SWP proves itself on Cape Flats 1 refurbishment

A series of refurbishment works in progress since 2022 are designed to extend the life of the Cape Flats 1 and 2 (CF 1 and 2) sewer lines, originally built in the 1960s. Alastair Currie speaks to Vuyo James – senior professional officer: Planning, Design and Projects at the City of Cape Town – about the experience to date.

Has the decision to go the SWP (spiral wound lining) route been the right one?

VJ Based on the profiling data we’ve captured so far on CF 1 and 2 – each measuring approximately 14 km in length and running in parallel – the SWP methodology is the most efficient and costeffective way of restoring the structural integrity of these precast concrete lines that vary in depth from 1-3 m below ground level.

The other option we did consider is a curedin-place pipe (CIPP) lining methodology. However, initial trials indicated that we weren’t going to achieve the meterage advance rate targets required.

From a City perspective, this is the first time we’re going the SWP route on this scale and the repair results to date (namely work packages 17 and 18) are of a very high standard. This underscores why

SWP is a preferred trenchless technology intervention worldwide.

What are SWP’s key benefits?

The SWP technique results in the installation of a watertight PVC liner within the diameter of the existing pipe under live conditions. The key advantage of SWP is that it improves the velocity of the existing pipe without reducing its capacity.

Plus, the installation technique is so efficient. SWP PVC profile strips are progressively fed through a manhole access point onto a winding machine inserted within the pipe, which progressively forms and advances the new ‘pipe within a pipe’.

The two SWP liner options are the R5 or R6. The latter is a slightly thicker liner well suited for pipe sections that show signs of extensive deterioration caused by intense hydrogen sulfide (H2S) build-ups. Plus, you need to factor in soil and traffic loads, where applicable, when selecting either option.

How is the project being split up?

The overall CF 1 project has been split into multiple works packages spanning a term tender of three years. Cleaning, inspection and profiling has taken place on both CF 1 and 2. Both lines start in Athlone and end at the Cape Flats Wastewater Treatment Works (WWTW) in Zeekoevlei.

The fact that Cape Flats 1 Bulk Sewers was originally encased in concrete was an amazing foresight of the 1960s designers in extending their longevity, but that’s not the modern approach. However, it has made SWP a practical approach, rather than incurring the massive cost of a complete new pipeline replacement using technologies like microtunnelling.

As per the original programme, we started on CF 1 Bulk Sewers first. So far, we’ve completed a 1.2 km section with a diameter of 1 100 mm, starting at Pinati Estate in Athlone and extending to Govan Mbeki Road in Wetton.

In the next and current phase, we’ve awarded three new works packages for CF 1 Bulk Sewers, covering a distance of some 5 km. Overall, we plan to complete the CF 1 relining by November 2023, in the current 2023/24 financial year.

In the meantime, the profiling of portions of CF 2 is almost complete, and we’ll then hand over the data to the appointed consulting engineers so they can do further planning.

What are some of the key observations so far?

To profile CF 2, we diverted the flows into CF 1. As per standard procedure, we ventilated CF 2 to dissipate the H2S. However, as we headed towards the Cape Flats WWTW, the H2S values remained high, especially at pump station discharge points into the line. What is clear is that the turbulence created by pump stations increases the H2S values and accelerates deterioration in these areas, especially at places where industrial effluent enters the line.

This has a bearing on the installation of new future bulk lines. It’s clearly crucial to ensure that precast concrete pipes specified have internal corrosion-resistant inhibitors built in.

In the original plan, all the flows from CF 1 were to be diverted into CF 2 on completion of the latter’s profiling. However, in practice, we found that some sections of CF 2 were completely or close to being blocked. That required more intensive cleaning than envisaged. We have removed all the blockages, and that means we can now divert to CF 2 as originally planned.

As part of the solution to deliver the overall project, we’ve installed four sluice gates on the CF 1 and CF 2 lines. This means we can isolate and divert lines and flows where required. Having two lines running in parallel has been a blessing, because we can minimise the expense of over-pumping though a coordinated flow diversion strategy.

Then there are the 331 plus brick constructed manholes that need to be addressed. Some are intact; others have partially collapsed and need to be replaced. Either way, they have been remarkably resilient structures. Where revitalisation is possible, restoring with calcium aluminate cement is an internationally accepted benchmark option. The City will be releasing a separate works package for this manhole phase in due course.

And in closing?

The key takeaway is that establishing the groundwork on any project, particularly a trenchless technology one, requires detailed investigation. With both lines being more than 50 years old, some surprises are going to be expected as we verify the actual level of deterioration.

As we progress, the lessons learnt on CF1 – and, subsequently, CF 2 – will be invaluable for other towns and cities across South Africa needing to upgrade ageing pipeline infrastructure, where the benefits of trenchless technology applications are evident.