Subtitles section Play video Print subtitles When the Hong Kong-Zhuhai-Macau Bridge, or HZMB, was completed in January of 2018, it became the longest sea crossing bridge in the world. With a total length of 55 kilometers, the structure is impressive to behold, but the engineering behind it is even more incredible. The goal of the bridge was to connect the major population center of Hong Kong on one side of the Pearl River Delta to Macau and Zhuhai on the other. Normally to go between the cities requires driving around a large bay, and a more direct path could take two hours off the trip. Making such a long bridge required careful planning from the HZMB's designers. They had to build it to withstand the annual typhoon season, the occasional earthquake, and the ever-present sea. And their goal is to make it last for for 120 years. They also had to account for its location, cutting across five shipping navigation channels and the flight path of aircraft taking off and landing at Hong Kong International Airport. And finally, they had to consider the local marine life, like the chinese white dolphin, and the impact the construction and presence of the bridge would have on the ecology of the region. The first most obvious challenge when building over water is the fact that there's no land to build on. That's kind of the whole point of the bridge, so you need to start with a stable base. However the first solid ground you run into under water isn't usually ideal either. In this case, there were weak marine deposits that don't make for a solid foundation. To overcome this, builders drove long poles called piles down through the sediment. Piles used in the Hong Kong link road portion of the bridge could be as wide as 2.8 meters, and in a few cases were over 100 meters long to reach a stable foundation. With the piles in place, workers put a cap over them. HZMB designers placed many of these caps under the sea floor in order to minimize their impact on the flow of water. From the pile cap rises a single column called a pier, and spans with the road surface on them stretch from one pier to the next. To minimize the number of piers and reduce the ecological impact, long spans of up to 180 meters were used where applicable. These long spans are strong enough to support themselves in between piers, but they're still not long enough to leave a gap for the large vessels that use those five major shipping lanes into the bay. For those portions of the bridge, engineers had to switch up their design. Across three of the lanes, they used a cable-stay bridge design, where large towers are anchored with cables directly to the deck. The towers absorb the compression forces on the spans. For two of those navigation channels through, cable-stayed bridges were not an option. The tall towers would pose a risk to flights from the nearby airport. So the designers decided to take part of the bridge underwater. 6.7 kilometers of the HZMB's length is a tunnel, connected to the bridge with an artificial island on each end. This solved the air and sea traffic problems in one fell swoop, but posed another problem for the local marine life. The usual process of making artificial land starts with dredging out the muck on the seafloor and filling it with sand, then building a sea wall on top of that and finally filling in the area within the sea wall. But the process of dredging muddies the water with disturbed and dumped sediment. This suspended sediment can consist of organic material, nutrients, or contaminants that affect the ecosystem in positive and negative ways. When the dust settles, it can blanket and smother communities living on the seabed. Past projects that used dredging have played a part in the decline of the Chinese White Dolphin population in the area. To try and avoid harming the dolphin population further the builders used a completely different approach. First they installed a matrix of stone columns running down to the sturdier sand and clay. Then they drove hollow steel cylinders into the seabed. The steel cylinders were each 30 meters in diameter, and they were connected to form a continuous sea wall. After they were filled in, the rest of the island could be built, and the tunnel could be laid in 33 prefabricated sections from one island to the other before being buried. And that's the last piece of the puzzle. Add it all up, the piles of piles, the spans of spans, the cable-stayed sections, the artificial islands, the tunnel, and the link roads, and you get 55 kilometers of a record breaking bridge. For more engineering videos, subscribe! And check out my video here about another one of china's massive engineering projects. Fun fact, If you want to find the world's longest bridge, full stop, you don't have to go far. The Danyang-Kunshan Grand Bridge connects Shanghai to Nanjing and is nearly 165 kilometers long.Thanks for watching, and I'lll see next time on Seeker!