TECHNOLOGY: WSP Cape's Innovative Solution
Recent Western Cape Business News
EARLY in 2009 WSP Structures Africa’s Cape Town branch was appointed by Australian construction company Civiltec to produce an efficient design for a series of five labyrinth Fusegates for the upgrade of the Little Para dam spillway near Adelaide South Australia.
Fusegates are a spillway control system patented by Hydroplus, a division of VINCI Construction in France and can be used to increase spillway discharge potential and/or storage capacity of dams. At Little Para they are used on a new auxiliary spillway to increase overall discharge potential for extreme floods.
“WSP’s role was to optimise the design of the Fusegates to enable the Hydroplus system to be chosen by South Australia Water over other systems, based on cost, carbon footprint and time economies. Such Fusegates have traditionally been constructed using heavy cast insitu concrete works with large costs and risks in terms of materials and installation time on site. Thick structural steel plate with stiffeners have also been used,” says Andrew Hakin, director of WSP’s Cape Town branch.
Several design options were considered, utilizing precast designs and offsite fabrication. The design option chosen was to utilize precast concrete construction off site for the construction of the reinforced concrete base chambers and an innovative stainless steel sandwich panel construction for the 6m high labyrinth shaped Fusegate walls.
The five base chambers were each precast in two halves. They were then transported to site, accurately positioned onto the spillway and each chamber made whole utilizing an insitu concrete stitch. The benefits of accurate individual fabrication and excellent quality control having been achieved through the precasting taking place in the precast yard.
The Fusegate walls were designed utilizing a new type of duplex stainless steel (LDX 2101) in 4mm thick plate to form the inner and outer faces of a 150mm thick sandwich panel. Each face was fabricated with stiffening ribs and the large 6m high sections of each labyrinth Fusegate were linked together in the workshop using a specially designed system of overlapping tabs, extending from the ribs of each panel face, through which drop in rods were located to secure the two faces together. The sections were transported to site and welded to the base chamber’s cast-in bearer plate and to each other to form the five separate Fusegates.
The hollow sandwich panels were then filled with a flowable low shrinkage grout mix on site to form the finished product. The thin wall sandwich panel design can be simply considered to operate structurally as a three dimensional composite beam. The stainless steel face plates act as the flanges of the beam whilst shear forces are transferred diagonally through the grout in compression and through the overlapped ribs with their drop in rods, in tension.
Lean duplex stainless steel (LDX 2101) was chosen for the project and supplied by Sandvik Materials Technology. This steel was selected for the following reasons:
• It provided similar corrosion resistance to grade 316 stainless steel
• It has a higher tensile strength that grade 316 stainless steel
• It was much more cost effective than other forms of stainless steel
• LDX is manufactured using approximately 65 per cent recycled material thus providing a large contribution to the extremely low carbon footprint of the project. Approximately 70 tonnes of LDX material was used in the sandwich panel which is estimated to be less than half that which would have been used in a traditional single plate construction. This further improved the low carbon design performance in terms of minimised material usage.
The combination of new technology, engineering innovation and lateral thinking applied to the safety upgrade of the Little Para Dam in South Australia by specialist construction firm CivilTEC and WSP Structures Africa (Pty) Ltd specialist consulting firm constitutes a significant step change in the design approach to the Fusegate System.
New ground was broken by fabricating the composite walls for the Fusegates buckets off-site in a fabrication yard, using a purpose-developed connection system.
The stainless steel/concrete composite construction was chosen ahead of the more usual concrete walls – in order to meet the cost saving, program and environmental targets of the main client, South Australian Water Corporation (SA Water).
SA Water wanted the Fusegates to have a 100 year maintenance-free lifecycle – a requirement that was met by choosing a duplex stainless steel for the structural elements.
On site fabrication is due for completion on site next month.
Principle : SA Water
Main contractor : Hydroplus Australia
Sub contractor : Civiltec Pty Ltd
Design consultant : GHD Pty Ltd
Structural designer : WSP Structures Africa (Pty) Ltd (Cape Town)
Steel fabricator : LWA Engineering
Steel manufacturers : Sandvik and Outokumpu
Precast concrete : Bianco Pty Ltd
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