Day 4: Los Caras Bridge

The Los Caras bridge links the centre of Bahia de Caraquez in the south with the town of San Vicente in the north spanning the long estuary that lies between them. With a total length of 2,000m the structure is divided into three parts, the two approaches and a central isolated span of 1,710m. 

The original design of the bridge did not consider seismic isolation. Some considered this feature unnecessary and expensive. But during construction, some savings were realised by letting the piles work by friction at around 50-75m of depth rather than driving them deeper to 120m to hit rock strata. When the resistance of the piles was confirmed to be sufficient, a part of the budget differential was spent installing friction pendulum seismic isolators for USD3m over a total cost of the bridge of USD102m. This 3% of additional investment delivered multiples in repair savings and maybe saved the bridge entirely.

Besides losing the USD102m infrastructure, failure of this critical facility would have resulted in significant increases in travel time between the north and the south of Manabi by about 2 hours. Resupplying north Manabi after the earthquake would have been a much harder task without this bridge in place. Therefore, the decision to ensure the safety of the bridge by implementing a seismic isolation system in its redesign was, in hindsight, fundamental.

The bridge did not suffer any visible damages. The neoprene covers of the isolators broke and there are clear signs that displacements reached an average of 30cm. The designed considered maximum displacements of 56cm so the performance was well within the design criteria, with the exception of Pier 12 where some evidence of settlement was found showing signs of lateral displacement (although not permanent). The soil around this particular pillar will be compacted and improved in the near future by driving piles around the pillar and the seismic isolators will be replaced.

The experience at Los Caras bridge needs to be an inspiration for reflection about seismic isolation in Ecuador and elsewhere. Rarely do we have an opportunity to realise the benefits of such a relatively small investment in such an evident context.

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