N25 South Ring Road
National Roads Authority/Cork City & County Council
SIAC Construction Ltd
As part of the ongoing improvement of the N25 South Ring Road in County Cork, Ireland the roundabout junctions at Sarsfield Road and Bandon Road on the N25 were to be grade separated. The proposed solution was a flyover consisting of a dual carriageway carrying the East-West traffic above both roundabout junctions. The main contractor for the N25 Bandon/Sarsfield interchanges upgrade is SIAC Construction Limited. Two single-span bridges will be used to carry the East-West traffic over the Bandon Road roundabout and a double span bridge over the Sarsfield roundabout. Reinforced earth embankments will form the approaches and the link between bridges. New on/off slip roads will be provided to the East of Sarsfield and midway between the two roundabouts. The central section of the scheme involved the construction of a new road alignment partly founded on the existing road and partly on the existing natural ground and consisted of embankments up to 8m in height and part at grade. The existing site geology consists of glacio-alluvial deposits, primarily silty or gravely with some peat deposits. Some sections of the proposed new road alignment are constructed over deposits of peat and silt to an 18 m depth.
The adopted ground improvement system proposed by the main contractor for these areas of soft ground was the installation of vertical mortar/concrete inclusions (known as “Rigid Inclusions”).
A geosynthetic reinforced transition layer is installed above the top of the inclusions to assist in the uniform transfer of the embankment loads into the composite improved soil below. Two orthogonal uniaxial layers of reinforcement are laid over the inclusions to transfer the load of the embankment. In order to comply with this minimum thickness and based on the spacing of the inclusions and their effective diameter a minimum thickness of fill above the inclusions of 1.53 m was determined to be required. This essentially meant forming the top of the inclusions below the existing ground level, for the at grade sections or close to the surface in embankment situations. The adopted construction sequence was as follows:
• Excavate down approx. 2 m below existing ground level
• Install a reinforced working platform comprising of reinforcement/separation geotextile and 600 mm of well graded granular fill
• Install Rigid inclusions to within 500 mm of the top of the platform
• Removal of the top 300 mm of working platform (200 mm above the inclusions heads) installation of the woven on the trimmed working platform surface & anchor at edges
• Install granular fill for the basal reinforced platform and road formation sub-grade
• Placement of a temporary surcharge on the road formation level to encourage settlement/reinforcement tensioning to occur prior to road surfacing
• Low Maintenance: Minimised deformation of the embankment after construction
• Elimination of effects on adjacent components, e.g. pipes, foundations, etc.
• Geotextile reinforcement provided optimal design objectives of high strength and low strain
• Customised roll lengths minimising waste