Stabilisation of a progressive railway embankment slip

Abstract

A walkover survey of a side embankment on the West Coast Mainline railway (UK) revealed significant movement of the twin track, a waterlogged trackbed and general deterioration of the slope. Ground investigations and monitoring indicated that in addition to shallow washout failures and formation pumping problems at trackbed level, the embankment was at limiting equilibrium with progressive downslope movement occurring along a deep-seated shear zone within underlying strata of glacial till and laminated clay (weathered mudstone/siltstone). Phreatic level increases during rainstorm events, in combination with winter groundwater maxima, induced artesian pressures on the underside of the steeply sloping shear zone which triggered step increases in the shear displacements. Other factors included the gradual post-peak decline in shear strength due to creep; internal weathering; cyclic, transitory stress increases caused by passing trains and densification of the overlying embankment fill. The upgrade works reduced the hazard of further embankment movement affecting the track and comprised the installation of stabilising piles along the mid-embankment slope in order to arrest the deep-seated slide (factor of safety, FOS = 1.2), the construction of an earth buttress and regrading works in order to reduce the upper slope gradient (preventing shallower slips and wash-out failures, FOS ≥ 1.3) and improved site drainage.

Keywords:

• embankment
• embedded wall
• pile
• instability
• slope
• stabilise

Acknowledgements

The authors would like to acknowledge the close collaboration with Edmund Nuttall Ltd. in the successful completion of this project. Network Rail is acknowledged for granting permission to publish this case study. A special word of thanks to David M. Green for assistance in compiling the monitoring data. The paper was written by the lead author while on sabbatical leave at the Urban Institute Ireland, University College Dublin.