Chiltern on Track

As part of an ambitious project to help expand the railway network and improve journey time for passengers travelling into London from Oxford and beyond, specialist engineers from our geotechnical arm BAM Ritchies are supporting the BAM Nuttall delivery team by stabilising newly widened cuttings and embankments that will help boost final rail capacity.

Andrew O'Donovan is BAM Ritchies' contracts manager for the team using ground nailing and retaining wall installation techniques to help provide support to cuttings and embankments near Gerrards Cross and Northolt as part of client Chiltern Railway's Evergreen 3 scheme.

He is overseeing the installation of more than 4,000 soil nails across the two sites as well as driving boreholes for and installing steel 'King Post' wall columns at over 160 locations at Northolt.
'There are three elements of the work we are carrying out on two different sites,' explains Andrew, adding: 'Soil nailing the realigned toe of an existing cutting near Gerrards Cross station and nailing to strengthen and widen an existing embankment at Northolt, as well as installing steel columns for a new King Post retaining wall at Northolt.'

Starting on site at the beginning of October, the team has begun work to stabilise the steepened slopes at Gerrards Cross where some 4m has been trimmed from the bottom of the batters with the slope re-profiled to 65°, helping provide more working space for the track realignment planned for the area. Completed during three weekend line possessions our team has installed 269, 14m long, 32mm diameter hollow stem soil nails, as well as 14, 39mm diameter nails of a similar length. Some 19, 15m long, 38mm diameter nails have also been installed across the site.

'In all we have installed almost 4.5km of soil nails at Gerrards Cross,' says Andrew. 'They were placed during possessions. In one weekend we managed to install 1.7km – more than a mile.'

Road / rail excavator mounted drilling equipment was used to install the nails, which were simultaneously grouted while soil netting has been placed to help stabilise the re-profiled slopes.

It's that sort of installation speed and quality that has become BAM Ritchies' trademark and is being exploited further along the track near Northolt.
Here our engineers are part way through the first phase of a complex installation that will see them drill well over 3,500 nails by the time the second installation phase is completed in spring next year.

The bulk of those will be the 2,928 self drilling 32mm diameter nails while a further 904 of a 38mm diameter will also be placed. Both types will be installed at 14m lengths.

These are being used to stabilise the slopes of an extended existing embankment and also boost its load bearing capacity.

The site at Northolt is split into four quadrants – north-east, northwest, south-east, south-west – and throughout the northern sector work is predominantly to strengthen the existing embankment.

In the southern sector, though, nails are being installed through existing gabions with extra fill being placed behind them to widen the embankment. The nails are drilled through the extra fill and into the existing material. Ground conditions are well suited to self-drilling nails according to Andrew.

‘It’s all London Clay other than the fill material which is pretty good for our installation. It drills well and we get a good return of material when grout flushing,’ he says.

BAM Ritchies is also in the process of drilling more than 160 boreholes along the length of a proposed King Post wall. Using a drilling rig equipped with 450mm diameter, 1m long augers our team will bore holes 6-8m deep and install 254 x 254 x 107mm steel universal columns to help form the King Post retaining wall at the site.

Andrew is sure the team will deliver the geotechnical stage of the project to our main project delivery team with time to spare.

‘It is challenging but we have been able to hit the ground running and output has been good. But then we are good at what we do,’ says Andrew.