The Sunderland Strategic Transport Corridor (SSTC) is a large infrastructure project to create a new section of dual carriageway between the A19 and Sunderland city centre. The £40M investment from Sunderland City Council serves to improve connectivity, reduce congestion and open up the area for regeneration.
Aarsleff Ground Engineering worked with civil engineers Esh Group to value engineer and redesign a retaining structure and slope stabilisation system for a section of the highway passing between the existing Pallion shipyard and the A1231 that runs above the site and over the adjacent Tyne & Wear Metro line.
Covering a total surface area of 2,000m², the Pallion soil nail retaining wall construction is one of the largest earth retaining structures in the UK. It is formed at a 70° angle, rising up to 12m high and stretching a length of 250m. The face required an aesthetically pleasing rock/gabion style finish and a solution to prevent rainwater penetrating the stone front, whilst still allowing groundwater to drain away and preventing erosion down the soil slope.
Working from the top down, the slope was cut to profile (typically in 1.5m high benches) to enable the soil nails to be installed. Before placing a structural facing of galvanised steel mesh, a single cuspated and impermeable ABG Deckdrain geocomposite layer was placed against the slope. The open side of the cuspated drain faces inwards to receive groundwater, and the geotextile thermally bonded to the top of the cuspates prevents washout, blocking larger soil particles from entering and clogging the drain. This allows water to filter freely into the void and down to a collector pipe at the base of the wall.
Three in one functionality: Deckdrain geocomposite provides drainage, erosion control and a barrier to rain water ingress at the front of the wall
The impermeable outer face prevents penetration of rainwater onto the steep soil slope and is drained separately down to the toe drain. This provides a barrier against clean rainwater entering the front of the wall and coming into contact with muddy groundwater that would result in staining to the ornamental stone facing.
The ABG Deckdrain 700S geocomposite has proven compression strength (BBA approved) in order to maintain in-plane flow and withstand the earth pressure at the base of the wall. The flexibility of the ABG Deckdrain geocomposite also ensures close contact with the soil over the whole slope, preventing erosion of fines and providing a highly efficient channel for the dispersion of groundwater.
The light-weight ABG Deckdrain rolls are easy to install, and were simply unrolled into position from the top of the slope onto the bench below. The structural steel mesh is placed over the geocomposite layer and bolted to the soil nails using large square washers, ensuring the composite is pressed into close contact against the underlying soil.
The remainder of the roll was pinned out of the way and dropped down to the next bench. In this way, the drainage core provided cover against erosion during construction, separating groundwater and rainwater from weakening the soils as each new tier was created.
The ABG Service
ABG Deckdrain is commonly used in Highways construction behind concrete retaining walls and is BBA approved. In this instance, further product functions were realised with the assistance of the ABG technical design service.
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