CAN-doing Ashley Hill

The Filton Bank Four Track project is one of Network Rail’s flagship projects and is, according to reports, essential to “unlocking the potential of the wider network to deliver the new Intercity Express Programme”. The project, located in a highly constrained urban environment, runs for 6 km through Bristol city centre. The project aims to remove a significant bottleneck in Bristol by increasing the number of tracks from two to four between Dr Days Junction and Filton Abbey Wood Station.

Geotechnical specialists CAN completed several cutting and embankment earthworks projects to facilitate an increase in the width of the trackbed over the scheme length. In the 1980s, two of the original four rail tracks had been removed, but major improvements were needed to restore 15 km of twin track, including signalling, track bed and infrastructure work.

Ashley Hill south embankment is 300 m in length with properties restricting access to the toe. The proposed new track alignment extended beyond the crest line. So, engineers sought to widen the crest of the embankment with a retaining wall and to reinforce the slope below with soil nails and a TECCO netting facing.

CAN’s working area involved navigating a narrow access track along the crest of the cutting, which demanded a complex programme of works to take account of the limited space and inability to transport large machinery to the worksite. The adjacent live rail lines added additional complexity, thus requiring much of the pile driving to be completed during the night.

CAN engineers installed more than 1400 soil nails (up to 20.5 m long) in difficult mixed ground conditions along the embankment using A-frame drill rigs on the slope and underreaching excavator-mounted drill rigs from the crest. Up to six drill rigs were operating concurrently, working efficiently in a carefully planned sequence.

The retaining wall consisted of 178 steel H-section driven piles up to 7.5 m long, installed to extremely tight tolerances by CAN’s specialist subcontractor SPI Piling. Several 8-ft raking soil nails were drilled through the waling beams and clearance holes in the concrete planks to provide additional restraint against overturning.

Despite the difficulty in access and logistics, CAN completed its programme on time, on budget and to stringent quality standards.