Bridge goes up in just four days
A new bridge was erected at the Hammarsdale interchange on the N3 within four days.
In an impressive feat of engineering and organisation, the bridge was erected across both carriageways of the highway at Hammarsdale, between Pietermaritzburg and Durban. Traffic on the N3 from Durban to Pietermaritzburg was diverted for two consecutive weekends to make it possible for the bridge to be erected.
The development of the Keystone Park Light Industrial, Warehousing and Logistics Precinct at Hammarsdale has significantly increased the volume of traffic – especially heavy vehicles – using the intersection of the N3 and MR 385.
The South African National Roads Agency SOC Ltd (Sanral) decided that the interchange must be upgraded from a simple diamond interchange, to a new semi-parclo (partial clover leaf) free flow interchange to prevent a back-up of trucks on to the N3.
Sanral Eastern Region project manager Stephné Wilmot said, given the high volumes of traffic using the N3 and the urgent need to construct the intersection as soon as possible to ease heavy vehicle congestion, it was decided to use time-saving construction measures.
Ian Jackson, resident engineer at Hatch consulting engineers, said to minimise delays, precast beams were used.
“With short to medium span bridges, you basically have three types of decks – cast in-situ; precast beams and deck slab; and a launched deck.
“The cast in-situ is the cheapest but will entail closing a carriageway for approximately three to four months and diverting both directions of traffic on to the one carriageway and then repeating the procedure for the next deck. Thus, it is time consuming and severely affects traffic.
“A launch deck is cost prohibitive because it is expensive to set up the launch yard. The geometrics of the bridge would only allow the launch of two decks. Thus, there are practical and cost constraints.
“Thus, a precast beam deck and slab has the least disruption of the traffic – two days in which to place the beams across each carriageway – and is also the most cost effective.”
Jackson said the beams were cast in a specialised casting yard five kilometres from the construction site.
The beams were transported to site with special low-bed vehicles.
As each beam weights 37 tons, an impressive crane was used to lift and place each beam in position.
It took two days to position 23 beams per deck. Due to the positioning of the crane to lift the beams and the potential danger to traffic, each carriageway was closed during the lifting and placing operation.