Construction World May 2015

PROJECT PROFILE

CONSTRUCTION of the NORTHERN AREAS SEWER

The Northern Areas Sewer (NAS) is a major collector sewer currently serving an area of approximately 4 100 ha of the northern suburbs of Cape Town. The total future catchment area

to be serviced by the NAS is approximately 4 600 ha of the bigger Cape Town metropole.

Main components of the silent piler.

to the system. Specially designed braces are used to brace the upper part of the shoring system, spanning the width of the trench between the sheetpiles. CSV also acquired two Japanese manu- factured Giken Silent Piling machines for this project. These machines are used to install Nippon Type FSP IV Sheetpiles, the same as for the vibrating hammers, but can also be used for other types of sheet pile installations. The silent piler makes use of a press-in method to install piles. The silent piling technology is applied where the 7 m deep trenching is carried out as close as 1,5 m from existing sensitive high voltage cables that could not be exposed to any vibrations or movement. Corporate social investment Although this project mostly involves special- ised work and is more machine orientated, 10% of CSV’s labour is sourced from the local ward council’s job-seekers database. Local businesses and service providers such as fuel stations, hardware stores, plant-hire companies, construction supply companies and other local companies are well supported due to the close proximity of industrial areas to the project and we are also saving on fuel cost by doing local business. With the employment of about 130 workers on the project the local take aways and grocery stores also benefit a lot from this project due to its close proximity to their business. A section of the project traverses the grounds of a technical college with whom CSV has built up a very good relation- ship. The company is currently setting up interviews with civil/building technology students from this college for employment as site foremen and supervisors. The aim is to employ 10 students from this college.

pipe jacks were done during this contract with the pipe diameter at these sections varying from 1 470 mm to 1 680 mm and the length of the jacks varying from 22 m to 84 m. Horizontal directional drilling technology was also implemented at one crossing which consisted of a 630 mm diam- eter HDPE pipe 60 m long. A section of the existing NAS, a 762 mm diameter concrete pipe with an approximate length of 1,2 km will be rehabilitated using ‘cured in place pipe’ (CIPP) technology. This rehabilitation work comprises cleaning the sewer, isolating the sewer and over-pumping the existing wastewater, CCTV inspections and the installation of the new pipe liner. Curing of the resins is achieved with the recycling of heated water. Construction innovation technology CSV Construction has specifically invested in technologically advanced shoring equip- ment for this contract which includes two High Frequency Excavator Mounted Vibrating Sheetpile Hammers fitted to 30 ton Hydraulic Excavators and it also imported a PTC high frequency Excavator Mounted vibrating pile driver. The steel sheetpiles that were imported from Asia are of the Nippon Type FSP IV Sheetpiles and the ones being used on this specific project are 7,5 m long. These sheet- piles are interlocking resulting in a solid wall capable of deflection and providing a very effective water cut-off which is much needed with the ground water level being generally 1,5 m below NGL but in places as high as 500 mm above NGL. Sheetpiles are driven so that the bottom tips (toes) of the piles are 1,5 m below the trench floor level. This is to provide adequate toe stability

The project entails the replace- ment of the original NAS using concrete pipes with a Calcium Aluminous Cement (CAC) sacrifi-

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cial layer, including construction of the asso- ciated manholes. The total length of the NAS Phase II is approximately 5,2 km. The three pipe sizes used for the NAS are 1 650 mm, 1 500 mm and 1 350 mm nominal diameter respectively. The pipes are 2,44 m in length. The depth of installation of the NAS Phase II typically varies between 4 m and 7 m and in terms of bedding preparation, trenches are dug up to a depth of 8 m. Having to deal with a water table as high as 500 mm above ground, adds to the complexity of the contract. In addition to the construction of the new outfall sewer, manholes are constructed at approximately 90 m intervals, and also at changes to the vertical and horizontal align- ment of the sewer, to allow for chambers (with benching) constructed to terminate below ground level with access chimneys constructed to ground level making use of precast concrete manhole rings. Construction of the sewer at major road and railway crossings are carried out by means of pipe jacking. Two of the pipe jacks cross some of Metrorail’s main railway lines and no disruption to commuter services were allowed while these sections of the construction were done. Strapping and extra support structures had to be designed and installed to prevent any subsidence of the rail tracks and measures were put in place to monitor this in conjunction with the fluctuating water table levels which had to be maintained. A total of eight

CONSTRUCTION WORLD MAY 2015