As part of the work, five drainage chambers are being built under the railway at New Mills.
Ground investigations were carried out at the site in early April, confirming that immediate work was required to repair a slope that supports tracks.
Network Rail said there was a risk of 50,000t of earth slipping from the embankment into the River Goyt below unless stabilisation measures were implemented.
Main remediation works got underway in May but engineers have since discovered “more complex issues” with drainage.
As a result, five chambers will be created under the railway to collect excess water and direct it to the river. This will also make the railway more resilient to climate change.
A Network Rail spokesperson told GE in May that the embankment was situated on “sidelong ground between New Mills and Marple, with a steep, large catchment above and the River Goyt located beyond the toe of the slope”.
They added at the time: “The embankment is constructed on a natural hillside comprising glacial till overlaying mudstones and siltstones of the Millstone Grit Group.
“The failure is understood to be a deep rotational failure within the glacial till and weathered bedrock. Water has been a key factor in the overall stability of this embankment, and it was observed by the route engineering team to deteriorate following periods of extreme weather.”
Network Rail worked with specialist geotechnical consultant and designer Coffey Geotechnics on the repair project.
Network Rail project manager Vitesh Solanki said this week: “I’m sorry for the disruption the rail closure will cause passengers. We have received a tremendous welcome from the community in New Mills and we are grateful for your patience while we carry out this important work.”
A spokesperson for Northern added: “We’d like to thank our customers for their patience while this important, safety-critical work is carried out by Network Rail. We’ll do everything we can to make sure customers can still get where they want to be – but they should allow more time for their journeys.”
Buses will replace trains between Marple and Chinley until 23 September while the railway line is closed.
