With the provision of 22 000 new low income houses, the N2 Gateway project is set to change the lives of many households living in the N2 informal settlements and is one of the most talked about assignments in Cape Town with a budget of R2,3 billion.

Some of the factors that led to the N2 Gateway housing project were the densification taking place in the catchment area of the Zandvliet Wastewater Treatment Works as well as overflow problems experienced in the existing sewer system. This led to the City of Cape Town inviting tenders for the design and construction of a new Delft Bulk sewer.

Sobambisana Community Developments, a consortium, was the successful tenderer and appointed a Joint Venture Contractor consisting of Asla Construction and Power Construction, two of the leading construction companies in the Western Cape.

"We were the leading consulting engineers for Sobambisana on this Project and handled about 75% of the design while Bergstan designed the remaining 25% of the pipeline. The City of Cape Town also appointed Stewart Scott International to act as project managers on behalf of the City," explains Project Manager Adrian Coetzee from Kwezi V3 Engineers.

“The pipeline would have adequate capacity for the new 22 000 homes together with the future sewage flow in the whole drainage area to the Zandvliet Waste Water Treatment Works. Since the tender has been awarded to the Sobambisana Consortium in April 2005, we have been actively involved with the feasibility phase for the sewer line and also the final designs on the project.”

One of the challenges during the design and construction of the sewer pipe line, was to allow adequate drainage for all 22 000 homes. Based on the data obtained in the previous reports, this would require a pipe of a proportionately large size to be constructed due to the ground constraints. “We followed the existing sewer line as this was the most viable route. This led to the crossing of many large existing services with diameters of between 800mm and 2400mm.

THE INVESTIGATION PHASE

Collecting information for the preliminary designs on the project was problematic due to the fact that most of the team members that worked on the previous plans no longer work for the council. “We were lucky to retrieve some of the original plans from Tygerberg, Oostenberg and even Stellenbosch Municipalities. We then used the as-built information together with GIS data of the total catchment area to create a model with the use of interactive design package, Civil Designer."

"One of the challenges that we faced was constructing the sewer line above and below the existing services while still being able to connect to the works and invert levels. The whole project was based on the same principle as the existing gravitational pipeline,” notes Kwezi V3 Technical Director Andre Prins.

The investigation of the venture commenced in July and lasted till November 2004 during which time the City of Cape Town applied for MIG funds from national government. Once these funds were granted, further investigation continued on the pipeline’s capacity. The cost estimate and planned time span on the existing as well as the adjacent pipeline were based on many assumptions. According to the technical report, it would take a period of two years to upgrade and construct the proposed and existing sewer line. National government however, wanted the pipeline to be completed in one year.

“The timeline for the pipeline’s completion was a 12 month construction period which only got going in December 2005. This meant that the contractor had to more than double the number of teams in order to meet the required deadline. This was a challenge but in our opinion, definitely achievable. At the time of the interview, the contract was on schedule to be completed within 12 months,” explains Adrian, remembering his feedback meeting with the Mayor of Cape Town in 2005.

CONSTRUCTING THE NEW PIPELINE

According to Adrian, the new pipeline had to be adequate for flows and it was therefore important to plan with the assumption that the entire catchment area would eventually be populated. “The old pipeline, being between 25 and 30 years old is linked at various key positions with sluice gate manholes so that certain sections of the existing manholes could be shut down to minimize flow."

"This was done in order to allow the existing pipeline to be refurbished so that it could act as a backup in the event of an emergency. The sewer pipe will eventually handle a maximum capacity of 4623 litres per second in peak wet weather flow at its bottom section,” says Adrian who worked extensively on the design.

One of the tender requirements stipulated a quality assurance plan for each phase of the project, a document that was stringently followed by the team during the project's execution. "Our Quality Manager Pieter Kriel devised a quality control plan for every task with corresponding checklists and audits with all the relevant stakeholders. When you are working on an assignment of this size a QMS document is vital in order to ensure that tasks are completed correctly," explains Andre.

Besides the design challenges, there were numerous environmental restrictions on the project. “We had to obtain special permission if we wanted to remove any of the Heritage, old White Gum trees as well as alien trees that crossed our path. We therefore used the services of a botanist, fresh water / river / wetlands consultant and a dune specialist in order to obtain the various permissions.”

A section of the housing project has since been completed with the first few occupants having received their keys to their homes in the last month. According to engineers Adrian Coetzee and Johan Prins, projects like the N2 Gateway come but once in a lifetime and will be remembered for years to come.