Macking a difference

By Kim Kemp

The City of Cape Town had a major challenge: The Black-Mac outfall sewer, over time, had succumbed to corrosion and disintegration and required a complete overhaul, as well as increasing the hydraulic capacity for the growing population.

Afriline6Final DN 1200 pipeline connecting the DN 1470 diameter pipes that were pipe-jacked crossing Macassar Road to the Macassar Waste Water Pump Station.

Laying a new sewer line comes with multiple challenges, more so when in the midst of a developed area: environmental concerns, traffic, and inconvenience to communities, and more. The Black-Mac project in Cape Town had a multitude of professional players to ensure that carrying out the necessary work considered all of these. The main contractor for the construction of the new Black-Mac Outfall Sewer was Afriline Civils.

Stephan Kleynhans, Aurecon technical director in the company’s water unit and project manager for the Black-Mac Outfall Sewer project, explains the motivation behind installing the new pipeline.

 Project motivation

The Black-Mac sewer runs for a few kilometres upstream of the Macassar pump station, says Kleynhans, and, “It picks up some of Blackheath, Blue Downs, Croydon, and Macassar’s sewerage. It was constructed in the 1980s and it discharges at the Macassar pump station.

“Along the Black-Mac sewer, there was a screening station that was in a state of disrepair, so what happened was that the City diverted flow from the Black-Mac sewer to the Zandvliet Waste Water Treatment Works (ZWWTW). But this resulted in the ZWWTW becoming overloaded, as the population increased,” Kleynhans explains.

When Aurecon was appointed for the upgrade of the overloaded ZWWTW, it was also tasked with recommissioning the Black-Mac screening station and to undertake a condition assessment of the existing outfall sewer.

While the project started during the peak of the Cape drought, in November 2016, imposing water restrictions on the project that also required dust control in the Cape winds, Kleynhans believes that the drier conditions made it easier for pipe installation. John Reed, contracts manager for Afriline Civils

From the screening station to the Macassar pump station comprises about 8km of pipeline. After conducting CCTV inspections of the entire length of the pipe, structural techniques were used to assess the remaining life of the existing sewer.

“It’s roughly 4km from the screening station to where we crossed the Eerste River with the siphon,” Kleynhans explains and adds, “That length of pipe still had a reasonable remaining life, so it was agreed that we will just reline that section. The section from the Eerste River to the Macassar pump station, however, was problematic in terms of remaining life, as the wall thickness of the old asbestos cement pipes had eventually eroded or corroded. Along with this, future developments were planned in the area that would feed into that sewer, so increasing the hydraulic capacity was required.”

For this reason, the 4km section from the Eerste River to the Macassar pump station was replaced with a new sewer line, consisting of HDPE-lined concrete pipes with internal diameters of 900mm and 1 200mm, respectively. The pipes were manufactured by Infraset (900mm) and Rocla produced the 1 200mm pipes and supplied within 40 weeks.  

While removal of asbestos can be a hazardous process, the length of pipe removed was not of a significant length. Nevertheless, the necessary permits and approvals were obtained, and main contractor on the project, Afriline, appointed a specialist asbestos subcontractor, Drizit Environmental.  

Kleynhans elaborates: “We only had two areas that were tied into an existing 800mm asbestos cement pipeline, at the Eerste River crossing and at the Macassar pump station, and only had small sections of asbestos that required removal. At the start was about a 10-metre section and at the end, about a 50-metre section. The new outfall sewer followed a different alignment to the existing outfall sewer, effectively on the northern side of the existing sewer.”

The project was under way at the time of writing and work had started with the tie-ins, Kleynhans explains. “We first had to lay the pipelines in parallel and then we connect the new one right at the pump station, with both the old and the new pipelines ‘live’. Thereafter, every upstream connection is tied in from the old pipe to the new pipe. Once the connections are completed, the old 3 800m, DN 800 asbestos cement sewer pipeline is decommissioned entirely.”

The old pipe then remains in the ground, Kleynhans explains. “Where we have the pipe going underneath existing roads, because we are concerned about traffic load, those sections are grouted up to ensure that the pipe doesn’t collapse structurally over time. The section that is in an open field is about one-metre deep and does not have any traffic load; therefore, grouting is not necessary. We do, however, seal up the manholes to mitigate the risk of people falling into the holes where covers have been vandalised or removed.”  

In a contract comprising so many elements, challenges are inevitable. John Reed, contracts manager for Afriline Civils, discusses some of those encountered.

Challenges

With a large section of the site bound between Macassar Road and existing residential areas, the working width of the site was very limited, with some of the pipes being installed at depths of between seven and eight metres deep, so ensuring adherence to Health & Safety compliance posed a challenge, says Reed. “The depth of the excavations required that they be battered, and staff had to work within a ‘drag box’ at invert level to create a safe working environment for our staff.”  

As the project took place in a residential area, the local community, children specifically, entered the construction area unauthorised. “With all the pipes and mounds of sand, for the children, it’s a playground,” Kleynhans noted. Coupled with this challenge, the project experienced continuous theft and vandalism of safety barriers, traffic signs, plant, equipment, and materials, Reed describes.

“We put up fences to prevent entry, but in the morning, they were gone,” Reed adds, necessitating full-time security on the site for the duration of the contract, as well as enlisting safety marshals from the local community, “which was more effective as they know the people,” he points out.

Reed says that during the excavations, while minor quantities of rock were encountered, it was the subsurface water that was a major obstacle and challenge in the pipe installation and chamber construction, with the need to deal with water in the trenches and structures (groundwater and surface run-off).

“That the gradient of the pipeline to be constructed was 0.76mm per metre, made precision surveying and installation of utmost importance,” he adds. “Dealing with the crossing of existing services such as high-voltage overhead power lines, fibre optic cables, water and sewer pipelines, and constructing adjacent to these services, also posed challenges,” he observes.

While the project started during the peak of the Cape drought, in November 2016, imposing water restrictions on the project that also required dust control in the Cape winds, Kleynhans believes that the drier conditions made it easier for pipe installation. He explains that the groundwater is relatively high in the area of the sewer installation, but because of the drought, this was less problematic during the installation. The recent rains have since changed the scenario, but the majority of pipe laying has been completed.

Technical scope of work

Reed explains that as the main contractor on the project, Afriline was tasked with excavating, installing, and backfilling 1 200m DN 900 HDPE-lined concrete pipes at depths of between two and nine metres on a bidim-wrapped 19mm stone bed and cradle, as well as excavating, installing, and backfilling 2 485m DN 1200 HDPE-lined concrete pipes between four and nine metres deep on a bidim-wrapped 19mm stone bed and cradle.  

“The DN 1470 HDPE-lined concrete pipes that went through Macassar Road were installed at depths of between four and six metres and required pipe jacking,” he says. “We were also required to weld the HDPE capping strips to seal the HDPE-lined pipe joints and construct 49 in situ cast, watertight concrete chambers of 3 600mm in length by 3 100mm in width, with either 2 600mm or 1 800mm precast rings as well as interim and top slabs, between two and nine metres deep,” he adds.

The existing siphon structure had to be demolished and replaced with a new watertight siphon-connecting chamber, while the remainder of the work included breaking into an existing live concrete chamber to connect the new DN 1200 pipeline; coating all internal surfaces of the new structures to protect the concrete against biogenic corrosion; and shoring/battering of pipe trenches to protect adjacent infrastructure and services, all while working in restricted working widths, Reed adds.

Included in the contract was connecting the existing live sewer systems to the new main outfall sewer, as well as modifying existing structures to accommodate the new or relaid sewer pipes up to DN 400.  

Construction included a new DN 250 uPVC treated sewage effluent (TSE) rising main of 900m in length; a new DN 250 uPVC pressurised water main of 350m in length, and connecting the new water and TSE pipelines to the existing relevant reticulations.

“Bulk earthworks were necessary to lower the ground level of the identified areas, necessitating the temporary relocation of fences and minor structures,” Reed adds.

Once everything is completed, the pipes, manholes, and joints will be tested to ensure that they are liquid-proofed.

Reed concludes that the proudest moment was: “Actually completing a very challenging contract with above average quality and project specifications. Backfilling of the trenches and chambers in December 2017 and the completion of the pipe installations and chambers in April 2018,” he beams. 

 Pipe specifications

“A total of 1 508 pipes were laid, comprising 2 500mm in length,” says Reed, adding that the logistics of transporting the pipes to site was undertaken with Afriline’s own crane truck, a 22m-long Mercedes 3335 Axor truck and trailer.

  • DN 1470mm – concrete jacking pipes – HDPE-lined 270° – CH3583 to CH3669 – for Macassar Road crossing by pipe jacking method — supplied by Concrete Units — jacked by Wepex.
  • DN 1200mm 100D – concrete pipes – HDPE-lined – CH1200 to CH3770 — supplied by Rocla — manufactured and supplied in 35 weeks.
  • DN 900mm 75D and 125D – concrete pipes – HDPE-lined – CH0 to CH1200 — supplied by Infraset.
  • DN 2600 and DN 1800 precast concrete manhole rings for 48 chambers — supplied by Rocla.

Project info

Client: City of Cape Town, Water and Sanitation

Engineer: Aurecon South Africa (Cape Town)

Main contractor:  Afriline Civils

Main suppliers, Cape Town: Rocla, Infraset, Concrete Units, Akwasol, and Lafarge ready-mix concrete.


Project management

Client: Keith Olsen

Engineer: Stephan Kleynhans

Contractor: Johan Marais (contracts director), John Reed (contracts manager),Chester Adams and Dino Cupido (construction managers), Arno Grobbelaar (safety manager), Christopher Reed (general foreman), Angelo Fortuin (senior surveyor), Duran Young (concrete foreman)

While the project started during the peak of the Cape drought, in November 2016, imposing water restrictions on the project that also required dust control in the Cape winds, Kleynhans believes that the drier conditions made it easier for pipe installation.


* Photos by John Reed, Afriline

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