1 Introduction
2 Subgrade fluidisation under undrained cyclic triaxial tests
2.1 Factors affecting subgrade fluidisation
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Atterberg limitsThe upper and lower bounds of water content where soil exhibits plastic behaviour are defined as the liquid limit (wLL) and the plastic limit (wPL) respectively [11]. These limits are collectively known as the Atterberg limits. The subgrade soils that were reported to have already pumped are mainly in the low-to-medium plasticity region on the soil plasticity chart [5, 12]. However, several of the mud pumping locations in New South Wales, Australia, are along the east coast, so they consist of low compressible estuarine clays with water levels that are often close to the liquid limit [5]. Therefore, this subgrade soil is in a fully saturated state and the continual passage of trains results in serious deformation followed by the subgrade becoming softer.
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Presence of finesIndraratna et al. [13] reported that this problematic subgrade soil has an overall fines fraction (< 75 μm) of about 30%. As well as fines, this soil also has a filter ratio (D15coarse/D85fine, where D15coarse is the diameter that corresponds to 15% finer by weight of coarse particles and D85fine is the diameter at 85% finer by weight of fine particles, respectively) of 5.8, this indicates there is a high susceptibility towards internal instability [14‐16]. Since these clayey fines have a larger specific surface area, they adsorb more water, whereas other studies reported a large amount of fines in the subgrade [4, 17‐19].
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Hydraulic gradientSubgrade fines cannot migrate unless there is a certain level of hydraulic gradient over the track foundation, but the repeated passage of trains causes a large increase in excess cyclic pore pressure in the saturated subgrade layers. Alobaidi and Hoare [20] carried out finite element simulations to study the pore pressures generated near the top of the subgrade under static loading. As Fig. 2 shows, the pore pressure at the end of the simulated subbase particle dissipated from 10.0 to 0.6 kPa within 0.25 s. This rapid dissipation of pore pressure between the centre and the end of the subbase particles amounts to an equivalent hydraulic gradient of 147; therefore, a high hydraulic gradient is considered to be one of the main forces which drive the migration of subgrade slurry.×
2.2 Experimental setup
2.3 Undrained cyclic triaxial testing
Soil property | Value |
---|---|
Liquid limit, wLL | 26% |
Plasticity index, PI | 11% |
In situ moisture contenta | 14.6% |
Specific gravity, Gs | 2.63 |
Maximum dry density (kg/m3) | 1814 |
Initial dry density ρd (kg/m3)a | Relative compaction (RC) (%) | Initial void ratio e0 | Loading properties | |
---|---|---|---|---|
Frequency f (Hz) | Applied CSR | |||
1600 | 88 | 0.644 | 1, 2 and 5 | 0.1–0.4 |
1680 | 93 | 0.556 | 1, 2 and 5 | 0.2–0.5 |
1790 | 99 | 0.469 | 1, 2 and 5 | 0.2–1.0 |
2.4 Results and discussion
2.4.1 Critical CSR
2.4.2 Loading frequency, f
2.4.3 Relative compaction (RC)
2.5 Mechanism of fluidisation
2.5.1 Based on the physical changes within the specimen
2.5.2 Stiffness degradation
3 Influence of mud pumping on the performance of ballast
3.1 Reduced drainage capacity of ballasted tracks
3.2 Reduced shear strength and friction angle of ballast
3.3 Degraded resilient modulus
3.4 Substantial track deformation
4 Role of vertical drains in alleviating cyclic excess pore pressure
4.1 Laboratory investigation
4.2 Field investigation
4.3 Numerical predictions
5 Conclusion
- The soil plasticity, fines content and the hydraulic gradient generated by the cyclic excess pore pressure plays a crucial role in the pumping of subgrade soil. In general, soils having low-to-medium plasticity are susceptible to mud pumping.
- Track performance is affected severely when soft subgrade is pumped into the ballast layer. The large-scale permeability tests revealed that as the void contamination index (VCI) increases, there was a significant reduction in the hydraulic conductivity of ballast fouled with kaolin.
- The peak strength of fouled ballast can be expressed as a function of the VCI by using one empirical parameter which depends on the effective confining pressure. Moreover, the resilient modulus Mr varied linearly with the VCI (when the number of cycles N < 4000).
- The soil plasticity, fines content, and the hydraulic gradient play a crucial role in pumping subgrade fines, and soil with low-to-medium plasticity are generally more prone to mud pumping.
- At a high cyclic stress ratio (≥ CSRc), the internal redistribution of water and the migration of fines under cyclic loading caused the laboratory specimens to become fluidised; this was accompanied with the softening of the soil and significant drop in the stiffness degradation index.
- The inclusion of prefabricated vertical drains can help delay the onset of subgrade fluidisation by reducing the magnitude of cyclic excess pore pressure that will accumulate in the saturated subgrade.