Trenchless Technologies, in conjunction with Africoast Consulting Engineers, recently completed Phase 2 of a contract for the Mandela Bay Development Agency.
This involved the rehabilitation of two existing concrete parallel sewer pipes located beneath the heavily trafficked Govan Mbeki Avenue in the heart of Port Elizabeth’s CBD.
Trenchless Technologies managing member, Sam Efrat, says that to date the company has been involved in two phases on this contract, ranging between 2007 and 2011. In Phase 1 rehabilitation of parallel 450 mm and 840 mm concrete sewers had to take place concurrent to an urban environmental upgrading project involving decorative paving and resurfacing works. In Phase 2, the 450 mm pipe increases in size to a 525 mm sewer which is located underneath a heavily trafficked roadway, while the 840 mm pipe – as it flows downstream – increases to 1 050mmlocated underneath the newly constructed Bus Rapid Transport lane. Considering the urban environmental upgrade project and that the bus and road lanes are utilised every day, only non-destructive trenchless techniques could be employed for the rehabilitation of the pipelines.
At the preliminary stage a thorough condition assessment of the two sewers was undertaken, which involved a theoretical analysis of the sewers based on effluent and operating conditions. A CCTV inspection and cutting of windows from the sewers was carried out so that they could be inspected.
“The condition assessment ascertained that the circumference of the 450 mmsewer was severely corroded due to acidic effluent, with pH values as low as 3.3. This sewer was very old and was cast in two sections, with horizontal joints running along the full length. The mortar had corroded out of the construction joints and it was no longer water tight,” explains Efrat.
What’s more, the 840 mm sewer was severely corroded above the water line and the reinforcing was exposed and corroded away at places. The most severe deterioration occurred at the sides of the sewer due to a combination of corrosion and erosion, which was particularly severe along the sections of sewer where the velocity was high. As a result, there was a 50 to 60 mm wide sill on either sides of the sewer, just above the low flow level. Efrat says that this corrosion was typical of what occurred in a sewer downstream of a rising main where there had been an accumulation of gas due to long retention times. It was estimated that the sewer would collapse within ten years.
Efrat highlights that a number of trenchless technologies were considered for the project, including sliplining, cured in place pipe (CIPP) and Ribloc Expanda, Ribline and Rotoloc – an Australian brand of trenchless products for which Trenchless Technologies is the sole distributor in Southern Africa.
“Sliplining appeared to be the most economical, but there was limited space for launch pits and storage of long lengths of high-density polyethylene (HDPE) pipes. The CIPP option would not have influenced the hydraulic capacity and lining could be continuous through the manholes. However the CIPP process was more expensive and the sharp edges of the sills running along the sewer did not allowfor a technically sound design.
“Ribloc Expanda, Ribline and Rotoloc solutions allowed for a complete no-dig lining solution that could be installed through the existing manholes with limited disruption. In addition to no excavation, the solution was design-compliant and the risk was lower as the process could be reversed in the event of unforeseen problems,” describes Efrat.
Ultimately, Ribloc Expanda was selected as the technology of choice for the 450 and 525 mm pipes in both Phases 1 and 2, while Ribloc Ribline was grouted in position for the 840 mm existing sewer in Phase 1.Ribloc Rotoloc was used in Phase 2 to line the 1 050 mm sewer.
The overall scope of work included the following:
- 450 mm sewer: 570 m of RiblocExpanda with a spirally wound unplasticised polyvinyl chloride (uPVC) profile expanded to a close fit
- 840 mm sewer: 560 m of RiblocRibline with a spirally welded steel reinforced HDPE profile grouted in place.
- A total of five different trenchless techniques were used, namely:
- CIPP ambient cure: 525 mm sewer, lining of a 90 degrees bend, 15 m length (undertaken by sub-contractor Tuboseal)
- RiblocExpanda: 525 mm sewer, 620 m of spirally wound uPVC profile expanded to a close fit
- CIPP UV cure: 800mm sewer, reinforced liner impregnated in factory 43 m in length, where the 1 050 mm sewer reduces to 800 diameter (undertaken by sub-contractor Tuboseal)
- RiblocRotoloc, 1050 mm sewer, 720 m, tight-fit spirally wound-in-place uPVC profile
- pipe bursting: 225 sewer, 16 m of 225 HDPE installed using 60 t static chain puller employed from within the manhole
“Preparatory works included cleaning the existing sewers by both high pressure water jetting and the pull-through of cleaning buckets using mechanical winches, after which a CCTV inspection of the pipelines was undertaken. In Phase 1, a sewer diversion was possible to divert the 450 mm sewer flows into the 840 mm sewer, whereas in Phase 2 overpumping of sewer flows by a surface by-pass pipe was necessary.” discusses Efrat.
Once the cleaning of the sewers had taken place, the manhole benching and portions of the manhole walls had to be broken to allow access for the Ribloc Expanda equipment in the 450 and for the Ribline equipment in the 840 mm manhole. “While the 450 mm breaking was straightforward, this proved to take two to three days per manhole in 12 hours shifts to prepare ahead of the Ribloc crew,” concleudes Efrat.
Efrat says that despite what may have seemed like insurmountable challenges, particularly on Phase 1, the team managed to overcome the following challenges with the following solutions:
- To maintain continuous sewer flows, the team constructed a by-pass to divert flow from the 450 mm sewer to the 840 mm sewer.
- To remove the encrustations from a section of 450 cast iron pipe, the team had to backream using horizontal directional drilling (HDD) and a 430mm OD backreamer
- To accommodate the winding equipment in the 840 mm manholes that had no concrete base, the team had to reline the manhole base with quick set mortar and mesh to prevent erosion of the earth base during sewer flows.
- As bends were found in the 840 mm sewer, which interrupted the winding of the MH to MH sections, shorter sections had to be wound that could be pulled through bends and hand welded in position using extrusion welders.
- As concurrent surface works competed for work space, night and weekend work became modus operandi.
- As there was a diameter reduction in the 450 mm line where it crosses over the 840 mm line, a new manhole was constructed on the 840 mm sewer to tie the 450 mm sewer into the 840 mm sewer, as a permanent solution.
Despite these challenges, Trenchless Technologies, consultants, Africoast Consulting Engineers and specialist consultants, PIPES, managed to pull off the successful rehabilitation of the two sewer pipelines underneath the heavily trafficked Govan Mbeki Avenue without any disruption to traffic and the public. Efrat says that this contract highlights the large range of trenchless techniques and expertise available in South Africa today.
“I believe that the overall success of the contract can be attributed to the combined management team that worked together and contributed their ideas and solutions to overcome the challenges, both cost-effectively and efficiently. Testament to the success of the project is Trenchless Technologies’ being awarded runner-up in the South African Society for Trenchless Technologies (SASTT) Award of Excellence in 2011,” concludes Efrat.