Reliability-Based Assessment of Ballast Bearing Capacity Under Aggregate Degradation in Railway Tracks

Abstract

The long-term performance and safety of ballasted railway tracks are significantly influenced by ballast degradation, which occurs under repeated train axle loads and results in diminished particle angularity, eroded surface texture, increased fines content, and reduced bearing capacity. This study assesses the effects of ballast degradation on bearing capacity using a reliability-based design framework that explicitly accounts for material and loading uncertainties. Experimental investigations comprised Los Angeles abrasion tests to quantify particle degradation using Marsal's breakage ratio and large-scale direct shear tests conducted under simulated axle loads of 20, 25, and 30 tonnes (equivalent contact stresses of 192, 240, and 288 kPa) to determine changes in shear strength parameters. Stochastic models of key design variables were developed from experimental data, and a limit equilibrium performance function was formulated, defining resistance as ballast-bearing capacity and load as the stress at the ballast–sleeper interface. Reliability analysis was conducted using the first-order reliability method (FORM) to estimate reliability indices and failure probabilities. The results show that increasing ballast degradation and axle loads significantly reduce shear strength and reliability and increase failure probability, particularly under 25- and 30-tonne loads. The study integrates laboratory-derived degradation indices with probabilistic reliability analysis, providing a practical approach for incorporating uncertainty into ballast design, maintenance planning, and railway safety management while extending track service life.