MORELETASPRUIT OUTFALL SEWER EXPANSION AND UPGRADE BEGINS:

Draining a 113 km² catchment area bounded by Rigel Avenue and Moreleta Park on its southern perimeter, Pienaars River catchment area (east), Waterkloof Ridge (west) and the Magaliesberg range (north), the project has been commissioned by the City of Tshwane Metropolitan Municipality (CoT).

Johann Wessels, Deputy Manager in the City of Tshwane’s Water and Sanitation Division, says greater population densities in the catchment area, especially in its southern suburbs of Morelata Park, Valley Farm and Garsfontein, have placed increasing pressure on the existing system.

The decision to proceed was taken after hydraulic and structural assessments had been undertaken by consulting engineers, Africon Engineering International (Pty) Ltd. These were conducted in the context of Tshwane’s sewer master plan, compiled by Cape Town-based consulting engineers, GLS Consulting. The plan includes a hydraulic model of the entire catchment area which Africon used to determine future potential total flows and additional capacity requirements. The areas ear-marked for upgrading are highlighted in green (see map).

When the entire upgrade has been completed the combined system will accommodate the catchment area’s ultimate peak dry-weather flow comfortably at no more than 70% capacity. The remaining 30% is reserved for handling storm water ingress during the wet summer months.

The existing pipeline, installed in the late 1950s and 1960s, splits into two at Derdepoort and these drain into the waste water treatment works of Zeekoegat and Rooiwal.

A physical examination of the existing system using CCTV cameras was used to diagnose structural failures and root ingress at the joints, although some sections of the line could not be properly examined owing to consistently high flow rates.

When the initial assessment had been completed, Africon conducted a feasibility study and a detailed survey of the whole route to chart the optimal layout of the new pipeline.

Thereafter, it was decided that the project would be implemented in three phases. Phases 1 and 2 involve the laying of new pipelines comprising CAC-lined reinforced concrete pipes. When these have been completed in 2014 the existing and new pipelines will operate in tandem as a dual-line system.

Phase 3 will address non-hydraulic capacity issues on the existing line, such as fractured pipes, structural failure, root ingress and water infiltration.

According to Kobus Prinsloo, the Africon engineer in charge of the project, budget constraints on the R74.8m Phase 1 contract will delay its completion until the second half of 2009.

“The reason for this was the huge and unanticipated escalation in South Africa’s
construction-related costs during recent years. As a result, the estimate on which the original budget allocation was based, was too low,” said Prinsloo.

Much of the new pipeline along the Phase 1 alignment comprises 900mm diameter concrete pipes, although stretches of the lower sections will include pipe sections in diameters of 1 050mm and
1 200mm. Pipe diameters ranging from 900mm at Val de Grace to 525mm in the upper reaches of the catchment area in Garsfontein will be used for Phase 2. Joints will be sealed internally on all sections of the project.

Phase 1 was begun in February and is being constructed from Derdepoort in the north to Val de Grace in the south, a distance of approximately 8.4km.

Most installation work in Phase 1 is based on open trenching, apart from seven instances where pipe-jacking is required, six under existing roads and one under a rail crossing. The pipe-jacking work is being done under a separate contract by Borwa Construction cc.

Cerimele Construction Company (Pty) Ltd was appointed as the main contractor on Phase 1. General manager, Kobus Grobler, explains that the trenching, which runs to a depth of up to eight metres, is Cerimele’s major challenge. 

“Although all construction work is important on such a large sewer pipeline, the pipe cradle is of critical importance to ensure structural integrity. We are using conventional drilling and blasting to remove rock, and this entails controlling the peak particle velocity of ground vibration to avoid damage to adjacent properties and services. Trenches must be adequately battered to ensure stability, or shored where working space is limited. We haven’t experienced much sub-surface water infiltration yet, but this is likely to change during the rainy season,” notes Grobler.

Work on Phase 2, which covers 7 400m from Val de Grace to Garsfontein, will probably commence in 2011 and is due for completion in 2014.

Many areas in which the new line is being laid, especially those in the southern sections, are heavily developed. Prinsloo observes that in previous decades the enforcement of flood lines was less strictly applied than today, and many of the properties are located below the
1:50 and 1:100 floodlines.

“This has resulted in the encroachment of properties right onto the banks of the spruit, a factor which imposed severe constraints on the route of the new line. This necessitated our opting for pipe-jacking under two townhouse developments at Val de Grace, one of the last green areas east of Pretoria.

“As pipe-jacking involves a considerable amount of blasting, we commissioned an environmental impact study and embarked on an extensive public participation drive before this option was chosen. Part of the process included meetings and negotiations with residents over a six-month period before the decision to tunnel under their properties was reached.

“The Val de Grace pipe-jacking operation entails drilling and blasting through some very hard rock at a depth of nine to 20m below the surface and over a length of 275m. Two 500 ton jacks with inter-jacking stations are being used on the project by Esor Ltd, the main pipe-jacking contractor.  Jacking commenced at the beginning of April and should be completed by February 2008.

“Although a connection to the rest of Phase 1 will only take place in 2009, we decided to go ahead with the Val de Grace pipe-jacking project as soon as there was consensus to go underground. While the blasting and pipe-jacking operation is taking place we are being as pro-active as we can in addressing any issues of concern to property-owners,” comments Prinsloo.

On the subject of piping material, Prinsloo says it was almost a foregone conclusion that the choice would be reinforced concrete.

“We didn’t really consider other options as the size and application of the pipes precluded us from doing so. The existing line is also comprised of concrete piping. It has performed well during its 50 to 60 year lifespan, and we believe it will continue to do so well into the foreseeable future.

“The real choice lay in selecting the most suitable pipe-lining option. Ultimately, we settled for CAC because of its good abrasion resistance and because much of the catchment area has fairly steep slopes. These result in higher flow velocities and a greater potential for abrasion. The lining itself is 13mm thick and considerably thicker at the pipe ends, due to the specific manufacturing technique adopted by SPC,” explained Prinsloo.

One of the design principles adopted for the upgrade was the positioning of the new pipeline on the opposite side of the spruit to that of the existing line. However, there are sections where the lines will be run in parallel on the same side of the spruit owing to space limitations.

“Another feature of the old line is that it crosses through rather than under the spruit in several places.  This led to the construction of what were essentially small weirs, which in turn led to scouring and erosion at the bottom of the weir walls. Consequently some pipe sections are in a poor structural condition at several crossings. However, by running the new pipe on the opposite side of the spruit many of the crossings will be made redundant. Those that remain could be upgraded to provide additional flexibility and to facilitate inter-connectivity between the new and existing systems in cases of emergency. 

“A total of five connections will be made between the new and existing pipelines along the alignments of Phases 1 and 2. This will facilitate flow divisions at each of these points via manually operated sluice gates. Under normal operating conditions a pre-determined flow position will balance flows between the two sewer systems,” says Prinsloo.

Another facet of the new line is the introduction of long radius bends to reduce frictional losses and minimize the hydraulic problems usually encountered at conventional in situ manholes. Smooth bends reduce potential blockages, and by so doing mimimise long-term maintenance requirements. 

“Long radius bends call for a contractor, such as Cerimele, which can build this type of line, as well as a pipe supplier who is willing to get involved with the design aspects. We were fortunate that Southern Pipeline Contractors were able to accommodate our design requirements with ease,” concluded Prinsloo.

A section of Phase 1 of the Moreletaspruit outfall sewer expansion programme clearly showing the long radius bends which have been incorporated to reduce frictional losses and minimize the hydraulic problems usually encountered at conventional insitu sewer manholes.

Owing to a lack of space or obstructions such as roads and rail tracks certain sections of the new Morelaetaspruit outfall sewer were routed underground using pipe-jacking. Most of them were minor operations, apart from a 275m section at Val de Grace, a suburb in eastern Pretoria. The picture shows workers at the rock face of the Val de Grace section where drilling and blasting took place eight to 20 metres underground.