Effects of vegetation on Earth Structures on the London Underground Network

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Earth structures on the London Underground (LU) network are over 80 years old, with some as old as 150 years. Most of the cuttings are formed in the London Clay Formation, and the embankments were generally formed from the excavated material from London Clay cuttings. The original construction of these earth structures was not scientifically undertaken, with over-steepened cuttings and poorly compacted embankment fill.  Many of them have been showing signs of movement and past instabilities since the early 1990s.  By the late 1960s, LU had introduced electric trains, which led to a reduction in the maintenance of vegetation on the earth structures, and the development of dense vegetation, including large mature trees. Over the last decade, the impact of climate change and extreme weather events have also become more frequent.  This has led to severe flooding and disruption of rail infrastructure, including some injuries and a few fatalities, as well as impacting serviceability of transport operation.  Detailed and deep monitoring of earth structures has provided improvements in the understanding of the behaviour of earth structures, as well as managing the risk of instability.  This paper presents the scope of the long-term monitoring undertaken on the LU earth structures, with a focus on the effects of vegetation. The results of the monitoring demonstrate the impact of vegetation on the porewater pressure regime within heavily overconsolidated London Clay cutting and embankment slopes, along with the associated slope and track deformation.  Removal of vegetation can lead to increases in porewater pressures, which in turn result in reduction in operating factors of safety.  The results also show the shrink-swell seasonal movements due to the presence of mature vegetation and the potential impact on track serviceability and performance.

3rd International Workshop on Soil-Vegetation-Atmosphere Interaction (RootS2025)

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