Managing the extent of tree removal from railway earthwork slopes

Abstract

Trees cover the slopes of many earthworks (embankments and cuttings) supporting the UK's rail network. Trees provide ecological and slope stability benefits to earthwork slopes but they can also cause seasonal shrinking and swelling of the soil. Seasonal shrink–swell movement in earthworks can affect the level and alignment of the track, causing speed restrictions, associated delays for rail passengers and a substantial cost for infrastructure owners. Guidance is required to identify and manage the removal of problematic trees, while avoiding full tree clearance on earthworks slopes.A study was undertaken on behalf of London Underground Ltd (LUL) to assess whether the National House Building Council (NHBC) guidance, considering tree species and the ratio of the distance of the tree from the track, Dt, to the mature tree height Ht, might be applicable to trees located on railway earthwork slopes. Excessive seasonal track movement was shown to correlate with the presence of high water demand (HWD) tree species located within a certain Dt/Ht ratio of the track, but not other tree species.Soil heave was measured on the slope of an instrumented railway embankment following removal of trees from the embankment slope. The magnitude and rate of soil heave was also estimated from five years of pore water pressure data, using a one dimensional settlement/heave calculation based on a linear swelling index. It was found that while the removal of HWD trees reduced seasonal shrink–swell movement, soil heave and upward track movement continued for at least 4 years after tree felling.