An unusually long railway blockade allowed critical slope reprofiling in Avon.
Maintenance on part of the South Wales Branch (SWB) Line near Chipping Sodbury in south west England presented a number of challenges.
The task was to ensure stability of the east cutting with steep rock slopes high above the railway lines.
Works included removal of loose large rock boulders between layers of compacted mudstone overlying hard limestone rock bands. The slope also needed to be cut back to a shallower, safer profile.
Safety on the £500,000 project was paramount and all the work had to be carried out during a nine day blockade. A Network Rail project manager commented: 'This project was two years in the making and it was imperative that the blockade works went exactly to plan, with risks effectively managed.'
Main contractor Alfred McAlpine was responsible for the project's delivery and awarded a £100,000 subcontract to WM Plant to remove more than 7000m 3 of overburden material and re-profie the rock slope along a 400m section of steep rail cutting. WM Plant specialises in slope solutions and used its superreach excavators, which can extend to more than 25m, to do the work.
The scheme is part of a much larger reinstatement and line upgrade project. There, other contractors are working on an about 16km length of the railway.
With a tight programme and work being carried out 24 hours a day, nothing was left to chance. The main challenges included the steep slope gradient and the need to create a shelf in the rock above the original retaining brickwork.
For the hard rock reprofiling, WM Plant used mechanical rock wheels attached to the excavator's extended dipper arm to grind away the rock face. Fitted with tungsten heads, the milling machines used power from the excavator hydraulics to cut the angle of the new face.
This competent rock work was sequenced to use three long reach excavators ranging from 45t to 90t.
Designed and built by Caterpillar for heavy duty applications, WM Plant used CAT 330, 345 and 375 excavators. Suitable for use with heavy attachments to tackle the rock, they have the power to lift more than 2m 3 of material in one bucket.
After initial clearance work, the rock wheel was put to work and the 60t CAT 345 with 24m reach manoeuvred it into position to grade the slope profi le. Working out over 15m and deep inside the cutting, the operator was unable to see the end of the dipper arm. To help the operator locate the attachment, a CCTV camera was fi tted on the dipper and connected to a screen in the cab.
To achieve the exact profile, a computerised dig monitoring system positioned the bucket or attachment relative to the slope. The system, installed by Prolec, enables the operator to work safely and control with precision the slope angle.
A worker was stationed on the bank for each machine to communicate detailed instructions to the operator by radio. To assess the stability of the rock faces, the banksman, attached to a safety line, climbed down the slope of the rock face to monitor the excavation.
With the reprofi ling of the main slope complete, a team of abseilers from Rock Solutions then worked alongside the machines to remove smaller fragments of rock as part of the clean up operations on the newly formed rock shelf at the top of the existing stone retaining wall inside the cutting.
Installation of a rock fence, using a 300mm diameter rock drill attached to the end of the excavator and driven by compressed air, will provide an additional safety measure. Sockets for the rock fence posts are drilled to a depth of 2m and subsequently backfi lled and attached to the rock mesh.
A fi nal clean up for the blockade works used road rail excavators to remove protective matting from the tracks, after which safety checks were conducted and the track was re-opened.