Publication Details

This article was originally published as: Indraratna, B, Rujikiatkamjorn, C, Wijeyakulasuriya, V, Shahin, M, & Christie, D, Soft Soil Stabilisation with Special Reference to Railway Embankments, in D. Chan & K. T. Law (eds.), Soft Soil Engineering: Proceedings of the 4th International Conference on Soft Soil Engineering, Vancouver, Canada, 4-6 October 2006, Taylor& Francis, 35-56. Book information is available here from Taylor & Francis Books.


Much of Australian railway tracks traverse coastal areas containing soft soils and marine deposits. Pre-construction stabilization of soft formation soils by applying a surcharge load alone often takes too long. The installation of prefabricated vertical drains (PVDs) can reduce the preloading period significantly by decreasing the drainage path length, sometimes by a factor of 10 or more. The analytical solution based on actual radial soil permeability is proposed considering the variation of vacuum pressure, and the Cavity Expansion Theory is employed to predict the smear zone caused by the installation of mandrel driven vertical drains. The predicted smear zone and the effect of drain unsaturation are compared with data obtained from a large-scale radial consolidation tests and the results are explained. When a higher load is required to meet the desired rate of settlement and the cost of surcharge is also significant, the application of vacuum pressure with reduced surcharge loading can be used. In this method, an external negative load is applied to the soil surface in the form of vacuum pressure through a sealed membrane system. The applied vacuum pressure generates negative pore water pressure, resulting in an increase in effective stress and accelerated consolidation, also avoiding the need for a high surcharge embankment. The analytical and numerical analyses incorporating the authors’ equivalent plane strain solution for both Darcian and non-Dracian flow are conducted to predict the excess pore pressures, lateral and vertical displacements and several selected case histories are analysed and presented. Cyclic loading of PVDs is also examined in the laboratory in a manner appropriate for railway environments. It is shown that short PVDs can dissipate excess pore pressure as fast as they are built up under repeated loading conditions. The research findings verify that the impact of smear and vacuum pressure can significantly affect soil consolidation, and these aspects need to be simulated properly in the selected numerical approach. Finally, the use of native vegetation to stabilise soft soils in railway environment is discussed with the aid of preliminary suction models developed on the basis of evapotranspiration mechanics applied to tree roots.

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