Construction World May 2015

PROJECT PROFILE

Installation of sheet piles with excavator mounted vibratory hammer.

Design innovation The design of this project is based on a 50 year design life. Pipes were designed with an addi- tional special 25 mm thick Calcium aluminate cements (CAC) layer inside the pipe which extends the life span of the pipe even more. CACs are cements consisting predominantly of hydraulic calcium aluminates which is a very hard material and can withstand severe chemical attacks. Pipes are designed to a very flat grade with 1:600 being the lowest and a tolerance of 25 mm are allowed for the instal- lation. The pipes are manufactured by the horizontal-spun method. Manholes are in-situ cast concrete structures and generally 4 m deep and are located every 90 m or less. Manholes are also designed with a 2 mm layer of Sika Epocem 720 applied to the inside of the concrete manhole which extends its life span by a further 25 to 30 years. The new pipe services an area of approx- imately 4 000 ha. Many years of planning and consultation with various services depart- ments, local residents and other concerned parties were done before the designs of the

wellpoints dewatering systems would also have been required. The open cut method would have had a huge impact on the environment giving the amount of soil to be handled and keeping in mind the close proximity of residential homes and other critical infrastructure like power lines, roads and railways. WithCSV’smethodof having the two sheet piled walls driven each side of the trench, the excavation is confined to an average width of only 4 m wide. Therefore CSV is handling less than a third of the amount of soil it would have with the open cut method resulting in a much happier environment. A 450 m section of the new pipe line runs about 1,5 m away from six 132 kV electrical

cables conveying 15% of Cape Town’s power supply. These are oil filled cables and are about 50 years old. Any vibration or move- ment of heavy machine close by the cables can cause a failure of the cables. Therefore CSV proposed the method of sheet pile installation by means of silent piling. The proposal was accepted by the engineers and client and the two machines were imported. Thus far 300 m of this section is completed successfully without any damage or negative impact to the cables or the surrounding resi- dential buildings. The silent piling equipment has a reduced impact on the environmen as it is far less noisy than the conventional construc- tion plant.

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Silent pilers busy with sheet pile installation.

system could be done/finalised. Environmental impact consideration

With excavations generally 6 m deep in clayish sand conditions and water table at 1,5 m below NGL (Natural Ground Level) like CSV is encountering on this project, the typical open cut method will result in an excavation width of minimum 20 m wide or more. The installation of continuous multi-tier

CONSTRUCTION WORLD MAY 2015