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Monday, July 12, 2010 |
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Geotechnical @ Kusile
:: News
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Geotechnical @ Kusile
Source: Don Layfield
The contract to install some 61km of piling was awarded to the Stefanutti Stocks-Franki Africa joint venture (SSF JV). The 4 160 augered piles vary in diameter from 800mm to 1200mm.
SSF-JV started at the Kusile site in Mpumalanga in January 2009 and the first pile was installed in mid March when access and drawings became available.
The geological interpretation on the site plan shows three different rock formations on the 600m long footprint of the power station:
1. Diabase with large core stones present in the weathered zones.
2. Rayton Shale formation.
3. Tillite & Shale, Dwyka Group (overlies Rayton Shale at depth).
The shale formation is lying at an angle of 60 degrees falling towards the Diabase, and severe water seepage is common in the contact zone.
“In the contact zone we have variable conditions with side wall collapses and a lot of water to contend with,” says Don Layfield, of Stefanutti Stocks. “Some of the piles are 25m deep, and a mere few metres away, only 12m deep.”
When there is water in the pile, a tremmie pipe is used to through which the concrete is pumped. As the concrete is pumped in, the water is forced to the surface and good concrete is left behind.
Due to the stringent settlement requirements and the obvious dangers associated with differential settlement on power stations, the preferred method of load transfer is end bearing and the quality control procedures to ensure load transfer is comprehensive.
Specially trained employees are lowered into the dry holes to clean up the loose material thereby ensuring 100% contact. To facilitate safe cleaning we installed a temporary steel casing to within 1m of the hole bottom to protect the cleaner. The cleaners further participated in a training program about working in confined space. Training includes testing for claustrophobia, safety measures that need to be in place before their descent and training in how to react should a crisis occur when they are underground. Each cleaner has a secondary winch attached to his harness should there be a problem with the crane. Before entering the hole the air is tested for dangerous gases and each cleaner carries a gas monitor with an audio alarm should the underground gas levels rise while he is working.
In Unit 6, where the sixth generator will be built, a high level of carbon monoxide in the trial holes made it impossible to clean the bottom of the pile by hand. The “base grouting” method is used here, which entails fixing four pipes into the pile while it is being installed. The bottom of the pile will be flushed with high-pressure water and once only clean water comes back through the pipes, grout will be pumped down to the bottom of the pile to fill the cavity formed by the water.
“The benefit of working in joint venture is that we are able to draw resources from two of the major players in the piling game,” says Don. We have a total of eight piling teams; six are working in various sections moving around as the priorities from the main contractor dictate.
A seventh team is carrying out lateral support, tension anchors and the ongoing tests required by the client and designer. The eighth team is still carrying on with ground investigations.
Weather patterns have changed drastically in the last few years and with the increased rainfall, working conditions have at times been difficult. “By mid March we had completed 2 250 piles,” says Don. “The work we are currently undertaking will be completed by the end of July. There is a strong possibility we will pile the Turbine and Air Cooled Conditioners in Unit 6, with another 1 000 piles, which would extend our stay until the end of the year.”
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