A number of geothermal power stations have been constructed in Kenya since 1985. Franki Africa (a Keller company) is currently involved in constructing the foundations for a further expansion at Olkaria, the largest geothermal power station of its kind in Africa.
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 Olkaria region is part of a large volcanic complex, which was formed some 25-30 million years ago and is located at the axis of the East African Rift situated in the floor of the Great Rift Valley. Violent subterranean forces tore apart the earth’s crust in this area and formed active and semi active volcanoes and lake basins along the rift valley.
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 were 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.
This impressive geothermal power plant facility is located within the famous Hell’s Gate National Park, surrounded by the magnificent scenery of Fisher’s Tower, Central Tower, Naivasha Lake and the Hell’s Gate Gorge. Abundant wildlife, such as zebra, buffalo, antelope, giraffe and baboons continue to graze idyllically in the area, particularly near grassy shores of Lake Naivasha.
Over recent years, the area has become a maze of pipes, which now form an integral part of Kenya’s geothermal power generation.
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 83.3 MW to the output capacity of Olkaria I, bringing the total output capacity of Olkaria I up to 274 MW.
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 valuable 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 the CFA (Continuous Flight Auger) piling method.
The 600 mm Ø CFA piles were designed to withstand loads of up to 1 200 kN, installed to depths of up to 15 m.
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 these particular 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 (PET) 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%, 150% and finally 250% 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,” comments 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.”
The Olkaria project endorses Franki’s commitment to supply the Sub-Saharan African region with quality geotechnical solutions.
Written by Roger Feldmann (Franki Business Development), October 2019