The ISERT (Improving the Stiffness of Existing Railway Track) project aims to devise a method for assessing and improving railway subgrade stiffness without disrupting operational services. The ultimate aim is to achieve a consistent level of subgrade stiffness along a complete route, enabling heavier loads to be carried and reducing maintenance costs. A smooth stiffness transition is required at the approach to underlying, usually much stiffer structures.
This is the second Engineering and Physical Sciences Research Council (EPSRC) LINK project to be undertaken by the Railways Research Group under the Inland Surface Transport programme.
The project is funded by industrial partners (see box) and the EPSRC and is due to be completed in September 2002.
Railway subgrade - the problem Repeated dynamic loading, fine-grained soils and high moisture content all contribute to railway subgrade problems directly beneath track components (Li and Selig,1995).
Progressive shear failure, the plastic flow of soil caused by excessive repeated loading at the subgrade interface, leads to heave at the track side and ballast pocket. Also, excessive plastic deformation caused by progressive soil compaction and consolidation leads to non-uniform track settlement and unacceptable track geometry changes. Remediation of excessive plastic deformation can also lead to increased ballast depth and associated ballast pockets. Finally, subgrade attrition with mud pumping, caused by high moisture content and repeated loading at the ballast and subgrade interface, leads to contaminated ballast and associated drainage problems (Li and Selig,1995).
All of these problems are recognised as direct causes of differential settlement and erosion and the subsequent repeated maintenance of track geometry (Awoleye, 1992). Differential track settlement can further exacerbate underlying railway subgrade problems leading to a rapid deterioration of track quality in the direction of traffic. Therefore, any chosen ground improvement technology needs to produce a uniform formation and subgrade stiffness. The causes of railway subgrade problems and the subsequent aims of any implemented ground improvement technology are summarised in Figure 1.
Railway subgrade - the solution
When acceptable track geometry is difficult to maintain as a result of weak or unstable subgrade, repeated ballast addition is commonly considered (Railtrack, 1997). Although repeated ballast addition is successful, it only temporarily compensates for track settlement problems and is a source of frequent track maintenance. Ballast addition can exacerbate ballast pocket and water entrapment-related problems.
Alternatively, a sand blanket and geotextile could be installed. The sand blanket acts as a filter barrier and gives increased resistance to shearing, while the geotextile directs water away from the subgrade, preventing further deterioration. Geotextiles also prevent slurry migration and subsequent ballast contamination (Awoleye, 1992). However, sand blanket and geotextile installation requires the costly and timely removal of track components and does not directly address the causes of settlement. Although this technique has been successfully implemented in the UK, there is scepticism regarding its performance in an industry with increasingly stringent time restrictions and demand for higher freight loads and line speeds. Economic analysis of tamping on soft formation routes also suggests that modified substructure designs could be widely applicable, provided that sufficiently cheap and effective methods of treatment could be devised (Jenkins and Wiseman,1990).
There is therefore a need for cost-effective stiffness measurement and curative insitu ground improvement techniques, which can be implemented through track components in accordance with the physical and time restrictions on UK railways. The ground improvement technique must create uniform subgrade stiffness and must also allow the formation of a smooth stiffness transition in the approach to underlying 'high-stiffness'external structures.
The aims of the research are to identify means of stiffness assessment and cost-effective techniques for improving the stiffness of railway formation and subgrade. This will be determined via a series of laboratory and field trials. Laboratory trials of both existing and modified stiffness measurement techniques and ground improvement technologies will be conducted in a sleeperloading rig (Brough 1,2000). The rig will allow simulation of train loading and full-scale testing of ballast, sub-ballast, formation and subgrade layers.
The main focus of the ISERT project will centre on full-scale field trials. Existing and modified soil strengthening techniques will be used on trial sections of operational track, measuring the long-term change in stiffness and track geometry. Three test sites are to be established with the aim of using stabilisation techniques to solve the generically different types of trackbed support problems. Site investigation has already begun at a possible weak formation and clay subgrade trial site near Leominster station (Figure 2 (a) and (b)).
Measurement techniques include the falling weight deflectometer, ground penetrating radar, automatic ballast sampler, cone penetration test, continuous surface wave system and other handheld devices (Brough 2,2000). The research will also include a long-term monitoring programme using insitu sensors. The best means of routine stiffness measurement must be determined so that regular monitoring can take place in an accurate, consistent and safe manner, and importantly so that the effectiveness of track maintenance works can be determined.
Ground improvement techniques to be considered include densification of the soil, construction of displacement minipiles and injection/mixing techniques using chemical or other additives. Techniques showing promise include bottom feed vibro replacement, soil mixing, soil nailing and slurry pressure injection. These technologies have been the subject of a preliminary assessment for application to improving railway track subgrade stiffness (Brough et al,2000).
Guidelines will be produced that relate ballast/sub-ballast/soil condition to improvement techniques and methods of application. They will be established according to three principal output objectives:
Establish potential for mechanisation of the preferred ground improvement technique.
Produce preliminary formation/subgrade design guidelines.
Produce guidelines on techniques for track subgrade stiffness measurement and long-term monitoring.
Awoleye EOA (1992). Trainload freight: cost-effective substructure design: half sleeper scale tests, British Rail Research, Civil Engineering Structures Unit, Ref. RR CES 020.
Brough MJ (2000a). Development of laboratory equipment for railway track and subgrade testing, confidential report, Improving the Stiffness of Existing Railtrack, Project No GR/M76508.
Brough MJ (2000b). Classification and assessment of railway subgrade, confidential report, Improving the Stiffness of Existing Railtrack, Project No GR/M76508.
Brough MJ, Stirling A, Ghataora G and Madelin K (2000). Improving railway subgrade stiffness - assessment of traditional insitu ground improvement techniques, Proceedings of the 3rd International Conference on Railway Engineering, London, UK.
Jenkins JA and Wiseman PW (1990). A statistical investigation of the effect of subgrade stiffness on rate of track geometry deterioration, British Rail Research, Ref TM TD 052.
Li D and Selig BT (1995). Evaluation of railway subgrade problems, Transportation Research Record, Vol 1489, pp 17-25.
Railtrack (1997). Track substructure treatments, Railtrack Line Code of Practice, Report No RT/CE/S/039.