Civils and stations

WM Plant provides foundation for rock cutting project

A recent nine-day blockade on the Wales Branch (SWB) Line near Chipping Sodbury presented a golden opportunity for Network Rail, whose engineers took advantage of the line closure to complete a wide range of maintenance and safety related work.

The safety critical operations were required to ensure stability of the east cutting with steep rock slopes high above the railway lines. Proposed works included removal of loose large rock boulders, between layers of compacted mudstone overlying hard limestone rock bands that also needed to be cut back to a shallower, safer profile.

Thanks to an open, collaborative approach which left nothing to chance, the line re-opened on time without any disruption. Nonetheless, site operatives had to work round the clock to get the work done without delays, and the demanding schedule left little room for error. Within their tight timeframe, those working on the site had to cope with a number of major challenges, such as the geographical obstacle provided by the steep slope gradient, and the need to create a shelf in the rock above the original retaining brickwork some 12m below and 15m out from the new slope profile.

As main contractor, Alfred McAlpine utilised the expertise of WM Plant to remove over 7,000m3 of overburden material and re-profile the rock slope along a 400m section of steep rail cutting. WM Plant’s super reach excavators were used to reach out over 25 metres to carry out the project, and, as a solution to 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 were positioned and, using power from the excavator hydraulics, effectively cut the angle of the new face. To cut through this competent rock, work was sequenced to employ three long-reach excavators ranging from 45 to 90 tonnes. Designed and built by Caterpillar for heavy duty applications, the CAT 330, 345 and 375 excavators were able to use heavy attachments, and offered the power to lift over 2m3 of material in a single bucket.

After the initial clearance work, the rock wheel was put to work and the 60-tonne CAT 345 (complete with 24m reach) manoeuvred it into position to grade the slope profile. Working out, over 15m and down inside the cutting over 10m, the operator was unable to see the end of the dipper arm. To solve this problem, a CCTV camera was used on the dipper and linked back to a screen in the cab.

To attain the exact profile, a computerised dig monitoring system was used to locate the bucket or attachment relative to the slope. The Prolec-installed system enabled the operator to work safely and precisely control the slope angle, thanks a display screen in the cab guiding the rock wheel.

Safety was of paramount importance and a dedicated banksman was used for each machine. To assess the stability of the rock faces the banksman would be attached to a safety line and communicate by radio with the operator part way down the slope.

With the re-profiling of the slope complete, a team of abseilers from Rock Solutions worked with 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 formed the final phase of the works, using a 300mm diameter rock drill attached to the end of the excavator. Driven by compressed air, this drill was able to carve sockets for the rock fence posts to a depth of 2m, which were backfilled and attached to the rock mesh.

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