Stabilizing a Failing Embankment in a Protected Area
Wrabness embankment, on the Manningtree to Harwich line in Essex, has been prone to slippage and movement for some time, affecting the track. DYWIDAG supplied and supported the appointed specialist drilling contractor on this project, with a brief to stabilise the embankment running through Stour Woods – a protected nature reserve.
Construction at the Wrabness site started with the temporary works package in December 2021, and completion of the permanent works was in April 2022. To maintain operational Rail movements the work was undertaken from the embankment slope side. The principle contractor J. Murphy & Sons Ltd, planned and managed the construction works to minimise the impact on residents and to satisfy Natural England’s requirements for working adjacent to the Stour and Copperas Woods: a SSI. J. Murphy & Sons implemented a monitoring strategy during construction, which recorded movements of the track, embankment slope and lineside structures, such as overhead line equipment. The monitoring data was shared with the project team. Atkins Global undertook weekly reviews of the site monitoring data and to highlight areas for concern.
The detailed design solution involved inserting DYWIDAG Hollow Bar R38 Galvanised soil nails to stabilise the deep-seated slope movements, as well as the construction of king post cess retaining structures to reinstate the cess at discrete locations.
The design of the soil nail solution was based on slope stability analysis of the embankment undertaken at critical sections, and the fact there are predominantly Cohesive soils with underlying terraces of Sands & Gravels. Soil nail positioning and spacing varied along the site depending on the slope geometry and the TAM risk categorisation. The design specified that the slope face be finished with a flexible facing, spanning between nail locations to minimise long term degradation of the slope face. To verify the ultimate bond resistance of the soil nail and identify any issues with the method of construction, sacrificial soil nail tests were conducted at the site in advance of the main work.
1388no. soil nails and 16000 mtrs of hollow bar, with 11.5 mtr average nail length, were installed into the embankment toe, using Ripamonti Excavator Mounted Mast Drilling Rigs, 2no. standard reach units and 1no. long reach unit. All rigs were operated from a temporary working platform constructed towards the toe of the embankment.