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BAM Ritchies and its contracts manager Andrew O’Donovan explain more about the ground nailing and retaining wall systems being used on Chiltern Railways’ Evergreen 3 project.

and improve journey time for passengers on the Birmingham – London line, special- ist engineers from geotechnical contrac- tors BAM Ritchies are supporting the BAM Nuttall delivery team by stabilising newly widened cuttings and embankments that will help boost fi nal rail capacity.


Andrew O’Donovan is BAM Ritchies’ con- tracts manager for the team, using ground nailing and retaining wall installation tech- niques to help provide support to cuttings and embankments near Gerrards Cross and Northolt as part of Chiltern Railways’ Evergreen 3 scheme.

He has overseen the installation of more than 4,000 soil nails across the two sites, as well as driving boreholes for and install- ing 163nr ‘King Post’ wall columns.

“There are four elements of the work we are carrying out on two different sites,” explains O’Donovan. “These are soil nail- ing the realigned toe of an existing cutting near Gerrards Cross station and nailing to strengthen and widen an existing embank- ment at Northolt, installing steel columns for a new King Post retaining wall at North- olt and installing steel columns for another King Post retaining wall at Neasden.”

Starting on site at the beginning of October 2010, the team has stabilised the steepened slopes at Gerrards Cross, where some 4m has been trimmed from the bottom of the batters with the slope re-profi led to 65°, helping provide working space for the track realignment and new drainage installed in the area.

Completed during three weekend line pos- sessions, the team 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-diame- ter nails were also installed across the site.

“In all we have installed almost 4.5km of soil nails at Gerrards Cross,” says

part of an ambitious project to help expand the railway network

O’Donovan. “They were placed during pos- sessions. 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 rock fall mesh with combined erosion mat- ting has been placed to help stabilise the re-profi led slopes.

That installation speed and quality is also being exploited further along the track near Northolt. Here, the team has completed the installation of 3,900 nails: the bulk of those were self-drilling 32mm diameter nails, while a further 904 were a 38mm diameter. The majority of both types were installed at 14m lengths. These are being used to stabi- lise the slopes of an extended existing em- bankment and also boost its load-bearing capacity.

The site at Northolt is split into four quad- rants; north-east, northwest, south-east, and south-west. Throughout the northern sector, the work is required to strengthen the embankment to allow the existing Up Main to be slewed across.

In the southern sector, nails were installed through new gabions with extra fi ll placed behind them to widen the embankment and allow the new Down Main to be con- structed adjacent to the slewed Up Main, thus enabling the enhanced line speed through alignment design. The nails were drilled through the extra fi ll and into the existing material. Ground conditions were well suited to self-drilling nails according to O’Donovan.

“It’s all London Clay, other than the fi ll ma- terial which is pretty good for our installa- tion. It drills well and we get a good return of material when grout fl ushing,” he says.

The majority of the nails were installed us- ing 6m masts, so minimal bar additions were required to reach the required depth of 14m, though where nails were positioned closer than 6m to the boundary a sectional

mast was used. The length of this mast could be varied so that the section length of each soil nail could be maximised by quickly altering the length of the mast. Up to 700nr nails were drilled using shorter sections of bar.

A peak of fi ve drilling rigs were used to meet the Easter weekend deadline to en- able the Up-line to be slewed over.

Over 7,000m2 of slope facing consisting of rock fall mesh with combined erosion mat- ting to stabilise the re-graded northern sec- tor was placed in under four weeks.

BAM Ritchies have also drilled 163 bore- holes along the length of a proposed King Post wall at Northolt. Using a drilling rig equipped with 450mm diameter, 1m long augers, the team bored holes 6-8m deep and installed 254 x 254 x 107mm steel uni- versal columns to help form the King Post retaining wall.

Six miles further towards Marylebone, at Neasden Junction, another King Post wall was built, to retain the higher level LUL relay room and fencing whilst the adjacent Up Harrow line was constructed 1.85m below. The retaining wall was constructed during three 12-hour possessions by drill- ing 600mm-diameter, 8m-deep boreholes for the installation of 41 galvanised 254 x 254 x 167mm steel universal columns. The drilling was carried out using a telescopic Kelly bar Geax DTC50 drill rig.

On both King Post wall sites, the column was secured by grouted single sized lime- stone.

“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 O’Donovan.


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rail technology magazine Jun/Jul 11 | 95

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