Chiltern on Track
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 has overseen the installation of more than 4,000 soil nails across the two sites as well as driving boreholes for and installing 163nr 'King Post' wall columns. 'There are four 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, installing steel columns for a new King Post retaining wall at Northolt 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-profiled to 65°, helping provide working space for the track realignment and new drainage installed in the area. Completed during three weekend line possessions 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 diameter nails were also 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 rock fall mesh with combined erosion matting 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 the team has completed the installation of 3,900 nails: the bulk of those were self drilling 32mm diameter nails while a further 904 of a 38mm diameter. The majority of both types were 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. 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 fill placed behind them to widen the embankment and allow the new Down Main to be constructed adjacent to the slewed Up Main, thus enabling the enhanced line speed through alignment design. The nails were drilled through the extra fill and into the existing material. Ground conditions were 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.
The majority of the nails were installed using 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 five drilling rigs were used to meet the Easter weekend deadline to enable the Up-line to be slewed over.
Over 7,000m2 of slope facing consisting of rock fall mesh with combined erosion matting to stabilise the re-graded northern sector was placed in under four weeks.
BAM Ritchies have also drilled 163 boreholes 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 universal columns to help form the King Post retaining wall.
Six miles further towards Marylebone at Neadsen 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 12hour possessions by drilling 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 limestone.
‘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.
Article courtesy of rail technology magazine June / July 2011 |
