Subgrade Soil Fluidization under Cyclic Loading of Heavy-haul Trains and Preventive Measures
- Publication Type:
- Conference Proceeding
- Citation:
- 2026
- Issue Date:
- 2026-06-14
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ABSTRACT: Soft soil deposits along Australia’s low-lying coastal regions pose significant challenges for the safety and stability of
rail infrastructure. The undrained instability of these often saturated formations exacerbated by excess pore water pressure (EPWP) in
tandem with upward hydraulic gradients under dynamic wheel loading is the primary cause of soil fluidization (mud pumping). This
paper identifies critical ground (subgrade) conditions prone to fluidization and proposes novel solutions to ensure an efficient and safe
operation of rail tracks. Using an iconic custom-built Dynamic Consolidation Apparatus (DCA) capable of assesiing the fluidization
potential, laboratory testing under cyclic loading was conducted to investigate: (i) the occurrence of subgrade instability under various
drainage conditions and intermittent cyclic loading with rest periods, (ii) the role of drainage geotextiles in stabilising the subgradeballast
interface, and (iii) the effectiveness of a combined system of a prefabricated vertical drain (PVD) and a geocomposite (i.e. an
impervious membrane sandwiched between two drainage geotextiles). Experimental results indicated that prior to fluidization, the
water content in the upper soil layer approached its liquid limit due to internal moisture redistribution, effected by very fine particles
from the bottom half of the test specimen migrating towards the top surface. This unique failure mechanism, characterized by fluidised
soil (slurry) being pumped to the surface under high EPWP gradients, differs from traditional cyclic undrained yielding. The inclusion
of the geocomposite at the ballast-subgrade interface could effectively impede particle migration by reducing the EPWP gradients. The
findings also revealed that longer rest periods between loading cycles could reduce the likelihood of mud pumping. Additionally, PVDs
on their own significantly reduce EPWP build-up in thicker soil layers. A case study at the town of Sandgate, NSW, demonstrated the
effectiveness of relatively short PVDs (approx. 6m) in enhancing the stability of a track built on deep estuarine clay deposits (> 15m).
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