Traffic has been kept moving during the reconstruction of Northern Ireland’s busiest motorway thanks to innovative reinforced soil, retaining walls from Anchor Wall Systems.
The Anchor "Landmark" retaining walls have been installed at the Stockman’s Lane Junction of the Belfast Westlink, one of the City’s principal arterial roads and part of a £115m, M1 motorway improvement scheme being undertaken by Contractors HMC [ Highway Management Construction] for the Northern Ireland Department for Regional Roads Services.
The Stockman’s Lane project is one of three junctions on the Belfast Westlink, all of which required major reconstruction to improve traffic flow and increase capacity to cope with over 65,000 vehicle movements per day.
Sited within a heavily built up area of the city, HMC was faced with the problem of widening the road from 2 lane dual carriageway, to 3 lane plus hard shoulder, whilst maintaining the same overall highway footprint - no additional land take was possible. Two new bridges were required to span the Stockman’s Lane circulatory system beneath the motorway and slip roads had to remain in use throughout the work
HMC’s solution was to install near vertical, masonry-faced reinforced earth retaining walls between the new carriageway and the slip roads. Four walls are to be constructed, each approximately 150m long and up to 4m high. Detailed design was undertaken by Geotechnical Specialists Maccaferri, who specified Anchor Wall System’s Landmark Segmental Retaining Wall blocks for the wall facing.
The Anchor Landmark system employs an innovative positive connectivity device that locks polymeric geogrids to the masonry facing blocks. These geogrids are installed in conjunction with layers of compacted backfill behind the wall to form a highly efficient earth retaining structure.
Explaining the choice of Anchor’s Landmark System, David Crowther, Technical Manager for Maccaferri said: “Landmark is one of only a handful of retaining wall systems that carry British Board of Agrement [BBA], Roads and Bridges Accreditation. Landmark gave us the assurance of a proven system with a design life of 120 years. It is also approved by Northern Ireland’s Road Services for use in structures requiring TAS approval so we were confident of its performance.”
Unlike traditional retaining wall structures, Landmark is a dry-built system requiring no concrete foundations or shuttering work.
It uses geogrids that work with compacted backfill to create a mass structure of immense strength and stability.
Bulk excavations behind the wall extended 3.5m to accommodate Enkagrid Pro 60 & 90 geogrid, supplied by Maccaferri. Over 11,000sqm in total were installed. During construction, the geogrids are placed into a receiving channel on the top face of the block and held in place by means of a polymer locking bar. Corresponding tongues on the underside of subsequent blocks hold the locking bar in position preventing pullout of the geogrid under load and give the system immense strength.
Geogrids were placed at every alternate course of blocks and were sandwiched between 250mm thick compacted layers of type 1A1 backfill. A 300mm wide layer of un-compacted, free draining granular material was installed behind the wall face to act as a drainage medium. This leads to a porous drainage pipe at the foot of the wall and prevents the build up of hydrostatic pressure behind the wall.
The M1 Westlink project is the first major highway infrastructure use of the Anchor Landmark system in the UK and Ireland. The Westlink reconstruction is a PFI, DBFO package [Design Build Finance & Operate], and is also the first of its kind in Northern Ireland. Construction work began in early 2006 and will continue until early 2009.