Why China Split a Mountain in Half to Build a Bridge

This is the highest bridge on the planet. At 625 metres above the Huajiang Canyon in China, you'd have to climb to the top of this country's tallest building to get this far off the ground. Construction crews faced unstable mountain terrain and powerful winds in the race to open the structure earlier this year. But it's not only that it's dizzyingly high up. This structure features some of the most impressive bridge engineering I've ever seen. There's a reason that the phrase no three feet of level ground is a saying synonymous with the locals in Guizhou. More than 90% of the province where the Huajiang Canyon Bridge is located is covered in mountains and hills. It's also stunningly beautiful. Described as the Switzerland of China, it's recognised worldwide as a travel hotspot because of its cultural offering and stunning views. But that doesn't mean it's always been easy to get to. Historically, Guizhou has been something of an isolated island and while accessing it was difficult, moving around it was even harder. And that's why the Chinese government are spending billions to try and better connect this region with the rest of the country and, indeed, the wider world. In the late 90s, China opened the Guiyang Longdongbo International Airport and a high-speed rail line and expressways now run through Guizhou. But the nation's mountain mission trudges on. To enable travel through this area, Guizhou is now home to a crazy 32,000 bridges, providing direct routes through the challenging landscape. And the most impressive one of all opened earlier this year. The world's highest bridge is now open in China. Almost 200 metres higher than the Empire State Building. It will reportedly slash commute times across a canyon. China has now officially opened the highest bridge in the world. Take a boat ride on the Beipan River and you'll start to understand the scale of this incredible piece of engineering. Huajing Bridge stretches across the Huajing Canyon, hundreds of metres above the ground. Originally, to get from one side to the other, you'd be driving around winding mountain roads for up to two hours. Now, it takes just two minutes. Now, the bridge forms a key part of the Guizhou S57, which is basically a 150 kilometre highway that slices right through the heart of a mountain. And I mean that very literally. A mountain has indeed been cut in half to allow for the construction of a road through this region. Using explosives and massive drills, engineers created a V-shaped cut in the rock. The expressway connects the Liuzhou Special District and Anlong County via 14 interchanges to make driving here somewhat manageable. So, raise your hand if you're surprised to see China splitting a mountain in half to build a highway, only to connect that highway to the world's highest bridge a bit further down the road. Never mind the engineers actually working on this stuff. It takes me a huge amount of brainpower just to make the videos, to work out what the hell they're doing and how the hell they pulled it off. Luckily, I keep myself sharp using our video partner, Brilliant. Their series on mathematics is the perfect antidote for keeping up with the sheer number of calculations made by engineers for a bridge of this scale. But don't worry, their courses only focus on the most useful and applicable maths concepts, building up over time so you never feel like your efforts are being wasted. In short, I can honestly tell you that Brilliant makes you a better thinker and problem solver thanks to their thousands of lessons in maths, data analysis, programming, science and AI. To learn for free on Brilliant, head to brilliant.org forward slash the B1M, scan the QR code on screen or click the link in the description. They've also given our viewers 20% off an annual premium subscription, which gives you unlimited daily access to everything on Brilliant. Now, let's get back to that mountain of a project. Huajiang Bridge is the giant puzzle piece that connects the highway on both sides of the canyon. Engineers studied the area 5 to 10 kilometres up and downstream to find the right location. Now, that involves a number of considerations like finding the right soil conditions to stabilise the towers, navigating the incredible height of those canyon walls and of course, the width of the gap that you're trying to bridge across. And with all of that considered, it so happened that what was needed was a record-breaking bridge. A fun coincidence that China seems to keep running into. This is where they landed, with the bridge stretching 2,890 metres with a main span of 1,420 metres. Now, there were a couple of obvious design options for the architects to consider. Cable staid or suspension. A cable staid bridge looks like this. You need to very tall towers with cables that come down to support the bridge deck. The more practical design for long-span bridges, and what was agreed for Huajiang, is a steel truss suspension bridge. It's a well-proven system in rocky, mountainous terrain like this as the support cables can be anchored into hard rock, offering incredible stability. Before construction could start, engineers used a virtual pre-assembly technique to map out how the structure would come together, a bit like a practice run. The steel girder structure consists of 93 truss sections featuring cords and deck panels. Altogether, it weighs 22,000 tonnes. Now, bear in mind that the steel used for the Eiffel Tower weighs 7,300 tonnes, so the Huajiang bridge is three times heavier. And all of that material is being manoeuvred at a ridiculous height. And while steel is incredibly sturdy, a 625-metre drop onto rock might just challenge its structural integrity, not to mention the material cost. So, accuracy is key. The biggest crane in the world is Big Carl, a 250-metre piece of kit built by the Belgian company Sarans. You'd need three Big Carls stacked on top of each other to hover above the top of this canyon. And while he wouldn't put it past China to try and go after that record too, they've actually gone for a more intricate solution. What we have here is using what's called a cable crane, which is an overhead system. So, it's like an overhead cable runway, where the units are delivered to the ends, where you've already built the abutments and concrete foundations and so on. And then you lift it up and it travels along, almost like a monorail, but it's a cable, not a rail. And then you can deliver it to where it needs to be and then you attach it to the cable system. It's an incredible answer to what is the ultimate test in engineering at this height. Those steel girder pieces are placed in the middle and then the bridge is built outwards from there. And you do it like that to balance the structure and to avoid placing undue pressure on one single support tower. The steel sections are then slotted correctly into place using the Chinese equivalent of GPS called the Beidou Navigation Satellite System. Now, we can't forget that by nature this canyon has sloped sides which are trickier to build on than a flat surface. Just look at what happened to the Hongqi Bridge in Sichuan recently when cracks appeared on the nearby roads and slopes. Given the uneven terrain, configuring structural foundations to get even distribution is a real challenge. And that's why the towers are actually different heights. The north tower stands 262 metres tall while the south tower is 205 metres tall. And the cables are stabilised using an asymmetrical anchorage design. The mountainside is anything but symmetrical. And so your two cables at the back coming down from the tower tops, coming down to ground where the anchorages are, will very often meet the ground at completely different levels or different places. So the asymmetry will be in the anchorages. Some might be deeper than the others, depending again on the local geology, exactly where the cable meets. Now, the landscape is the most obvious hurdle to overcome with the Huajiang Bridge. But what about when that landscape moves? While not known for facing the biggest earthquakes on the planet, Guizhou does experience some seismic activity. It's important that any bridge like this is ready for that eventuality and, as Ian explains, it was a key consideration. A bridge like this, a long span bridge, of course you've got the weight of the bridge deck which is hanging there like a sort of glorified pendulum and an earthquake comes along and it's going to try and move. Sometimes we use hydraulic devices, big sort of shock absorbers, if you like, at each end so that they kind of absorb the movement and slow it down but still allow the bridge to move under thermal change, for example, temperature change. That's very common. Then we have one of the biggest challenges of all. Whenever you build major infrastructure, the wind is right near the top of the list of challenges to overcome, not least when designing a bridge and especially one standing this high above a massive canyon. The geographic formation of a canyon has a huge influence on wind patterns through something called the Venturi effect, named after the Italian physicist Giovanni Battista Venturi, and he found that when wind passes through a narrow passage its speed increases and its pressure drops. It's often experienced between tall buildings in cities and, of course, in a canyon like this where there are narrow sections where wind gets trapped, its behaviour can be pretty volatile. Gusts are reported to reach staggering speeds compared to an express train. Now, regular viewers of the B1M are going to be familiar with the testing method. A certain amount can be done with a computer but ultimately you really need to head to a facility with a wind tunnel. There, a scaled down model of the bridge and its surrounding elements are subjected to recreated wind patterns to see how they perform. And while that might sound familiar, you might have heard of that bit before, what you may not have heard of is another system used on this bridge, Doppler LiDAR. Doppler is a system which we we know from most familiarly when a siren goes past you, you know, the pitch changes and that's just the frequency shift when something's moving. LiDAR is a way, it's a laser system for representing in three-dimensional space surfaces or voids, objects and so what this is doing is using a sort of scatter of laser beams into the clouds, into the air and measuring the frequency shift, the Doppler bit, measuring the frequency shift of the reflected signal. The signal is reflecting off moisture particles in the air so what that's doing is building up a three-dimensional image of the moving moisture particles in the sky and that gives you a picture of what the wind is doing. It was crucial to account for the geological conditions, seismic events and the wind. But just because those conditions were factored in at design stage doesn't mean monitoring ends when the bridge opens. As strong as the cables are that support the great weight of the steel, they're under tonnes of strain. Engineers keep an eye on them by attaching fibre-optic strands into the cables themselves. As the metal wires expand under increased load, the fibre-optic strand expands too. Then, by passing light down the fibre-optic system, you can measure reflections and the extension using a fibre-optic strain gauge. As you can imagine, it's crucial to keep an eye on the tension. After all, the cables are the most critical aspect of a suspension bridge. They're usually made using pre-formed parallel wired strands where stacks of wires are bundled together and then collated into one big hole. And then, to connect all of the upper elements you need to have a serious stomach for heights. A catwalk sits on top of the structure to allow workers to move about. Rather them than me. When you look along the river, you can see where the heights of engineering used to peak. Now we have extraordinary structures like the Huajiang Bridge, costing up to $300 million and built in just three and a half years. But while it is a record breaker, and that would usually come with a certain level of acclaim, this is just what China does. The country has more high-speed rail than anywhere else on the planet. It has more bridges and a higher number of dams. But what's happening in Guizhou is about more than just setting records. Since 2012, the nation has been spending hundreds of billions to develop infrastructure projects in its poorest regions. It's all part of China's War on Poverty to better connect and develop revenue generation outside of the key city hubs like Beijing and Shanghai. Although it's worth pointing out that Guizhou is part of China's Belt and Road Initiative, which is a strategy launched by the nation to create economic corridors around the world. China's keen to show how developed each section of the Belt and Road route has become. And that brings us back to why the Huajiang Bridge is a whole lot more than what it seems. This thing was first built with you, me and the rest of the world in mind. Like bears to honey, China's hoping that Huajiang will become a tourism trap. The bridge features an 800-metre glass elevator that takes visitors up to a two-storey coffee shop in one of its support towers. On the underside of the car deck, a glass walkway lets you look down to the canyon if you dare. There's even an artificial waterfall that spans hundreds of metres across the bridge. But aside from engineering fans and coffee lovers, China wants Huajiang to become a hub for extreme sports. There's a bungee jump here so that adrenaline seekers can fling themselves from a platform on the world's highest bridge. Plans are being formulated for professional base jumping events and there's even talk of a centre for extreme sports in the canyon below. You see, Huajiang Bridge is an amazing spectacle of modern engineering. It's testament to the development of this industry that we can construct infrastructure like this even in the most inhospitable of environments. Hundreds of millions and even billions of dollars are spent creating travel routes that generate money from visitors far and wide. Bridges like the one in Huajiang are literal sightseeing destinations in their own right. This video was sponsored by Brilliant. Don't forget to go and check them out the link below because it really helps support the B1M when you take the time to go and have a look at our video sponsors. And as always guys, if you enjoyed this video and you want to get more from the definitive video channel for construction, make sure you're subscribed to the B1M.