43 / 68
41
CONSTRUCTION WORLD
MAY
2015
PROJECT PROFILE
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
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
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.
Installation of sheet piles with excavator
mounted vibratory hammer.
Silent pilers busy with sheet pile installation.