One of the challenges to the railway industry is to develop more cost-effective methods of renewal of track and formation material. Since privatisation of the UK's railways the growth in passenger and freight traffic has increased pressure on achieving timely renewals of track in order to avoid speed restrictions, to reduce line and levelling maintenance requirements and to improve ride quality and performance.
A focus on asset management of infrastructure has enabled whole life costs to be examined in more detail. Since privatisation, costs have become much more transparent.
These include the cost of train delays caused by track condition and the cost of track renewals, reballasting and formation improvement.
Improvements in track investigation techniques have helped to better focus expensive formation work such as sand blanketing, installation of geotextiles, impermeable membranes or geogrids. Such techniques can also identify sites that have significant variations in track support stiffness, for instance in soft soil areas or sites where transition arrangements around structures are inadequate.
Common track substructure problems include:
Ballast attrition leading to development of wet spots.
Subgrade strength failure (bearing capacity failure).
Formation too soft.
Formation too stiff.
Variable formation stiffness.
On very soft subgrades the train speed may approach the 'critical velocity'of the track subgrade (the propagation speed of Rayleigh waves within the subgrade layer) and vertical displacements of track can develop in a bow wave.
This can lead to the necessity for ground improvement works in areas where increases in line speed are required over soft soils.
There are remedial methods for each of these modes of failure but there is a need to develop more cost-effective solutions, for example based on the use of geomembranes and geogrids. New methods of track maintenance and renewal are increasingly being used. An example of a new track renewal method is the use of steel sleepers to extend the life of ballast and formation - as steel sleepers are more tolerant to poor ballast conditions than concrete sleepers.
The use of steel sleepers was tested on the North West Zone and is now being used elsewhere. Stoneblowers (which pneumatically inject small aggregate to lift sleepers to the required level) are increasingly being used for maintenance of track line and level as they give a longer maintenance interval and cause less damage to ballast than tampers.
Traditional methods of track investigation have usually involved excavation of trial pits, sampling and laboratory testing. The testing regime establishes particle size distributions of granular materials, undrained shear strength of cohesive materials and sometimes stiffness measurements of subgrade by repeat loading of samples in triaxial cells or by plate bearing tests. Experience has shown that track on very soft ground results in poor track geometry.
This can be improved by installation of geogrids or increasing the depth of granular construction.
Developments in track investigation methods include ground radar, rapid stiffness measurements taken from track level and rapid sampling methods of ballast and formation by coring.
Recent work on the development of a track deterioration model as part of the Eurobalt project will help optimise track designs. Eurobalt is a European project, part-funded by the European Commission, and includes Railtrack as well as French, German and Swedish Railways. Its aim is to provide a numerate model to aid specification of the optimum track renewal. This provides an opportunity to optimise the track form in terms of whole life costs for the train traffic mix expected on a line.
The model and experimental results demonstrate the relationships between track stiffness, variability of track stiffness, dynamic forces from train traffic and track geometry deterioration. The model has been shown to give a very good prediction of geometry deterioration rate (the main driver of maintenance requirement). Parametric studies using this model will enable optimisation of track forms for minimum maintenance and life cycle costs.
A research project on improving railway subgrade stiffness is being carried out by a consortium of consultants, a railway maintenance contractor and the School of Civil Engineering at the Un ivers ity of Birmingham.
The objectives of the project are to provide design guidelines for subgrade assessment and treatment of railway subgrade.
Eifion Evans is senior asset manager for Railtrack's Track Asset Group