Railway infrastructure is essential to the European economies for several reasons. It provides a safe, efficient, and affordable means of transporting passengers and freight. It also plays a vital role in connecting major economic centres and regions and supporting international trade, thus generating immense social benefits. These benefits are why SNCF Réseau (the French railway infrastructure manager) and the French government have invested heavily in modernising existing rail infrastructure in recent years. One of the methods for limiting the environmental impacts of trackbed renewal is to use geogrid stabilisation to reduce trackbed thickness while maintaining similar performance to standard trackbed designs. However, the exact effects of geogrid stabilisation on track service life and maintenance requirements are still poorly understood. These phenomena develop over such extended time frames that it is crucial to design geogrid stabilised trackbeds and forecast their behaviour using numerical modelling.

Geogrid stabilisation refers to the use of geogrids to improve the mechanical performance of unbound aggregate layers. Such reinforcements are polymer-based geosynthetic materials, consisting of open meshes composed of ribs (working in tension) linked together by more or less rigid junctions. The presence of these apertures favours the interlocking mechanism between aggregates and the geogrid.

The viability and effectiveness of geogrid stabilised trackbeds, relative to the needs of SNCF Réseau, has been demonstrated via cyclical loading test in a laboratory and validated on instrumented track sections. These experiments led to the development of simplified design guidelines and numerical models. However, their usefulness is limited by the scale of the existing laboratory testing rig (relatively small box, with unquantified border effects) and the uncontrolled parameters of the instrumented track (driver behaviour, weather, etc.). Both weaknesses can be remedied by reproducing cyclic loading tests in a larger testing rig with more extensive measurements. Thus, the CyLo RT project aims to assess the long-term behaviour of railway trackbeds under cyclic loading, with and without geogrid stabilisation, in the controlled conditions of the TU Darmstadt Geotechnical Test Pit. The acquired data will be coupled with the results of other ongoing research and used to improve the numerical modelling of SNCF trackbeds. This approach will contribute to the evolution of SNCF policies regarding trackbed design and service life.

The experimental campaign took place between October 2024 and March 2025. The resulting data is currently undergoing post-processing for subsequent interpretation.

Large-Scale Model Test

The test-setup developed during this project (Figure 1) aims to replicate track conditions close to those found in service. It includes a subballast layer made of gravel, placed over a weak semi-elastic subgrade composed of sand and rubber granulate. A stabilisation geogrid was installed at the subballast-subgrade interface in one of the two tests. Cyclic loading representative of railway traffic under simplified conditions was applied using a vertical loading system positioned centrally above the test pit. The applied loading was designed to simulate the passage of approximately 50,000 regional passenger trains.

Extensive instrumentation was deployed to enable a detailed analysis of geogrid stabilisation mechanisms during cyclic loading (Test 2) and comparison to the test without geogrid (Test 1). This included strain gauges installed on the geogrid, earth pressure sensors strategically positioned, and Distributed Fibre Optic Sensors (DFOS) placed at two levels below the geogrid.

The instrumentation was structured into two complementary sub-systems, enabling high-resolution monitoring of the track bed response during cyclic loading:

• The first sub-system comprised 28 sensors for Test 1 or 37 sensors for Test 2, including: force and displacement sensors embedded in the hydraulic actuators; 4 load cells into the loading rig to measure the applied vertical loads; 6 displacement transducers to measure the vertical displacement of the two blocks of the loading rig; 14 earth pressure sensors distributed installed in the sub-soil to map vertical stress fields; 9 strain gauges attached to the geogrid to measure its deformation along two perpendicular directions (in Test 2 only); 2 temperature sensors.
• The second sub-system focused on distributed strain monitoring using 6 DFOS sensors, each approximately 40m in length (Figure 2). These were embedded in a grid pattern at two distinct levels beneath the subballast layer. The DFOS technology provided detailed insight into the strain evolution and deformation profiles of the subsoil.

Preliminary results from Test 1, indicate a progressive accumulation of surface settlement at the loading rig level over repeated loading cycles, with greater settlement observed during the initial stages of the test, as expected.

GEOLAB – Transnational Access Project CyLo-RT: Cyclic loading of railway trackbeds with and without geogrid stabilisation

Olatoundé, Yaba; Liaudat, Joaquín; Kochnev, Alexander; El Ayoubi, Ahmad; Jenck, Orianne; Emeriault, Fabrice; Vollmert, Lars; Szymkiewicz, Fabien; Zachert, Hauke (2025)