Effective drainage of trackbed is critical in helping to ensure the structural integrity of the ballast and subgrade layers.
Much has been written about how ineffective drainage increases saturation of the ballast and subgrade under wet conditions. Saturation results in a reduction in trackbed stiffness leading to increased rates of trackbed failure associated with factors such as ballast degradation, subgrade erosion and mud spot development. High moisture levels will also accelerate the rate of wear of timber sleepers and rail corrosion.
Well maintained and correctly designed drainage will extend the life of the ballast and the subgrade, reducing maintenance intervals and the likelihood of emergency interventions.
This blog offers a shallow dive to explore the influence of both regional drainage patterns, defined by terrain and climate, and trackside drainage systems on the condition of the track subsurface and the risks to other track infrastructure. Understanding how both can impact the trackbed facilitates maintenance prioritisation on a network scale and the development of suitable drainage treatment designs at the local scale.
Context
Railways traverse diverse environments with widely differing climatic and hydrogeological settings. If trackbed is set in a landscape with irregular intense rainfall patterns, the effects of weathering and erosion can be a significant hazard to the railway. Regional context is key to understanding flood risks to natural water courses and trackbed drainage assets such as culverts and drainage ditches.
Context can be provided by combining regional remote sensing methods, such as satellite-based synthetic aperture radar and manned or unmanned airborne lidar, and site-scale lidar surveys to derive digital terrain models (DTMs), which can be analysed to characterise and model drainage pathways.
The vulnerability of assets such as bridges, culverts and embankments can be assessed using regional DTMs, coupled with high density lidar point cloud data acquired using a mobile terrestrial laser scanner (MTLS) system. Mapping drainage paths, runoff slopes and blocked drainage helps identify areas where drainage capacity is limited and therefore most prone to the effects of flash flooding, information that can be used to drive network scale infrastructure (including drainage) upgrades.
Monitoring trackside drainage infrastructure and prioritising local-scale maintenance
Trackbed metrics derived from MTLS and ground penetrating radar (GPR) systems can be combined to implement a trackbed drainage assessment tool. The tool combines GPR-derived ballast fouling levels and an estimation of moisture likelihood, with automated assessment of cross-track and along-track drainage capability based on a suite of parameters derived through analysis of the MLTS point clouds. The latter include detection of shoulder ditches, determination of along-track gradient within the ditch and identification of outflow locations. The results are used to score the quality of the internal and external track drainage. The scores are then codified to provide the railroad with a rolling assessment of drainage condition, alongside identification of locations where maintenance is required.
Integrating GPR subsurface and surface terrain data in this way represents a step change in identifying interactions between track substructure properties and terrain factors. This change improves the efficiency of the diagnosis of root causes of track geometry top and alignment irregularities, the prioritisation of ditch maintenance locations, and the development of high-level treatment designs, and communication of these to asset managers and stakeholders.
The case study below illustrates how a combination of GPR-derived moisture distribution and an MTLS-derived lidar point cloud where used to develop a formation reconstruction treatment for the location. Determination of the base of ballast using GPR additionally enabled the development of a drainage solution using aggregate trench drains as a treatment. Changes to the trackbed condition from subsequent GPR and MTLS data capture runs can be used to monitor the effectiveness of any treatment carried out.
If you’re interested to learn more about combining GPR and MTLS surveys take a look at this explainer video.
Stay tuned to our blog series on the benefits of railway asset condition mapping.