GÚodynamique et Structure was contracted by Gefyra to provide conceptual foundation design for the piers of the Rion-Antirion Bridge, in the Gulf of Corinth, Greece.
With a length of 2,252m, it is the longest cable-stayed bridge in the world. The foundations presented particular challenges.
The water depth at the pier sites is about 65m and the soils consist of weak clays, silts and sands to a depth of several hundred metres. In addition to substantial self-weight loading, the bridge is required to sustain wind speeds of 250km/h, impact from a 180,000t tanker and earthquakes of up to seven on the Richter scale.
The foundation concept to accommodate the seismic loading was to permit limited permanent displacements.
The bridge could sustain lateral displacements of its piers of a few tens of centimetres. However, tilt of the piers had to be constrained to less than 0.1%.
To deliver this performance, a pier with a base diameter of 90m was designed.
Sliding was to be permitted under the extreme seismic loading condition between the base of the pier and the foundation soils. Foundation design was developed with the aid of numerical analysis (using macro-elements) and centrifuge model testing.
To restric t the seismic displacements to the sliding mode, potential seismic bearing capacity failure had to be avoided. This required strengthening of the weak surface soils with groups of driven tubular steel piles (Figure 5).
Design was optimised with piles at 7m centres driven to a depth of 25-30 m. A gravel mattress was provided to cap the pile group and provide predictable performance for the sliding interface.
Figure 6 shows a section through the 3D moment - horizontal force - vertical force stability envelope for the foundations.
The inner surface shows the capacity that could have been mobilised without strengthening of the soils. The outer surface shows the substantial increase in capacity provided by the piles. Permitting sliding at the base of the piers limited the peak lateral demand on the strengthened foundation.