Wet Ground Conditions Overcome
Published in The Civil Engineering Contractor in September 2006
A R200 million sewer contract is a sizable undertaking, especially when wet ground conditions pose many construction challenges. This is the case at the N2 Gateway Delft Bulk Sewer project, Robin Hayes learned.
Sobambisane Community Developments (of which ASLA Construction and Power Construction are members) was appointed the handle this design-and-construct project. The tender contract followed International Federation of Consulting Engineers (FIDIC) turnkey contract conditions. While a fixed price was provided for the job, the fast-track nature of the project meant that, from the outset, there would be extras and additions to be accommodated in the works. ASLA Construction and Power Construction have partnered before in other aspects of the Gateway project as contractors to Sobambisana Conmmunity Developments but not in joint venture. So, while the two companies knew each other, in this case extensive planning was required and frank discussions on strengths and weaknesses lead to apportioning of work and contract responsibilities. Once that was settled, a team approach was adopted to address the seven major contract activities, which included excavation and pipe laying, concrete work, materials store, workshops and training, among others. Extensive use is made of local and emerging contractors with training support provided to engineers from the City of Cape Town.
In addition to the appointed contracting team, Sobambisana Community Developments included consulting engineers Kewzi V3 and Bergstan in the tender process and Chand environmental consultants in association with Ecosense to supervise construction all of whom have permanent technical staff in site offices. Looking after the client’s interests is project management consultant Stewart Scott International.
Deviation from original plan
The new pipeline follows the route of the old fairly closely, through Delft and sections of the Kuilsriver flood plain, but that doesn’t make its construction wany easier, according to Tony Araujo, site agent for Asla/Power joint venture (responsible for the technical aspects of the project). “The route traverses existing services, protected wetlands, information settlements, private farm land and crosses the Kuilsriver twice, the R300 main road and the N2 Highway”, explained Araujo.
Ground conditions are typical of the Cape Flats: sand dunes, fine sandy soils with isolated areas of calcareous sandy material mixed with some clayey/calcareous boulders.
There have been a number of administrative and planning issues that have caused the construction programme to deviate from the original plan. Great urgency to complete the pipeline in time for occupation of the houses, which were scheduled for handover at 100 per week, led to the adoption of a fast-tracking approach, which culminated in a tight contract period of just 11 months. The contract period-commencing in December 2005 – is particularly challenging considering the high water table expected during the wet winter months. While the project had received approval from the Department of Water Affairs and Forestry (DWAF) technical approval and an environmental impact assessment (EIA) record of decision (ROD) was imminent, several other legal and administrative issues had not been resolved prior to the contractors establishing on site.
Examples included the removal of informal settlements by city officials and the granting of permission by land owners to cross their farms. These issues and a late but unsuccessful appeal lodged to the Ministry for Environment, Planning and Economic Developments for a change in the approved pipeline route also resulted in substantial re-programming of the construction schedule. Implications of the route change, upheld more than a month after contract commencement, meant the partial destruction of seasonal wetlands, which required permissions from DWAF in Pretoria. At the same time, concerns were being expressed by environmentalists and others over the impact of the Kuilsriver crossings and a special team was formed to examine the issues, comply with the dictates of the National Water Act and recommend a method of construction that lessened the environmental impact.
The team’s proposals were accepted by DWAFm which issued authorisation for the lower and upper Kuilsriver crossings during February and March 2006 but a decision on the seasonal wetlands would take a little longer. Three types of wetland were affected: imperate cylindrical, degraded and Renosterveld – each requiring a separate rehabilitation regime. To minimise impact, it was a condition that construction work had to take place during the dry season. The upshot was that method statements were devised and issued and an environmental control officer appointed to ensure compliance with the Construction Environmental Management Plan. DWAF in the Western Cape approved the measures in June 2006, allowing construction to proceed in sensitive areas. Notwithstanding these impacts, the flexibility of the contracting organisation accommodated these changes while maintaining in other less sensitive areas.
Why a new sewer?
The much-publicised and, at times controversial, N2 Gateway housing project, involves the construction of 22 000 housing units and was scheduled for completion in June 2006. Approximately 15 000 of these units were construction in Delft Symphony, Delft extensions 7, 8 and 9 and Driftsands. It was clear that the existing bulk interceptor sewer built many years ago, was inadequate for the increased flow. The capacity was often exceeded, especially in the wet season when the level of groundwater peaked above the level of the sewer. In such cases, stormwater and groundwater entered the pipelines through damaged couplings and cracks that had developed over the years. Sewer overflow was not uncommon causing a major health risk to communities and environmental concerns.
Under a Municipal Infrastructure Grant, the new sewer pipeline is under construction for the City of Cape Town by Sobambisana Community Developments, sub-contracted to the ASLA/Power joint venture. The pipeline has been designed to address capacity requirements not only for the Gateway project, but for the greater Tygerberg area. The project pipeline starts in Delft at Sumphony Wat and terminates at the Zandvliet wastewater treatment works – a pipe run of about 15,66km.
The project comprises two parts: Section 1 is known as the Delft Interceptor Sewer that runs from Hindle Road to Kuilsriver (designed by Bergstan) while the second section is known as the main sewer, commencing at Kuilsriver (Mfuleni), running along Old Faure Road, turning to run along Baden Powell Drive and then connecting into the Zandvliet wastewater treatment works, which was designed by Kwezi V2 Engineers.
Pipe-jacking at road crossings
A number of pipe-jackings under roads and services was specified and the work sub-contracted to specialist firm Escor, which was contracted to handle four crossings, according to Escor director Arthur Field. HIndle Road, the R300 and N2 are all road crossings between 50m to 70m in length and at a depth between 4m and 8m, he said, adding: Hindle was 1 350mm diameter and the other two were 1500 mm and 2 250 mm.
Further jacking took place under services adjacent to the R300 where the pipeline corsses under existing large-diameter water mains, a 750 mm-diameter sewer, bulk stormwater and power cables. “While the Hindle Road, N2 and services crossings were fairly straightforward, except for the problem of groundwater, the R300 presented a particular challenge as unidentified concrete structures were found directly in our path at a depth of more than 8m,: explained Field. “Our conventional vacuum pump dewatering techniques proved ineffective at these depths and well-point dewatering was employed,” he elaborated. “We opted for an additional jacking pit in the central median.” The underlying ground conditions and removal of the concrete structures has led to some unexpected subsidence on the R300, which, although stable, will form part of remedial work by the contractors during off-peak traffic conditions. Ironically, the contractors requested that the Department of Transport allow this crossing to be done by open-cut rather than jacking bu the requested was declined due to heavy traffic volumes on the R300.
