Steep reinforced soil slopes will maximise the area of a massive land raise around the Stratford Box, which will house the new Channel Tunnel Rail Link station in east London.
In one of the UK's largest ever landscaping projects, 2.5M. m 3of soil excavated from the box and adjacent tunnels will raise the surrounding area by about 7m for future development, bringing much needed regeneration to the rundown area.
The work forms part of main contractor Skanska UK's Contract 230 of CTRL Section 2.
Geogrid for the steep slopes of the land raise and for bridge abutments on the site is supplied by Tensar International. A total of 1,860 linear metres of reinforced soil slopes will support the new development, including about 1,600m 2of slopes up to 8.9m high.
All but the most polluted soil is being cleaned and reused on site.
'Raising the site provides a clean platform ready for development and means very little material has to be taken off site, saving costly disposal, ' says Graham Church, a geotechnical engineer for Rail Link Engineering, the company responsible for engineering design and project management on the entire CTRL scheme.
Skanska UK Civils land raise manager Daniel Mayhew says: 'A lot of work has gone into cleaning the polluted upper layers excavated from the box, but as work progresses, virgin ground is being excavated and can be used directly on the land raise.'
So far geogrid work has been focused mainly for the abutments for two bridges to carry construction traffic over live freight railway lines, known as the East and West Bridges.
A combination of permanent and temporary faces are being built, with the permanent faces forming steep boundaries and winding corridors for the existing railway lines.
The West Bridge abutments have side slopes of 70infinity. The inner slopes will be permanent but the temporary outer slopes will be covered when the land raise takes place.
As the land raise progresses, the reinforced soil abutments will be connected and continued to form 550m long embankments along the existing railway line. When the outer slopes are covered during the land raise, this will leave a corridor or 'false cutting' lined by the reinforced soil slopes.
Lengths of the geogrid start at 12m in the lower part of the embankment, founded on the alluvium, and reducing to 5.5m towards the top, using a range of 120 RE, 80 RE and 55 RE grid.
Geology of the site comprises made ground and alluvium followed by the Lambeth Group and then Thanet Sands.
'We are now starting to reach the Lambeth Group and the Upnor beds in the box excavation whose non-cohesive fraction can be used as a suitable fill in the reinforced earth, limiting the import of material, ' says Church.
The permanent and temporary faces are formed by folding the geogrid back on itself, with a Tensar Bodkin connector threaded through to form a loop in the end. This is then pushed through the steel mesh panel (SMP) facing and secured by R12 bars.
A fine facing mesh and topsoil is placed behind the steel mesh facing panels on the permanent faces and the slopes are then hydroseeded. Tensar special projects engineer John Dixon says: 'Up to 100m 2of slope is being constructed per day on the West Bridge abutments.
'As well as internal stability the slopes and abutments are being designed for global and compound stability using Bishop's simplified method with a long term factor of safety of 1.4 and a short term stability of 1.25, ' he adds.
'Tensar geogrids are BBA certified for use in bridge abutments and embankments and are designed to BD70/97m BS8006 and HA68/94.'
Before the land raise can start the site will have to be cleared, including the removal of a factory built to provide the contracts with precast tunnel lining segments.
The East Bridge abutments have vertical soil reinforced faces to reduce the span of the bridge.
There 120 RE uniaxial geogrid will be used at the base of the slope, changing to 80 RE uniaxial geogrid further up the wall. The grids are installed at 200mm spacings on the lower layers, progressing to 400mm spacings on the upper ones.
Lengths vary from 12m in the lower part of the abutment to 7.5m in the upper part. Granular fill material will be used between the reinforcement layers.
The main use of this bridge will be to transport tunnel segments from the factory to the east end of the box.
Mayhew says: 'The bridges are being built for construction traffic and supplies needed during the construction of the Stratford Box and are therefore only meant to be in use for four to five years. However they could stay depending on the access needs to the box in the future.'
The 40,500m 2of geogrid and SMP-facing systems designed and supplied in Tensar's £470,000 contract makes it 'one of the biggest reinforced soil projects in the country to date and probably the biggest one on a UK railway, ' Dixon claims.