The geotechnical behaviour of ballast materials for railway track maintenance

The search for an improved mechanised method of maintaining railway track to the correct level and alignment led to the development of the stoneblower, a machine that lifts the rail and attached sleepers and pneumatically injects the required amount of 20mm stone into the void created, creating a two-layer granular foundation. Stoneblowing minimises disturbance to the existing well-compacted ballast and reduces the amount of particle breakage normally associated with maintenance. By 1999 a fleet of stoneblowers was fully operational across the UK rail network and achieving target rates of track maintenance. However in some cases the track quality appears to have been left in worse condition than it was pre-maintenance. To investigate the deformation characteristics of the foundations directly below the sleepers a programme of large scale triaxial tests on 20mm stone, 50mm ballast and two-layer specimens was carried out under monotonic, cyclic and post cyclic monotonic loading. It was perceived that the amount of volume change, which was partly dependent on the amount of particle breakage, was critical to the overall permanent strain. From the laboratory testing it was found that particle breakage behaviour had an important role to play in the performance of the ballast, even at relatively low pressures. The specimens were particularly sensitive to the moisture content and to the variation in the subgrade modulus, both of which were related to an increase in the amount of particle breakage occurring in the test. In the two-layer specimens there was an increase in the resilient modulus and a decrease in the amount of breakage. Complementary to the laboratory work in-depth analysis of track quality data and of data downloaded from the stoneblower was undertaken. Detailed analysis of the data gave a better understanding of the performance and provided guidance for optimising the deployment of the stoneblower fleet and the stoneblowing procedure