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Grouting: Managing mainline mining risk

Innovative ground engineering design has helped Network Rail remove restrictions that had been in place for over 60 years on the West Coast Main Line

Temporary speed restrictions on the UK rail network are usually just that – temporary. They do not normally have to be accommodated in the train timetables. 

However, one imposed on a section of the West Coast Main Line in Scotland had, until recently, been in place since the1950s. Innovative use of drilling and grouting, micro-piles and rock fill buttresses means that the ground movement issues affecting a 65m section near Wishaw have finally been resolved.

The finished result means that Network Rail has lifted restrictions and the project team, formed of Cowi, QTS and Cementation Skanska, has developed a new approach to slope stabilisation.

Garrion Gill

Garrion Gill

The work involved stabilising a 45°slope

The steeply sloping Garrion Gillsite, which is covered by an ancient woodland, has long been a problem area for the railway line with records dating back to the mid 20th century showing ground movement. Anecdotal evidence also suggests that issues have existed at the site since at least the 1880s.

“A routine track inspection in February 2013 showed there was vertical movement of the track,” says Cowi regional director for Scotland Eva MacInnes.

“An 80km/h speed restriction had been in place since the 1950s becauseof movement, which was managed with ballast packing in the past.”

Previously the movement on the track was not linked with slope failure but MacInnes says that inspection of the site revealed a clear back scar at track level following the February 2013 movement.

“The back scar extended over 65m and there was up to 500mm of vertical movement on the scar,” she says.

Cowi was brought in at this stage to try and help understand what had changed at the site to cause the slope to fail.

Aspin Foundations worked with Network Rail’s contractor QTS to undertake a ground investigation at the site. MacInnes said that the ground investigation was extremely challenging, as parts of the slope were at 45° and unstable.

As a result, only a few boreholes were located on the slope itself but instrumentation was placed within these which was critical to understanding the ground movement.

The investigation identified that there were two parts to the failure mechanism – there was progressive collapse of historic coal mining below the track which was combined with creep movement of the slope. The coal mining was thought to have been the cause of the historic movement at the site and the slope failure was a more recent problem.

The twofold cause of the ground movement was not the only challenge the design team had to overcome –there were logistical and environmental issues too. The steep nature of the failed 40m high slope meant that access was difficult and limited possible stabilisation solutions.

exposed slope

exposed slope

Garrion Gill - the exposed slope

In addition, the woodland covering the slope is classified as a Special Area of Conservation – the highest environmental rating in Europe – due to the presence of protected ferns.

This meant that government and Scottish Natural Heritage approval of the plan was necessary.

“We looked at a huge range of options from piling along the crest and anchoring through to soil nailing,” says MacInnes. “Most of the options presented risks from working on a failed slope or would impact on rail services.

“Safe operation of the railway was the main focus for Network Rail, but the solution also needed to remove the speed restriction and the need for long term maintenance at the site too.”

The solution developed through an optioneering process led by QTS withCowi and Network Rail involved grouting the mine workings and construction of a rock fill buttress in five stages to give access for the grouting and provide long-termstability.

Nonetheless, MacInnes says that detailed analysis of the design using the Slope W software package showed that it was not sufficient to stabilise the slope in the short term, so the design was modified to include the placing of steel reinforcing bars within the grouting drill holes to create micro-piles.

“The bottom up approach to grouting and construction of the rockfill buttress addressed the instability from the progressive collapse of the old mine workings but not the creep,” she says.

Garrion Gill

Garrion Gill

The completed project

“Using Slope W, we looked at a number of cross sections through the valley shoulder to analyse the design using information from the inclinometers and model the groundwater. We also used the model to back-analyse the design during the early stages of the work.”

According to MacInnes, using the micro-piles in this way made the solution unique. In total 192 micropiles were installed in the 570 grout holes drilled during the main works. Taking the solution from design to reality was not simple either.

“It was a geometrical problem as the slope had a bowl shape to it,” says MacInnes.

Cowi and QTS undertook a number of workshops with its grouting partner Cementation to develop the methodology for the work.

Getting materials onto site –especially the rock for the buttresses –was the first hurdle, before the team even tackled the issue of getting grout and drilling flush to and from the rigs themselves.

“To allow the machines to work continuously without interrupting rail services, we needed to store up to 2,500t of stone on the site,” explains QTS design manager Mark Craig.

The former mine workings presented a risk to the planned laydown area and had to be grouted before work on the slope could start.To put the scale of this work into context, the overall scheme was expected to use 22,000t of aggregates and called for 15km of drill holes, split over 570 holes, taking 930t grout.

The lay down area called for 21km of drill holes, split over 590 drill holes taking 3,300t of grout. QTS also worked with Cementation to develop a trench system to carry drill flush to and from the rigs to prevent seepage into the rock and reaching nearby Garrion Burn.

The remote location of the site also meant that drill flush recycling was essential and Cementation works manager for Scotland, Ireland and northern England James Hamilton says that 1M litres was recycled during the course of the work.

“We needed to store up to 2,500t of stone on the site”

“We used Siltbuster settlement tanks to allow the water to be reused,” he explains.

Once all the preparation was completed, the effort put into the planning paid off and work on site went well.

The success of the work was also confirmed by instrumentation and monitoring of the site during the work.

“Monitoring was critical,” says MacInnes. “We had a huge monitoring system installed by Datum with tilt meters and rotation monitors in place, as well as an action plan for evacuation should the worst have happened. “We recorded a very small amount of creep movement during the work.”

Construction work took almost 11 months to complete and MacInnes says that the workshops were key to the success and smooth running of the scheme and helped to deliver the £4.7M contract on time and budget.

 

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