N1 widening underscores the value of planning

Completion of the additional lanes, shoulders and concrete median barrier. The original 20 mm UTFC layer was replaced with a 25 mm

UTFC layer

Over past decades, the N1 (TR9/1) connecting Cape Town’s Northern Suburbs has progressively evolved into a superhighway. The most recent intervention is the widening of an approximately 5 km section between Jip de Jager Avenue and Old Oak Road from a four- to a six-lane dual carriageway along with a range of associated works.

The demolition operation started on a Friday night and included both the demolition and removal of debris from the roadway. The road was reopened to traffic on Saturday afternoon – less than 24 hours after demolition began

The original fourspan Old Oak bridge consisted of a beam and slab deck supported on three piers and two abutments. The contractor elected to demolish the bridge using large excavators. A sand layer was placed on the existing roadway to protect it from falling debris L ed by the Western Cape Department of Transport and Public Works, SMEC South Africa was appointed in 2013 for the full scope of professional services. This ranged from project management, planning (including traffic modelling and analysis), detail design and procurement, to contract administration, supervision and close-out.

“When design work commenced in 2013, in excess of 120 000 vehicles were already using this route daily, making it one of the highest-trafficked roads in the province,” explains Michael Hendrickse, chief engineer: Geometric Design, Western Cape Department of Transport & Public Works. “This also meant that when the 36-month construction programme started in 2016, traffic flow disruptions had to be kept to a minimum. For this reason, key phases were often completed at night, such as asphalt resurfacing.”

The City of Cape Town’s Regional EMME Model was used to forecast traffic demand volumes based on a 20-year design horizon post implementation. This was further refined by employing a microscopic transportation modelling simulator to calculate anticipated travel times and average speeds during the morning and afternoon peak periods.

“The model that we used assumes a residential growth of 50% or 500 000 dwelling units in 20 years – i.e. by 2032,” says Hendrickse. “It also incorporates a modal shift towards public transport and takes into

The shaping of the upper selected layer for the additional traffic lanes and shoulders within the newly constructed concrete median barrier, as well as temporary concrete safety barriers Asphalt base patching in progress on the existing lanes. Since lane closures were not permitted during daytime hours, these works had to be carried out at night The new Old Oak bridge closely matches the adjacent bridge. Construction of the northern abutment footing is shown in the foreground with piling of the southern abutment shown in the background

account various new road links as proposed in the city’s CITP road network for 2032. We were also mindful to ensure that the project ties in appropriately with Sanral’s planned upgrade of the N1 east of Old Oak Interchange.”

Pipeline relocation and bridge demolition

A key hurdle that needed to be tackled at the onset was the relocation of a 60-yearold 1 200 mm OD Wemmershoek bulk water pipeline section from parts of the median on the existing dual carriageway. This was successfully replaced with a new 1 500 mm OD steel pipe positioned outside the newly demarcated road reserve. Had the older section been left in place, there was a real risk of construction-related damage and catastrophic failure. Also, repositioning the pipe made sense from a future serviceability perspective. “A key reason why the overall project was so successful is that sufficient time was allocated for proper planning,” says Cobus Hendriksz, functional manager: Roads & Highways, SMEC. “That included the comprehensive traffic studies so that we could study the different scenarios.”

Another key challenge was the need to remove the Old Oak West bridge to make way for a new and higher overpass structure. The way it was executed is remarkable. Instead of taking 48 hours as planned, the bridge was demolished and the freeway reopened to traffic 20 hours later.

“Reopening the N1 in less than 48 hours was an incredible achievement for the project team,” says Hendriksz.

Key works

With the road construction phases gaining momentum, the existing grassed median island was removed so that the new inbound and outbound inner lanes could be constructed. Additional road works included the upgrading of four interchanges at Plattekloof, Jip De Jager Avenue, Old Oak Road and Durban Road, and the construction of new auxiliary lanes between the interchanges on both inbound and outbound carriageways to improve weaving capacity.

Ancillary works entailed a new street light system, new traffic signals at all interchanges, the removal and replacement of all overhead sign gantries, the relocation of the existing Freeway Management System, and the installation of sleeves for multiple fibre operators along the northern boundary.

“Thanks to utilising a multidisciplinary approach and strong teamwork between the department and SMEC, this mammoth project was completed on time and within budget,” Hendriksz concludes.