Geotechnical Design of the Southern Motorway Underpass

The Southern Motorway Underpass takes the new South Eastern Highway beneath the Southern Motorway (State Highway 1), one of the busiest roads in Auckland. One of the major requirements for the construction of the Southern Motorway Underpass was to minimise disruption to the motorway traffic. This was done by building the northern half of the bridge and moving traffic onto it before building the southern half of the bridge and also by adopting the top down method of construction. The top down method was achieved by initially constructing the soldier piles for the abutment walls, tying back to deadmen with pre-stressed anchors and initially excavating just sufficient material between the bridge abutments to construct the piers and place the bridge beams. To reduce the depth of temporary sheet piling required between the north and south halves of the bridge the remaining bulk excavations beneath the bridge deck were only carried out once both halves of the deck were opened to traffic. The skewness of the bridge meant that the distance between the bridge abutments and deadman had to be unusually short to keep the deadmen within the motorway embankment. Because of the soft ground conditions, there was potential for the 6.5m deep excavations to induce lateral abutment wall movements which would not have been acceptable to the bridge or the pavement of the motorway. This paper address the geotechnical design and construction aspects of the project.