Track behaviour: the importance of the sleeper to ballast interface

The aim of this research is to develop a fuller understanding of the mechanical behaviour of thesleeper/ballast interface, related in particular, to the forces applied by high speed tilting trains onlow radius curves. The research has used literature review, field measurements, and laboratoryexperiments on a single sleeper bay of track. Theoretical calculations are also presented.Field measurements are carried out using geophones to record time/deflection for sleepersduring passage of Pendolino trains on the West Coast Main Line. Calculations are presented toquantify normal and extreme magnitudes of vertical, horizontal and moment (VHM) loads onindividual sleepers.Results from laboratory experiments, on the pre-failure behaviour of the sleeper to ballast basecontact area, show that lateral load/deflection behaviour is load path dependent and relations aredetermined for improved computer modelling of the sleeper/ballast interface. Further test resultsare used to establish the failure envelopes for combined VHM loading of the sleeper/ballast basecontact area. Tests show that the sleeper/ballast base resistance at failure occurs at a load ratio(H/V) of about 0.45 (24°) at 2 mm of displacement tending to 0.57 (30°) at greaterdisplacements. In addition, measurements from pressure plates within the testing apparatus areused to describe the development of confining stress within the ballast during 100 cycles ofvertical load. The development of confining stress is assessed with reference to a finite elementmodel of the laboratory apparatus and it is shown that the earth pressure ratio moves towards theactive condition for peak load and the passive condition at minimum load per cycle.The contribution to lateral resistance of the crib ballast and varying sizes of shoulder ballast isalso established and it is found that the shoulder and crib resistance can best be characterised bytaking the mean resistance over a range of deflection from 2 mm to 20 mm. Calculations arepresented, supported by the experimental data, to quantify the resistance from different sizes ofshoulder ballast and a chart is presented which can be used as the basis for shoulderspecification in practice