Tensar geogrids save time and money on M25

Tensar Technology to provide stabilised embankments with slopes of up to 60deg constructed with geogrids and local London clay or crushed recycled concrete fill at M25 spur..

Major highway projects between J12-15 of the M25, include the new spur to connect to T5 at Heathrow Airport and the M25 widening itself.

Both were faced with similar problems due to the local soils when building slopes and embankments in confined space, whilst trying to minimise land grab.

The solution employed Tensar Technology to provide stabilised embankments with slopes of up to 60deg constructed with geogrids and local London clay or crushed recycled concrete fill.

Additionally, Tensar geogrids delivered a cost and time saving alternative to a cement bound sub-base for some of the slip roads.

As well as time savings over conventional techniques, the ability of the Tensar solution to use local fill saved on the cost of imported aggregates and was a more sustainable approach.

Normal limits for the long term stability of an unreinforced slope in London clay are 1:3.5(V:H) but on the T5 spur roads there was in some places, insufficient space available for slopes of this inclination.

By applying Tensar Technology in a design and supply contract, a slope face of 1:0.6(V:H) was achieved using local clay fill on one 4m high and 200 metres long section.

Shallower slopes between 1:2.5 (up to 5m high) and 1:1.7 (up to 6m high) were constructed at other sections.

A Tensar slope system incorporating a steel mesh panel face, to which geogrids are attached, was used for the 1:0.6 slope, which avoided the need for external propping.

Topsoil was placed behind a special polymer liner mesh for later hydraseeding; the shallower slopes were faced directly with topsoil and grass.

Consultants were Brown and Root (now KBR), and CA Blackwell worked as sub-contractors for main contractor Laing O'Rourke.

Steep slopes of 1:1 were required when widening the M25 between J12 and J13 to minimise the land take, Tensar RE uni-axial geogrids were used here with Class 6I recycled aggregate to construct slopes up to 6m high, faced with a proprietary web system before topsoiling and seeding.

This section was constructed by McArdles for main contractor Balfour Beatty.

Designers were Gifford/WSP, who were also involved in construction on the River Colne Bridge between the M25 and T5.

Here two 90m long and 6m high embankments founded on landfill materials with 1:1 slopes were constructed using Tensar Technology around the piles which support the bridge deck.

Where slip roads around J14 passed over back-filled gravel pits, the primary support was a series of vibro-stone columns.

The illustrative design incorporated a granular capping layer, overlaid with a cement bound sub-base (CBM).

Gifford/WSP proposed an alternative granular sub-base construction incorporating two layers of Tensar biaxial geogrids within the capping layer.

This not only saved on the cost of cement, it also greatly speeded up the work time on site by saving the curing time required for the CBM.

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