JAES Learning

The longest sea bridge in the world, the HZMB, suddenly disappears into the sea. Welcome to a modern underwater tunnel. Such tunnels are built using ingenious techniques that join different concrete elements. How do the joints remain leak-proof under tremendous hydrostatic pressure? This experiment provides the answer. I have filled the meeting region of the tunnel elements with oil. I can easily separate them out, oops. Now, let’s remove some oil using this syringe. Wow, the elements are bonded with incredible strength. The same theory is applied under the ocean for these gigantic concrete elements. Observe very carefully; very soon, a water chamber is going to form between them. First, pulling jacks are installed between the new and the previous element. An external positioning system slightly lifts the new tunnel element. The pulling jacks are then activated, and the elements close. Now, you can see that some water is trapped between the two elements, forming a water cavity. Similar to the experiment, by simply pumping out the water, the negative pressure inside the cavity will automatically force the elements to bond more firmly. The geometry of these gigantic concrete elements is sophisticated and interesting. How are they fabricated? How are they transported? Let’s visit a concrete casting facility in the harbor. This area can be a purpose-built basin, or an existing dry dock is prepared near the water. They use reusable formwork, and a huge amount of scaffolding and support is needed to support these forms. Some forms are even hydraulically powered. Once the rebar and form arrangement is complete, the concrete is poured into the cavity. Once the concrete is cured, the forms are removed. The length of the section just made is around 20 meters. However, each tunnel element should be 100 to 200 meters long. The trick is to move this small element forward on tracks, and the next casting is done in the same location. In this way, the final tunnel element length is achieved in eight to 10 phases. You can see they have built huge tanks inside the concrete element. We will understand their purpose later. Once the concrete has cured, each end of the tunnel element is sealed with a watertight, temporary bulkhead. When the tunnel elements are ready, the basin is flooded with water from the adjacent waterway. At the same time, the ballast tanks inside are also partially filled. The partially filled water tanks help stabilize the elements and prevent irregular movements. These bulkheads ensure the inside of the tunnel element remains dry and air-filled. The concrete segment is designed so that when it is airtight, it floats on water—a clever use of the principle of buoyancy. Once afloat, the massive tunnel elements are towed out of the basin by tugboats. The element’s top surface is just 30 cm above the water's surface. The elements can weigh tens of thousands of tons. The elements are now towed to the tunnel site. To place these tunnel elements, the seabed has to be ready. The seabed needs to be excavated into a perfect trapezoidal shape with a well-prepared foundation. How do the engineers achieve this in the deep ocean? These huge dredges are performing the most crucial job of the tunneling project. They are preparing the seabed for the placement of the heavy tunnel elements. Clamshell dredgers are most commonly used for this purpose. They remove sediment and prepare the seabed in a trapezoidal shape. A layer of granular material, such as gravel, is often placed in the trench to create a smooth, level, and firm surface for the tunnel to rest on. The tunnel elements have reached the trench area. Large pontoons are waiting for them. Now, the handover of the element from the tugboats to the pontoons takes place. Before the immersion of the element, the pontoons must be placed exactly above the trench area. Look at this complex web of mooring ropes attached to the pontoons and the element. They are anchored to the seabed. We have made a simplified web of cables to control the position of this floating object. This is interesting, just by pulling these cables, I will be able to move this object in X direction, to control the Y direction just pull these two ropes. By pulling these two ropes you can control the yaw of the object. With this clever mooring rope control, the element is carefully positioned over its final resting place. The pontoons receive position signals from the GPS and robotic total stations. Now, can they simply fill the ballast tanks again and immerse the tunnel element? You can easily guess what will happen here. You may keep the tunnel element exactly above the resting area and drop it, but the reality is that it will never reach the target. Inside the water, objects move in a crazy way. What we need is a controlled immersion. To immerse the element with control, we again use a clever cable arrangement. Powerful hydraulic strand jacks support the element. Even if the ballast tanks are filled, the strand jacks will decide when to release the element. The clever cable arrangement again guides the position of the element. You may observe some measurement towers on the element.