The Engineering Marvel that Sits in Greece
The Rion-Antirion bridge is one of the world’s longest multi-span cable-stayed bridges and the longest of the fully suspended type. Crossing some of the most active earthquake fault lines in Europe, and with nothing solid at the bottom of the sea to build it on, the bridge seemed an impossible task at first. So, how did the engineers ever manage to build this 2,280m long bridge in the presence of such head-scratching challenges?
An Exhausting Detour!
The Gulf of Corinth slices deep into mainland Greece. On one side of the Gulf is Rion, and on the other is Antirion. However, the Gulf becomes very narrow at its western end. Crossing this small stretch of water hence became an extremely desirable alternative to an otherwise 280-mile detour around the mainland.
The Greece prime minister, Charilaos Trikoupis, had dreamed of a bridge to connect the two places, almost 100 years ago, but the wealth of Greece at that time did not permit its construction. However, the bridge plan was brought back to the table during the mid-1990s, and by August 2004, Charilaos’s dream was completely materialized, transfiguring a typical 45-minute ferry trip between Rion and Antirion to merely a 7-minute drive. The bridge transformed the lives of people of the northern and southern coasts, opening up a plethora of trade and travel opportunities from mainland Greece into the otherwise remote Peloponnese.
Without some unprecedented engineering, that bridge would still be a dream. Almost 3km in length, the bridge is long, but most incredibly, it is actively earthquake-proof. It can withstand shocks, measuring 7.4 on the Richter Scale — enough to annihilate an average bridge. Back in 2008, the bridge was hit by a tremor. The shaking ground had caused buildings to collapse less than 25miles away, killing two people and injuring many more. Amidst the chaos, the bridge survived unscathed, allowing emergency traffic to pass safely and quickly over the Gulf, but surprisingly shaking ground was not the first challenge earthquakes posed to the task of building the bridge.
What Lies Underneath?
Just getting started was a huge engineering problem, and the engineers first had to deal with what lay at the bottom of the sea. When they began, they did not know how they are going to lay the foundation of the bridge. The water depth of the sea at the Gulf of Corinth reaches about 65m, with the seabed comprising loose sand and silt for hundreds of meters down. With no solid bedrock, a solid foundation could not be designed. Solid foundations are, in fact, necessary to any structure, especially when they are shaking by frequent earthquakes. This is because combining a sandy seabed with seismic activity results in impending danger of ‘liquefaction’ from earthquakes. In a tremor, the soft, wet ground literally turns to liquid, which really is, as bad as it sounds. Nobody in the world had ever set out to build the bridge in these conditions. The engineers came up with a solution that seems to defy logic. But first why does a tremor turn sand and water into a liquid?
Lique-what?!
If you add an earthquake to loose, wet ground, something pretty strange can happen. The ground will hold the structure up fine before the earthquake, but as it shakes, the ground seemingly changes from solid to liquid, and then it becomes quicksand. We call this phenomenon liquefaction. When the sand particles are shaken, the spaces in between them (or the pores) try to get smaller and the water that is in there gets squashed and the pressure goes up. Once that happens, whatever is on top of the ground, sinks into it.
Learning from Flora
Ordinarily, engineers can drain the sand to get rid of the water or compact it to squeeze the water out. But one cannot drain the bottom of the sea, with a sandy layer extending to almost 500m thick. The designers hence came up with an ingenious solution, taking inspiration from the Vetiver grass.
This fragrant grass can grow up to 150cm (5 ft) high and have roots that grow downward 2–4m in depth. It is extremely effective in stabilizing and protecting soil against erosion, enabling the engineers to devise a similar solution. Similar to how the long, thin roots of the Vetiver grass holds the soil together and helps stabilize riverbanks and cliff edges across the world, from Africa to Fiji, beneath each pier, the seabed was reinforced and stabilized by driving 200 hollow steel pipes (piles) vertically into the ground. These piles help reinforce the upper layers of the soil strata that have to carry large seismic forces coming from structural inertia forces and hydrodynamic water pressures.
However, it is interesting to note two things: these 200 piles still do not reach the bedrock, and more surprisingly, the pier footings actually do not even rest on these piles. In fact, the top of the piles is almost a meter beneath the base of the pier. Yet, they are enough to prevent the sand underneath from liquidizing. Therefore, even though the four piers hold up the entire weight of the long bridge, the engineers had succeeded in keeping these piers from sinking in the ground during an earthquake.
One Problem After Another
However, the next challenge was to allow the piers to move laterally on the sandy seafloor, without their toppling over. In an earthquake, the earth moves vigorously from side to side and would take the gigantic piers with it, when shaking. These huge and heavy pier structures, weighing almost 171,000 tonnes, had to move freely and smoothly from side to side as well, but the problem was that, for a flat surface, when the structure begins to slide on top of the sand, the leading edge begins to dig in (or tow-in).
This towing in is because the leading edge puts more pressure on the sand beneath it until the pressure is more than what sand can support, causing the leading edge to dig in and the entire structure to topple over. To counteract this problem, the engineers decided to change the seabed with a layer of gravel. The pier footings hence were made to rest on a bed of gravel meticulously leveled to an even surface, allowing the piers to slide from side to side without any towing in. Gravels enough to cover two football pitches were used on the seabed, creating an almost 3m deep layer underneath. Without being affixed to the seabed to withstand the earthquakes, the piers of the bridge were designed to stand strong and long.
What Lies on Top?
The pending question now was what about the bridge deck. The deck, which has six lanes in total, is mammoth and if the engineer were to fix it to any of the piers, the deck could easily buckle or break once the piers start to move freely. The engineers had to build the deck in a way that allowed it to move independently from the piers. That is why the deck was designed to be fully suspended from the top of the bridge. As the piers move during an earthquake, the deck is able to swing independently. But had they completely tamed the impacts from the earthquake at this point?
Slow Down!
Allowing the deck to swing freely heralded good news but created its own problems. If the deck swung too far in either direction, there was a chance for it to smash in one of the four thin arms of the piers. This could fatally damage the bridge. The engineers could not allow that to happen. However, once an object as big as the road deck starts to move, it would require something extraordinary to stop it. The use of a viscous damper served as a promising solution to this problem.
Viscous dampers are powerful braking systems that use liquid to resist movement. Essentially, the damper turns some of the kinetic energy due to the movements into heat inside the damper, thereby controlling movements. Hence, the bridge is fitted with its own incredible liquid safety system. When an object, connected to the damper, starts to move, it makes the piston of the damper move through oil. The oil, being a viscous fluid, offers significant resistance to any movement, which ultimately enables the damping system to expend a significant chunk of that kinetic energy (from the object movements) into heat.
The viscous dampers in the Rion-Antirion bridge are the biggest in the world, and effectively control the movements of the deck. Therefore, slinging the road deck like a hammock allows the bridge deck to move independently, but the viscous dampers stop it from moving too much, preventing the deck from hitting the arms of the piers and shaking the bridge itself to bits.
The Rion-Antirion bridge, located in an area prone to strong seismic events, comprises a cable-stayed bridge, with the main bridge having four pylons and the longest suspended deck in the world. The main bridge is designed against earthquakes with 2000 years return period and a Peak Ground Acceleration of 0.48 g. The designers made this bridge uniquely earthquake-proof, with unprecedented solutions, which is how and why this engineering marvel sits proudly in Greece.
