Olkaria is located immediately to the south of Lake Naivasha in the Great Rift Valley of Kenya. This area is geothermally active, and the geothermal energy is being used to generate clean electric power.
The geology in the area where the Olkaria geothermal power station is located is characterised by steep-sided domes formed from pyroclastic rock and lava flows. The domes enclose an approximately circular depression that has been cut by the Ol Njorowa Gorge, which was formed by out-flowing water from Lake Naivasha.
Franki was recently appointed to design, install and test the pile foundations for Unit 6, which is the latest addition to the ongoing expansion to the Olkaria I geothermal power station.
Olkaria I, is one in a series of six sister power stations in the area. Olkaria I, II, III and IV are currently producing power, while Olkaria V is still under construction, with construction of Olkaria VI having been planned for 2021.
Design and construct
This latest expansion project (Unit 6) broke ground in December 2018, and piling commenced in May 2019. Unit 6 will add another 83.3MW to the output capacity of Olkaria I, bringing the total output capacity of Olkaria I up to 274MW.
A comprehensive geotechnical investigation was done on the site, and the results of this investigation formed an integral part of the foundation design. The Franki team was commissioned to provide a "design and construct" solution for the foundations. The in-house design team was able to use this information to provide optimal foundation solutions to meet the stringent settlement criteria set down for the various structures.
The design included foundations for the turbine building, cooling tower, hot well, and scrubber areas. Initially, a combination of two soil improvement techniques, (dynamic compaction and rigid inclusions) and a piled foundation were considered. After careful analysis and taking differential settlement into account, the team finally concluded piled foundations for all the structures would be the most suitable. The ground conditions were conducive to CFA piling.
The 600mm diameter CFA piles were designed to withstand loads of up to 1,200kN, installed to depths of up to 15m.
Volcanically active area
The mix design is one of the most important factors to consider for CFA piling projects. As this area is volcanically active, the ground temperature is substantially higher, which resulted in accelerated setting of the concrete placed in the ground. Flash-setting, together with the requirement for full- length reinforcement cages and high-strength concrete due to the high sulphate content of the ground, created significant challenges in the execution of the project.
To ensure that the project progressed smoothly, the Franki site team worked closely with the main contractor, Civicon, to develop a mix design suitable for the site conditions. Careful planning of the site operations was also important to minimize the time between casting of the pile and cage insertion to reduce the possibility of flash-setting.
The on-site Franki operations teams were able to achieve high productions rates - an excellent achievement, considering the challenging and confined site conditions.
Stringent testing regime
The pile design and load-bearing capacity was further verified by a stringent testing regime, consisting of four static load tests and a pile echo test on all piles. The static load tests were done according to the test procedure outlined in ASTM D1143-81: 1994 where the piles are loaded in three cycles, i.e. 100 per cent, 150 per cent and finally 250 per cent of the working load. The piles performed well and all settlements observed were well within the project specifications.
"The excellent production and test results achieved reflect the importance of choosing the right pile type to suit the ground conditions," said Dr Nicol Chang, technical director at Franki Africa. "We have equipment strategically located in East African countries such as Kenya, Tanzania, Zambia, Mozambique and Mauritius, which allows us to tackle any geotechnical project in the East African region."
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