Blue Mountains Emergency Works, Morpeth
- Location: Morpeth, Northumberland
- Client: Story Contracting Ltd. For Network Rail
- Year: 2022
- Sector: Rail |
- Specialisms: Geotechnical Engineering |
The Morpeth Embankment site, situated on the East Coast Mainline (ECML) near Morpeth Railway Station, has experienced slope instability in the past. In 2016, other parties conducted major works to address localised instability on the embankment side slope, which was initially reported after issues with the Down Slow line’s line and level. The remedial works in 2016 involved regarding the slope by constructing a berm on the embankment side slope. However, track quality continued to decline even after these works, and all four lines needed regular maintenance through tamping.
In 2020, Pell Frischmann was called upon by Network Rail to provide geotechnical expertise for a project. The goal was to conduct a geomorphological survey, develop a monitoring plan, and outline a ground investigation to observe any signs of instability. In 2021, a ground investigation was carried out to investigate the impact of ground movement on track defects. Inclinometers were installed which identified a deep-seated landslide beneath the ECML and the hillside below. Unfortunately, a period of heavy rainfall resulted in significant track defects, leading to the closure of one line and a 10mph speed limit being imposed on the remaining three lines. Consequently, Network Rail requested Pell Frischmann to design a permanent solution to remedy the situation.
What we did
The remedial works at the site were carried out in two phases due to the emergency nature of the scheme. The first phase aimed to reduce the impact of higher porewater pressures on landslide movement and instability before Winter 2021/22. It was designed as a temporary fix that was quicker to install and involved the installation of horizontal drains and short sections of staggered sheet piles (each pile up to 12m in length) intersecting the basal shear surface of the landslip. This interim solution was not intended to provide long-term stability to Network Rail/Eurocode and BS6031 standards but to improve stability enough to handle increased rainfall events while a longer-term solution could be designed, procured, and installed in the second phase of works.
Various options were considered for the long-term stability of a slope. Although reducing pore water pressures on the slip surfaces was deemed cost-effective for improving stability in the shorter term, it wasn’t considered a viable solution for long-term stability due to the size of the main failure, high groundwater levels, poor condition of existing slope drainage, and limited drainage outfall.
As a result, spaced piles were installed as restraining structural elements on the hillside to effectively stabilise the landslide. Two twin rows of concrete piles reinforced with circular hollow section steel tubes were installed at the toe of the ECML embankment and mid-way down the hillside below. Due to the unstable hillside and difficult access, smaller piling rigs and lifting equipment were preferred, which limited the pile bore diameter and space for reinforcement. As a result, traditional cage reinforcement was discounted in favour of CHS steel tubes, which were used with piles at 3D spacing in the proposed design.
Before construction began, the design of the pile reinforcement was reconsidered to adjust for the high cost and scarcity of steel. The solution was to use repurposed ‘non-prime’ steel from the oilfield industry, which was more readily available. This resulted in a reduction in construction costs and time, and a higher sustainability score for the project. The piles were made using low-carbon concrete, which further improved their environmental impact. In October 2022, the piling works were completed at the Blue Mountains Morpeth site. Since then, the piles have been able to bear the load and limit ground movement, leading to reduced maintenance requirements for the tracks. Pell Frischmann will continue to monitor the performance of the piles through instrumented piles and boreholes on site, as well as monthly track quality checks. As a result of these efforts, speed restrictions on site have been lifted, the track has returned to its design levels, and all lines have reopened as of Spring/Summer 2023.