Threshold loading effects on railway ballast

Abstract

The growing demand on railway infrastructure is compelling operators to run longer and heavier trains on existing networks. This raises concerns that the operational loads may exceed the thresholds at which ballasted tracks begin to experience accelerated deterioration. Consequently, this could lead to a higher frequency of maintenance cycles. Meanwhile, efforts to decarbonise the transport sector through the recycling and reuse of ballast materials require further research, particularly on how the settlement behaviour of reused and blended ballast compares to that of fresh aggregates.
This thesis aims to investigate the mechanisms of permanent settlement of railway ballast, applying knowledge of the mechanisms to quantify the prediction of settlement of full-scale ballast, quantifying the threshold stresses of the railway ballast and exploring the performance of the reused ballast and blended ballast in comparison with fresh ballast. This was achieved through large-scale cyclic and monotonic triaxial tests conducted mainly on full-scale fresh ballast and full-scale reused ballast on specimens 600 mm high and 300 mm in diameter. The cyclic triaxial tests on full-scale blended ballast on three different proportions between fresh and reused ballast were also performed. Monotonic tests were performed at the shearing rate of 0.5 mm/min, and the cyclic tests were performed by varying the deviator stress from the minimum value of 10 kPa to the specified maxima ("q" _"max" ) mainly under cell pressure of 30 kPa and loading frequency of 3 Hz. The complementary tests to investigate minimum and maximum densities of the ballast were conducted in the big box, 1 m × 1 m on plan and 730 mm high.
The results show that the rate of axial strain is directly proportional to the extent of lateral spreading experienced by the specimen during cyclic loading. Evolution of axial strain can be expressed in terms of logarithmic number of load cycles (N) and the axial strain produced at the first load cycle. The axial strain produced at the first load cycle (N = 1) depends on "q" _"max" , ("q" ⁄"p'" )_"max" , relative density of the specimen ("I" _"D" ) and grain surface conditions. The rate of increase in axial strain with the number of cycles beyond N = 1 depends on surface conditions of ballast grains. There is a threshold ("q" ⁄"p'" )_"max" which depends on specimens’ porosity and surface conditions of the grains (most probably roughness) to start causing a lateral spreading, dilation or failure. When a specimen is subjected to cyclic triaxial loading with progressively increasing maximum deviatoric stress ("q" _"max" ) applied in a stepwise manner, the strength and stiffness of the specimen improve as the size of each stress increment ("Δq" ) is reduced. Conversely, when the ballast specimen is subjected to cyclic loading for thousands of load cycles, dismantled and retested, the threshold ("q" ⁄"p'" )_"max" values appear to be reduced. The stress - strain relationship between cyclic triaxial testing and monotonic triaxial testing is provided.

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Identifiers

ORCID for Rashid Salum Abeid: orcid.org/0000-0002-0096-3312

ORCID for William Powrie: orcid.org/0000-0002-2271-0826

ORCID for Joel Smethurst: orcid.org/0000-0001-8175-985X

ORCID for Madhu Murthy: orcid.org/0000-0002-2570-5934

ORCID for David Milne: orcid.org/0000-0001-6702-3918

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