Abstract:
Foundations at Geneva are difficult and expensive due a large overlayer of clay and silt above competent
bearing material in the moraine. Common foundation schemes use deep barrettes or bored piles, first
passing large, weak layers of soil before finally reaching acceptable, solid moraine with reasonable values
of skin friction and end bearing resistance, thus requiring further deep penetration for the necessary length
for load transfer in the moraine. Depths of more than 60 m (200 ft) in Geneva are common in this region.
At the Onyx-Project Geneva Implenia Foundation Company also encountered these circumstances.
Despite usual solutions Implenia referred to an old and well known, but still intelligent and in today’s words
smart, efficient and economical solution. Using small shafts, passing the large layers of clay and silt, ending
with the pile toe in the underlaying moraine with a belled or underreamed base was the technic adopted to
save drilling time, reduce spoil and concrete, so finally reducing the CO2 footprint.
Piles with enlarged base (“belled†or “underreamedâ€) are reported at the beginning of the 20th century, for
example at Chicago, which has a quite similar geotechnical structure as Geneva with the need of deep
foundations. The first pile base enlargements were built in hand-dug caissons with wood lagging and steel
ring bracing. In the 50s and 60s piling works were more and more mechanized, driven by the development
of more powerful cranes and specialized drill rigs. Additionally the first tools for mechanized base
enlargement were invented around that period.
This report describes the project situation at Geneva, the drilling and enlargement technique of the pile
base, the verification of the design values with an in-situ bi-directional load test and the smart execution by
reduction of CO2 and costs.