M5 (Cape Town)
The M5 is an expressway in the City of Cape Town Metropolitan Municipality, South Africa. It connects Milnerton on the Western Seaboard in the north to Muizenberg in the south, and crosses both the N1 and the N2. For part of its length, from the N1 interchange to Plumstead, it is a limited-access freeway. From Mowbray to Muizenberg, it is parallel to the M4 Main Road.
Route
The M5 begins at the Potsdam interchange with the N7 highway in the suburb of Dunoon. It goes south-south-west as Potsdam Road and reaches a junction with the M14, where it becomes Koeberg Road and becomes a dual carriageway. The M14 joins the M5 for a kilometre southwards before becoming its own road eastwards while the M5 continues south as Koeberg Road. It proceeds southwards for 9 kilometres, through the large suburb of Milnerton, to reach the Koeberg interchange with the N1 highway in Brooklyn, where it becomes a limited-access motorway.It proceeds southwards from the N1 interchange as the Black River Parkway, crossing the Black River, through Maitland, to reach an interchange with the N2 highway near Mowbray, where it becomes the Kromboom Parkway. It proceeds southwards, through Athlone, Rondebosch East and Lansdowne, to reach the M68 interchange in Plumstead, where the M5 loses its motorway status but remains a dual-carriageway as Prince George Drive. It proceeds southwards, through Retreat, to enter Muizenberg on the False Bay Coast, where it terminates at a roundabout junction with the R310.
Koeberg interchange
The largest interchange on the M5 is the Koeberg interchange, connecting the M5 with the N1. The merging of lanes at this interchange used to result in multi-kilometer stop/start queues approaching the interchange on the M5.Elevated freeway upgrade
In 2008, work began to upgrade the Koeberg interchange with elevated freeways joining the M5 and the N1. The elevated freeway from the N1 onto the M5 was completed in June 2010 in time for the 2010 FIFA World Cup, while the other direction was completed in November 2011.Design
The elevated freeways embody some innovative design features and principles. For example, the elevated freeways had to be built without disrupting the daily flow of some 200,000 vehicles. The existing roads and bridges had already used up the available land into which to expand. The intersection is hemmed in by a canal, railway reserve, power transformer, and industrial buildings, etc. Thus, the possibility of overlaying the existing structures with a bypass had to be envisioned with a system that could fit into the existing interchange.Oval reinforced concrete pillars with "T" pieces atop, standing on foundations supported by concrete piles, could fit into the spaces between existing roads and bridges. These could in turn support post-tensioned concrete "U" beam spans, cast on the ground and lifted into place by crane. The road deck would be cast on top of the "U" beams. Five beams between each "T" topped pillar allowing for a double lane road deck. The beams resting on each side of the "T" piece support, are connected to each other and the deck by a concrete end which resists lateral-flexing of the deck between-beams.
The roadway on both elevated freeways narrows from a double lane to a single lane at the point where it connects with the existing N1 and the M5 motorways. At this point, the elevated freeway presents the motorist with a down-hill slope to encourage acceleration. The elevated freeway feeds into its own lanes on both of these existing freeways allowing acceleration and preventing congestion.
"U" beam spans
The "U" beams would have sloping sides like a "V" with a flat bottom. The forces at the centre of the span cause lateral inward compression at the bottom of the "V" and lateral outward compression transmitted into the deck as tension. The longitudinal tension, at the bottom of the "V" and at the centre of the span, is absorbed by potent post tensioned cable embedded in the concrete. Each beam rests on its ends, on purpose designed steel plates, which accommodate expansion and contraction and transmit the weight of the span into the "T" pieces atop the oval pillars.The roadway deck would extend beyond the edge of the outer beams and be lined with pre-cast crash barriers.
Box girder bridges
Box girder bridges were conceived, designed and built to span the railway reserve and the long stretches over the outbound N1 carriageway and the northbound M5 carriageway as it approaches the N1. These had to be supported by scaffolding in the case of the end bridges and by "I-beam" steel trusses in the case of the railway reserve spans. The railway reserve also includes the height of the electrification services. This height added to the height of the "I-beam" steel trusses, the scaffolding on top of the trusses and the height of the box girder bridge is the determining factor in the extraordinary height of the new elevated freeway, which at its highest point is at the same height as the 5th story of the adjacent old mill building.During construction of the end box girder bridges, the traffic had to be diverted round the supporting scaffolding. This was especially inconvenient on the outbound carriageway of the N1 where speed was severely reduced, adding heavily to congestion.