Science behind Rion - Antirion Bridge Construction and Design located in Greece

Cable stayed bridge in Greece

Need for the construction of Rion-Antirion bridge in the Gulf of Corinth.

The bridge unites two major roads which form a part of the European motorway. Before the bridge was built, It takes 45 mins from the ferry to cross the Gulf of Corinth. Now it only takes 5 mins that’s 9 times increase in speed. Every day around 10,000 vehicles travel on the bridge. Goods and Domestic travel become convenient to the people of Rion and Antirion.

Environmental challenges encountered by engineers.

1. No solid bedrock is found for hundreds of meters below the sea to construct foundations for the bridge. The soil is mainly composed of sand and silt.
2. The Gulf of Corinth is in the active seismic zones in all of Europe.
3. Winds at high speed gush through the Gulf.
4. The tectonic plates drift apart at a speed of around 1cm per year.

Engineering solutions to overcome the environmental difficulties.

Due to the absence of bedrock, The sea bed consisting of wet sand and silt on tremor undergoes Liquefaction. Since the location is an active seismic zone, There are high chances of tectonic plate movements or tremors. Liquefaction occurs when wet sand loses its strength and stiffness in response to shaking such as earthquakes. The soil behaves like a liquid that can sink the bridge.

To save the bridge from Liquefaction, Engineers came up with an innovative solution inspired by vetiver grass. Vetiver grass grows in the swamps of India. The volatile oil found in the roots is used as scent in incense sticks. The roots can grow up to 7 meters long straight into the ground holding the soil around hence preventing liquefaction. Engineers recreated this idea by placing 200 hollow rods beneath the soil of 25 meters long and 2 meters in diameter to stiffen the soil. There exists another problem in regards to laying the foundation. The tectonic plates move in all directions during earthquakes creating an unavoidable condition for the bridge to move freely on the seabed. If the bridge undergoes resistance offered by sand and silt, It might topple, Which is catastrophic. To attain the condition for the bridge to move freely side to side on the sea bed, Engineers used gravel layer of 4 to 7 meters deep to prevent the bridge from toppling during tectonic plate movements. When tremors occur, The edges of the bridge get accumulated by sand that causes the bridge to topple. By increasing the particle size, The bridge can slide over gravel smoothly when compared to sand, Hence a perfect solution to overcome toppling. Now that the bridge cannot sink as well topple.

Foundation and soil reinforcement with inclusions and gravel layer.

Well, this isn’t the end of hurdles. As of now, The bridge pillars can move freely sliding on the sea bed. Road deck of length 2883 meters cannot rest on the bridge foundation as it moves during tremors. The movement can cause breakage and rupture on the road deck. The engineers desired to construct the road decks independently of the foundations. They resolved this issue by using the principle of hammocks (a bed made of canvas or rope mesh suspended from two supports by cords at both ends). Sailors use hammocks as a bed to oscillate sideways that overcomes trembling by large waves and storms without disturbing sleep. Therefore the road deck is suspended by cables like a hammock and attached to the pillars but does not rest entirely on them. Now, as the pillars move in an earthquake the road deck swings independently. But when the tremors become more violent, The road deck could hit the pillars which support cables and break them. To avoid such severe swinging of the road deck, engineers used viscous dampers. It is used to stop the road deck from moving too much by offering resistance hence saving the entire structure of the bridge. These are frequently used in mechanical systems.

The road deck is suspended by cables and does not rest entirely on the pillars.

Here’s a catch, How do the viscous dampers know whether the road decks are swinging destructively or not?. Thanks to the predictive failure technique used in the fire sprinkler system. The sole purpose of the system is to detect fire and release water or otherwise sprinkle water. But how does it detect fire?. Well, there is a glass capsule connected to the water pipe containing volatile liquid whose boiling point is 68 degrees. Due to fire, the liquid reaches its boiling point, and the pressure inside the capsule increases, thus breaking the capsule and releasing water. The same mechanism is followed in viscous dampers. Inside every damper, there is a fuse that is designed to break at a certain limit allowing the viscous dampers to function only during earthquakes .

Viscous dampers with fuse inside laterally supporting the road deck.

The fan-like structures or cables hold the road deck allowing it to swing on either side. Due to an aerodynamic effect called vortex shedding, the cables tremble and the material might fatigue, which is catastrophic. Due to vortex shedding, low-pressure points are formed behind the cables causing them to vibrate. To minimize this effect, engineers used helical strakes, which change the direction of air along its helical path stabilizing the cables. The bridge has the largest expansion joints in the world, allowing for the two coasts to drift five meters away from each other.

Helical strakes on cables minimizing the effect of vortex shedding

Rion-Antirion bridge is one of the engineering marvels in history. It stands as an icon for upcoming engineers. The bridge would have been impossible without soil reinforcement, gravel layer, viscous dampers, fuse, and helical strakes. All thanks to science and hardworking engineers who came up with incredible solutions to achieve something remarkable.