Construction Techniques

The underpasses at Broadway and Grosvenor will be constructed by a method called top-down construction. In simple terms, the piles will form a wall onto which a roof slab is built. Once this is complete excavation is carried out from each end down to the base of the underpass. A base slab is then constructed, which will form the floor of the underpass. The piling technique is called secant bored piling. In this process primary un-reinforced piles are bored along the line of the walls. Between these piles secondary reinforced piles are bored which overlap with the primary piles. A water-tight joint is therefore created.

At Broadway we divert services and construct the secant bored piled walls North and South of Broadway round-about. The walls are constructed so that they extend through the sleech into the underlying boulder clay, which is impermeable and results in the formation of a water-tight three dimensional box. Because Broadway underpass is longer and deeper than Grosvenor the walls outside the permanent roof slab will have to be temporarily propped during excavation and construction of the permanent base slab which then props the walls.

The Blackstaff and Clowney rivers, which run under Broadway roundabout, have to be diverted to facilitate the construction of the underpass. The Blackstaff comes from the South and the Clowney from the West of the city. We are straightening the Blackstaff, which currently flows through a culvert and criss-crosses underneath the M1and the roundabout.  The Clowney, which is a faster flowing river, will be diverted from its current course, where it goes into a pre-cast culvert adjacent to the Park Centre, and will be taken south wards 200 metres parallel to the M1 and then across under the M1 to join the Blackstaff in a twin culvert to the east of the motorway and Broadway flowing northwards.

The culvert system has been designed to replicate the existing conditions whereby the two rivers meet in an overflow structure beneath Broadway roundabout. A new fully enclosed overflow chamber will be constructed to the east of the underpass to facilitate this.

Traffic management entails us moving M1 and Westlink traffic to one side as we build the buried walls, then the slip road to the outside of the wall, which are then used for mainline traffic during the construction of the underpass. The slip roads are constructed north and south of Broadway roundabout, the roundabout is then elongated to allow us to construct the walls, permanent roof slab and permanent road lay-out. Finally traffic is moved onto the permanent roundabout allowing us to excavate between the walls and below the roof slab.

Unlike Broadway, which is a full underpass, Grosvenor is a partial underpass/flyover, where the Westlink mainline carriageway is lowered by three meters and the Grosvenor Road raised by three metres. After we divert the services at Grosvenor we construct the secant bored piled walls and these walls are then extended above ground to retain the embankments required for the slip-roads linking Grosvenor Road to the Westlink.

Once the walls have been constructed to the South of Grosvenor Road a temporary bridge is erected over the Westlink to carry all of the Grosvenor Road traffic and pedestrians. This will separate Grosvenor Road traffic from mainline Westlink traffic. As Grosvenor Road traffic is moved onto the temporary bridge, Westlink traffic is moved to the outside of the proposed underpass. This facilitates the construction of the permanent bridge and approach embankments. These will be built along the line of the existing Grosvenor Road. This will also allow the excavation and construction of the underpass on the line of the Westlink.