Project outline

Wrabness Embankment is a 10m high earthwork on the Manningtree to Harwich line which has been subject to long-term deep seated slope movements affecting the track. The site has an extensive history of monitoring and repairs becoming Anglia RAM Team’s highest priority earthworks site for renewal.

Murphy were commissioned to validate the existing F001 soil nailing scheme to determine opportunities for optimising and rationalising the design. By using specialist in house geotechnical skills to engage collaboratively with Network Rail and our designer, Atkins, Murphy was able to reduce of the cost of the works by nearly 40% from the initial budget estimate.

The final scheme replaces the failing shoulder and damaged S&T troughing to provide a level standard cess walkway along the length of the site with improved access to a signal & lineside S&T structures.

The planning of the scheme involved careful environmental consultation to allow work within a SSSI and close to the sensitive Stour Estuary RAMSAR site. Extensive planting along the toe of slopes is designed to provide habitat replacement for clearance enabling works.

Project delivery and innovations

The design validation process compared a range of design approaches from betterment to a full EC7 design for each area of the site. Collaborative working between Murphy, our designer, Atkins, & the NR RAM Team allowed an appropriate design approach to be matched against the defects and failure mechanism identified, with significant cost savings.

Early sacrificial pull-out testing of nails was promoted as part of the detailed design, resulting in around a 30% saving in nail metreage.

Easy to cut and handle recycled plastic planks were used for cess support. These provide a similar performance to concrete planks but are safer to use, cheaper, more sustainable and with a lower visual impact. The use of ungalvanised H piles also provided a more sustainable, lower visual intrusion option.

The use of recycled plastic planks and GRP handrailing for the cess support removed the requirement for bonding.  The cess detail also incorporated ballast fill behind the cess wall to remove the requirement for excavation and compaction, thereby reducing the potential track risks and CRT issues associated with the works.

A photogrammetry drone survey was completed by our designer, Atkins, to capture the works in 3D once completed.

Key challenges

The site is remote from any major roads, so an advanced phase of enabling works was required to establish access routes and to clear the dense trees covering the slopes. Liaison with nearby residents and closing of a public footpath was necessary to allow plant access down narrow farm tracks.

Additional measures were required for access in the SSSI area to ensure protected Hazel Dormice were not disturbed and to protect rootballs to enable natural regeneration once the works were complete.

Once Vortok fencing was installed at the crest, a roped access harness system under IRATA supervision was used for cess support installation outside of possession. Working around existing lineside structures careful setting out, monitoring and liaison with the Network Rail maintainer to agree details for future access. An integrated monitoring plan was put in place to collate and review data from track, lineside structures, and slope monitoring.

To enable soil nailing of the steep slopes working platforms were constructed on the lower slopes so installation could be undertaken under ALO. On competition these platforms were then landscaped and incorporated into the permanent works as additional toe berms, minimising waste generation and transport. Any Type 1 material remaining was provided free of charge to the landowner improving local relations and further minimising transport impacts.

Key facts
  • Cost of the works reduced by nearly 40% from initial budget estimate due to in house geotechnical expertise and collaborative working
  • 377 lin m of cess support constructed
  • 241 No. H piles 3.2m & 4m long installed from track via RRV
  • 21,105m of soil nailing installed from the toe
  • 14901 No. self-drill soil nails, typically 15m to 17m long
  • 554 lin m of standard cess walkway constructed
  • 38-week programme, including advanced works to clear the embankment and to install access
  • Next phase of work will be replanting and landscaping as part of reinstatement
Wrabness work site aerial shot
Drone images courtesy of Atkins

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