the drain open. The separation element keeps the layers apart. Only a nonwoven to case 3.14 may partially replace a mineral protective layer - a nonwoven to case 3.4 does not reduce the protective layer thickness.
Technical benefit: drain gravel that is not filter-stable against the soil stays permeable.
Technical benefit: GRK 4 robustness for placing and compacting the protective layer.
Technical benefit: formation improvement without installing a full mineral protective layer.
The table compares the three DB HPQ-qualified Secutex® nonwovens by application case, position and function.
Final product selection is based on the subgrade particle size distribution and a project-specific assessment. For guidance, contact Naue Engineering & Design Department (EDD).
| Criterion | Secutex® 151 GRK 3 | Secutex® 251 GRK 4 | Secutex® R 901 DB |
|---|---|---|---|
| Application case | 3.3 - filter element in drainage installations | 3.4 - separation and filter element beneath base courses | 3.14 - nonwoven for formation improvement |
| Position | Around the drain gravel in trenches and slots | Formation level, beneath the protective layer | Directly under the ballast on existing lines |
| Function | Filtration | Separation and filtration | Separation and formation improvement |
| Opening size O90 | 130 µm (requirement 0.08-0.16 mm) | 100 µm (requirement 0.06-0.20 mm) | As per DBS 918 039 |
| Effect on protection layer | None | Thickness not reduced | Partial replacement on suitable existing lines |
| Installation | Drain body fully wrapped | On the prepared subgrade before the protective layer | Track-bound or track-free; ballast placed directly on top |
These benefits come directly from how the Naue products work in this solution, and what they mean in practice for your project.
DB HPQ for every application case
Secutex® 151 GRK 3 (case 3.3), Secutex® 251 GRK 4 (case 3.4) and Secutex® R 901 DB (case 3.14) come from one manufacturer. This removes approval uncertainty at tender and construction stage.
Drainage that stays open
Drain gravel is often not filter-stable against the surrounding soil. A Secutex® filter nonwoven around the drain body keeps fines out, so the trench can keep discharging water.
Clean layer boundaries
Beneath the protective layer, Secutex® 251 GRK 4 keeps subgrade and protective layer material from mixing under cyclic train loading.
Less mineral material on suitable existing lines
Where DBS 918 039 case 3.14 applies, Secutex® R 901 DB partially replaces the mineral protection layer. This reduces the material that has to be delivered and installed during the possession.
Filter and separation elements are nonwoven geotextiles installed in the track structure to keep fine soil particles in place while water passes. As filter elements, they wrap the drain gravel in drainage trenches. As separation elements, they keep the subgrade apart from the protection layer or the ballast. DBS 918 039 regulates them as application cases 3.3, 3.4 and 3.14.
Case 3.3 covers filter elements in drainage installations (Secutex® 151 GRK 3). Case 3.4 covers separation and filter elements beneath base courses (Secutex® 251 GRK 4). Case 3.14 covers nonwovens for formation improvement directly under the ballast on existing lines (Secutex® R 901 DB). A product needs a DB HPQ for each case in which it is used.
The trench is typically located at the toe of a cutting slope, with the formation falling towards it. It is lined with Secutex® 151 GRK 3, a fully perforated pipe (DN ≥ 100) is placed in the lower part, and the trench is filled with filter/drain gravel. The nonwoven is then closed over the top, so the drain body is fully wrapped, and covered. The trench lowers the water table beneath the formation.
Open-graded drain gravel is often not filter-stable against the surrounding fine-grained soil. Without a filter, fines are washed into the gravel and the pipe, and the drain gradually clogs. A filter nonwoven with a suitable opening size retains the soil and lets the water through, so the trench keeps working.
A nonwoven to case 3.4 is placed at formation level when fine-grained subgrade and protection layer material must be kept apart. DBS 918 039 requires an O90 between 0.06 and 0.20 mm and a thickness of at least 15 × O90 and 1.5 mm, and filter stability between subgrade and protection layer must be verified. Secutex® 251 GRK 4 meets these requirements with an O90 of 100 µm. The protection layer thickness is not reduced.
Partially, under defined conditions. A nonwoven to case 3.14, such as Secutex® R 901 DB, is installed directly under the ballast and can partially replace a mineral protection layer. It is intended mainly for lightly loaded, low-speed existing lines with sufficiently favourable geotechnical and hydrological conditions. The standard construction is described in RIL 836.4105 A03.
Secutex® R 901 DB is unrolled on the prepared formation using track-bound or track-free methods, overlapped as per the installation guideline, and the ballast is placed directly on top. This allows installation within planned possession windows on existing lines.
The characteristic opening size O90 (EN ISO 12956) describes the largest particles that can pass through the nonwoven. It must be small enough to retain the soil and large enough to let water through. DBS 918 039 requires 0.08-0.16 mm for filter elements in drainage installations (case 3.3) and 0.06-0.20 mm beneath base courses (case 3.4). Secutex® 151 GRK 3 has an O90 of 130 µm and Secutex® 251 GRK 4 of 100 µm.
On DB projects, DBS 918 039 (HPQ for each application case) and RIL 836 apply. EN 13250 defines the characteristics required for geotextiles used in railway construction, and EN 13252 those for drainage systems. The opening size is determined to EN ISO 12956, tensile strength to EN ISO 10319 and static puncture resistance to EN ISO 12236.
Filter and separation nonwovens are selected on the basis of the subgrade particle size distribution, the hydraulic conditions and the application case. The Naue Engineering & Design Department (EDD) assists with product selection and specification:
The following system benefits come directly from how the Naue products work within the layer structure:
