Thursday, 10 April 2014 14:06

Improvement Of N7 Between Melkbos And Atlantis Intersections Is On Track

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Improvement Of N7 Between Melkbos And Atlantis Intersections Is On Track

SANRAL’s 30 month project to improve National Route 7 between the Melkbos and Atlantis intersections in Cape Town is progressing on schedule towards completion in November 2014. The N7 connects the Western Cape to Namibia via the Vioolsdrif border post, carrying large amounts of traffic in volumes that are increasing at a rate of 3% annually. The section between the Melkbos and Atlantis intersections had become a notoriously high accident zone and one of the main objectives of this project is to improve road safety.

The upgrade is replacing dangerous intersections with grade-separated interchanges and closing all other access to the freeway. It will also increase road capacity by creating a divided freeway with a 120 km/hour design speed. The dual carriageways are 10,9 metres wide with a lane width of 3,7 metres. The road reserve is 60 metres wide.

Haw & Inglis, one of the few South African construction companies dedicated to road construction, is the main contractor on this R302 million improvement project. The upgrade extends from the existing Melkbos intersection at MR (Main Road) 199 to the Atlantis intersection at the MR217/304 for a distance of 9,5 km, including the interchange ramps. The route traverses a relatively flat topography with rolling terrain typical of the area, consisting mainly of cultivated farmland interspersed with pockets of natural vegetation.

The improvements being carried out by Haw & Inglis comprise four main components - upgrading the existing at-grade intersection of the N7 and MR199 to a grade-separated interchange with ramp terminals; upgrading the existing N7 single carriageway to a dual carriageway between the two interchanges; upgrading the Atlantis at-grade intersection with MR217, also known as Route 304, to a grade-separated interchange with ramp terminals and closing off and rerouting the existing minor road and property access to the N7 onto adjacent and new roads. The free flow interchanges follow a combined diamond and partial cloverleaf (parclo) design which is a modification of the typical clover leaf interchange.

At the Melkbos interchange, MR199 passes below the N7 which is being maintained at its current level to avoid disturbance of Eskom high tension power lines running from Koeberg nuclear power station. The on- and off-ramps by-pass the pylons and the vertical alignment accommodates the required clearance profiles. Several pylons in the area have also been protected by guardrail barriers.

Standard road building techniques are being used, but there has been a departure from the conventional in the design of the interchange bridges by the Aecom Group. Instead of a foundation underpinning a substantial abutment wall, there is an earth retaining wall with a very small abutment on top, which has ensured a shorter construction period. Reinforced Earth was responsible to the design and supply of the earth retaining wall whilst Hall & Inglis constructed the earth retaining walls.

“One of the primary challenges on this project has been keeping two-way traffic operational at all times on this highly-trafficked section of road,” Kirsten Myburg, Haw & Inglis contracts manager on this project, says. “We began by diverting the traffic from the existing road with a bypass at the Melkbos interchange that allowed us to start with the new bridges and ramps. We’re also building new sections of road alongside the existing road, and then switching the traffic from the existing road to the new sections. This makes it possible for us to reconstruct the existing road. We’ve taken exactly the same approach on the Atlantis interchange, temporarily rerouting traffic from the areas where the bridges and ramps needed to be constructed.”

The Haw & Inglis team has encountered challenging ground conditions in the project area, with sand on the south-western side and unstable clays in the north and east. As a result, most of the cuttings have been undercut and stabilised with rockfill or pioneer layer to ensure a better foundation in the clay areas. Clay overlayed by a thin layer of sand runs down the middle of most of the construction area resulted in groundwater seeping between the two layers. This has called for a substantial investment in underground drainage.

Asphalt is being supplied by Much Asphalt and all the aggregates needed for the layer works from a commercial source. AfriSam is supplying about 650 000 tons of layer works material and 750 000 tons of overburden which is used for fill and selected over the project period, as well as about 15 000 m3 of readymix concrete for bridge construction at the two interchanges.

On any given day, the concrete giant sends out an average of 3 000 tons of material to the project site. This is in addition to its day-to- day supply to other customers in the Western Cape.

AfriSam’s contract with Haw & Inglis requires it to maintain stock levels up to 10 000 tons of each type of layer works material to ensure that, in the event of a breakdown, these critical supplies will not be interrupted. AfriSam has also needed to factor in any delivery delays between its quarry and the project site, typically caused by bottlenecks at the N7 weighbridge.

Prior to commencement of the project, AfriSam worked alongside Haw & Inglis to develop durable readymix designs that complied with the specifications for construction of the interchange bridges. Mix testing was carried out by AfriSam’s technical laboratories in Cape Town, and test beams were cast using various mixes and sent for independent verification by the Engineering Department at the University of Cape Town to achieve durability criteria as specified. Test beam verification was initiated three months before the project began, because the compressive strength characteristics of the concrete had to be validated over 28 days. Since construction began in May 2012, AfriSam continues to provide regular back to back testing with Haw & Inglis’ on-site laboratory to ensure the specifications are maintained through this highly specialised project.

Stringent environmental management was required on this project to conserve an expanse of Cape Flats Sand Fynbos and the Swartland Shale Renosterveld found in the area of the Melkbos interchange.

In terms of the Record of Decision (ROD), SANRAL agreed to the development of an alternative alignment where the first section of the dual carriageway was constructed to the west of the existing road, crossing over the existing alignment and back again. This has saved an area of about 25 000 m2 of the critically endangered Cape Flats Sand Fynbos and Swartland Shale Renosterveld.

Vula Environmental Services was appointed to manage the vegetation rehabilitation plant including harvesting and relocation of many plants which had to make way for the project.Haw & Inglis is spending at least 16% of its contract value on services provided by local small, medium and micro enterprises (SMMEs), typically for ancillary work. Local labourers recruited for the duration of the project are receiving ongoing skills training in competences such as installing gabions and shuttering, flagmanship, concrete lined drains, road signs and fencing as well as operating chainsaws and troxlers.

Contract completion (before extension time) is November 2014, and AfriSam’s contribution will end in October 2014.

Last modified on Thursday, 14 September 2017 16:06

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