Keywords: subgrade, vegetation, total suction, solute suction, resilient modulus.

This paper details seasonal monitoring of the soil suction distributions and the resilient modulus of plastic clay subgrades across a ballasted rail track in two quite different climates within Australia. At both sites, sub-sites were set up in reasonable proximity to one another, with and without established trees in the rail corridor, since vegetated rail sites were observed to require far less maintenance. In this study it was shown that the vegetated sites were significantly drier and therefore provided stiffer subgrades, as evidenced by substantially greater values of resilient moduli of undisturbed soil samples recovered from below the track. Furthermore, the vegetated sites experienced less seasonal variations in soil moisture conditions and were therefore more stable, particularly at the site with more distinct seasons. Both observations indicate that vegetation can be beneficial in the rail corridor, particularly on flat sites, which are difficult to drain. Resilient modulus, up to 350 MPa, was observed to depend on total suction and stress state.

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