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27.11.18

Monitoring the performance of earthworks

Source: RTM Oct/Nov 2018

Dr Joel Smethurst, associate professor in geotechnical engineering, and Professor William Powrie, professor of geotechnical engineering, both of the University of Southampton, discuss the work and research that’s helped the rail sector in its approach to earthworks and managing vegetation.

There are many hundreds of kilometres of earthworks (cuttings and embankments) on the UK rail network. Most were not built to modern engineering standards and with many now well over 100 years old, the earthworks continue to age and thus require maintenance and remediation.

Problems include shrinkage and/or differential movement of clay earthworks caused by the seasonal cyclic removal and replenishment of water from the soil, and slope failures (landslides) caused by elevated groundwater pressures during wet winter periods. Slope failures represent a major hazard to railway safety in the form of the potential for train derailments, and may cause long closures of the railway while earthworks are rebuilt.

Until the 1960s, vegetation was heavily managed on many earthworks to reduce the risk of fire from steam locomotives. With the demise of steam traction, active vegetation management all but ceased, and mature trees became established on embankment and cutting slopes. Seasonal cycles of summer water removal by the trees and winter rewetting by rainfall cause shrinkage and swelling movements in high-plasticity clay fills, which affect the line and level of the railway track. Trees root much deeper than grass, and are capable of removing a considerable amount of water over a summer season, causing localised vertical surface settlements of 50mm or more.

For railways on clay embankments subject to summer drying, speed restrictions may need to be imposed or regular re-ballasting or tamping carried out to maintain consistent rail level and alignment. To reduce the need for these, asset owners have been removing vegetation from the side slopes of earthworks. However, there is a concern that the removal of trees may lead to re-wetting within the core of an embankment and the loss of roots which help to bind the soil, thereby impacting adversely on the stability of the slopes.

The effect of vegetation on earthworks performance has been investigated through carefully instrumented field studies and by analysing wider observations of track movements close to mature trees.

As part of a study carried out for Network Rail in 2006, instrumentation was installed to measure changes in soil water content, groundwater pressures, and soil vertical and lateral movement in an old tipped London Clay railway embankment at Hawkwell, Essex. Shrinkage of the clay embankment caused by large deciduous oak and ash trees was giving rise to poor track quality and the need to apply train speed restrictions. The embankment was monitored for a year with the trees in place. The trees were then removed from the upper two-thirds of the embankment slope within the instrumented section, leaving trees only on the bottom third of the slope.

After tree felling, monitoring continued for a further four years. Tree removal was effective in substantially reducing seasonal shrink/swell movements at the crest of the embankment, although the low groundwater conditions that were beneficial to slope stability were lost as the embankment rewetted. A careful balance therefore needs to be struck between maintaining stability and reducing seasonal displacements. This may require a more limited or selective vegetation clearance, such as the managed removal of just high-water-demand trees, than was carried out in this case.

A further study used track level monitoring data for a series of sites that were either at-grade on a clay foundation, or on a clay embankment. Data on the locations, heights and species of trees present at each site were obtained. The species of trees and their distance horizontally from the track were correlated with seasonal track displacements. Results were assessed with reference to the National House Building Council (NHBC) guidelines on building near trees, which categorises trees species into high, medium and low water demand.

The NHBC guidance defines a distance of influence over which a tree may damage shallow foundations as 1.25 × the tree height from a building foundation for high water demand trees, and 0.75 and 0.5 × the tree height for medium and low water demand tree species respectively. Plotting the track displacement against the distance of the tree from the track, normalised by tree height, showed that most high-water-demand trees within 1.25 × the tree height of the track were causing appreciable (up to 30mm) seasonal vertical movements. As mature trees can be up to 20m tall, this would suggest that all high-water-demand trees (including species such as oak, poplar and willow) should be removed if possible, from a zone up to 25m laterally from the track, if damaging ground/track movements are to be avoided.

In some cases, much of this zone will be on land not owned by the railway, making practical application difficult. Medium and low water demand trees were generally found not to cause very significant ground movements, suggesting that where possible, high-water-demand trees should be replaced by species of low and/or medium water demand.

Field instrumentation data has also been used to investigate the wet winter conditions that may cause slope failures in embankments. A series of groundwater level measurements made during the very wet winter of 2000-1 were re-analysed, alongside contextual data such as embankment fill soil type, foundation material, slope aspect, borehole/peizometer location on the slope, slope angle, and slope vegetation type. Most of these factors were found not to have any correlation with the measured water pressures; the only significant relationship was between groundwater pressure and the foundation material on which the embankment was constructed.

Clay embankments constructed on a higher permeability (gravel or chalk) foundation soil were found to maintain low water pressures throughout even a very wet winter. Embankments founded on a clay soil, of possibly lower permeability than the tipped clay fill, were generally found to have much higher groundwater pressures, corresponding to a groundwater level just below the slope surface. So even during an extremely wet winter, clay embankments on a foundation that helps the embankment to drain will maintain low water pressures, and are more likely to remain stable than those built on natural clay.

Insights into the factors influencing the changes in soil water content and water pressures in old railway earthworks have the potential to allow us to better understand their behaviour and inform how they should be managed. The field observations described here are continuing to be used to produce better models of earthworks behaviour and performance, guiding earthworks and vegetation management.

 

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