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“I have spread my dreams under your feet”*…

Brunel’s tunnel shield. The isometric on the right shows an individual frame of the shield

From Brian Potter, a fascinating look at the history of a technology, tunnel boring, the products of which we tend to take for granted…

Tunneling is an important technology for modern civilization, as a tunnel is often the only reasonable way to create a direct path between two points. When the Hoosac tunnel was completed in 1875, it turned a difficult, 20-mile railroad route along “precipitous grades” into a direct 5 mile route, connecting Boston with the Upper Hudson Valley. Large infrastructure projects such as hydroelectric dams often require tunnels to function. The Hoover Dam required more than 3 miles of tunnels 56 feet in diameter to divert the Colorado River around the construction site. And a tunnel can be used to create new land beneath dense urban areas, making it possible to build large-scale horizontal infrastructure like sewers or mass transit that wouldn’t be feasible to build above ground.

A common way of building a tunnel today is with a tunnel boring machine (TBM), particularly in urban areas where other construction methods such as drill-and-blast or cut-and-cover would be too disruptive. Of the 89 transit projects around the world that required tunneling in a dataset compiled by Britain Remade, 80 of them used TBMs. But tunnel boring machines are a comparatively modern construction technology. The first successful rock tunneling machines weren’t invented until the 1950s, and into the late 1960s most tunneling was done using other construction methods. But as TBMs have improved, they have increasingly been the method of choice for tunneling through a wider variety of ground conditions. And while many construction tasks have resisted automation and mechanization, tunneling machinery has steadily gotten more automated, to the point where a modern TBM is akin to a mobile factory that burrows through the earth and constructs a tunnel behind it.

Soft ground TBMs evolved from unmechanized tunnel shields. The first tunnel shield [see illustration above] was designed by Marc Brunel, father of famous engineer Isambard Kingdom Brunel and an accomplished engineer in his own right, and built by Henry Maudslay for tunneling under the Thames in 1825. Brunel’s shield, which was inspired by the action of shipworms boring through the wood hulls of ships, consisted of a large cast-iron structure, 38 feet wide by 22 feet tall, which was broken into 12 separate “frames,” each consisting of three individual compartments stacked on top of another. Within each compartment was a series of horizontal boards, called “poling boards,” that were placed against the face of the tunnel. A worker in the compartment would remove a board, dig out the earth behind it to a depth of around 6 inches, and then proceed to the next board. Once all the soil behind the boards in a frame had been dug out, that section of the shield would be advanced forward using screw jacks, and the process would repeat. Behind the shield, masons would construct the brick lining around the sides of the tunnel, which prevented the tunnel from collapsing and provided a structure for the shield to push off against.

When the Thames Tunnel was completed, it was the first tunnel under a body of water in the world. But the project proved to be incredibly difficult, encountering “almost overwhelming problems” (West p115). Excavation was slow, advancing at around 8 feet per week on average, and the tunnel flooded repeatedly. Gas occasionally filled the tunnel, which caused “collapse and blindness of the workmen” (West p109), and at one point the entire shield needed to be replaced. The tunnel wasn’t completed until 1843, 18 years after it was started, and it was never a commercial success, though it is still in use today. Tunneling via shield wasn’t tried again for over 25 years…

[But it was tried again… and again, and again, being improved and enhanced each time. Potter describes (and illustrates) the steady mechanization of the process– up to and including The Boring Company]

… The arc of tunnel boring machinery looks much more like the progression we see in other industrial areas, and that we don’t often see in construction. Construction operations often remain craft-based and labor intensive, and have been performed in similar ways for decades (or centuries). With tunnel boring machines, we see gradual automation and “factoryization,” where the work increasingly takes place in a highly mechanized, factory-like environment. New technology comes along and displaces the old technology, even in an environment of high risk aversion. And the process gradually converges on the “continuous flow,” where the machine continuously transforms solid ground into a lined tunnel, and continuously removes excavated material with the use of conveyors, the same sort of development we see in things like Ford’s assembly line, chemical process industries, and the Toyota production system.

The Evolution of Tunnel Boring Machines,” from @_brianpotter.

* W. B. Yeats, “Aedh Wishes for the Cloths of Heaven”


As we go deep, we might recall that it was on this date in 2017 that work on the Ryfast Tunnel, connecting the Norwegian city of Stavenger with the the village of Tau on the other side of the fjord, entered its final stage. It became the longest undersea road tunnel in the world; its 8.9 mile length was greater than the Eysturoyartunnilin in the Faroe Islands (at 7.0 miles), the Tokyo Bay Tunnel in Japan (at 6.0 miles), and the Shanghai Yangtze River Tunnel (at 5.6 miles) in China. It is also currently the world’s deepest subsea tunnel, reaching a maximum depth of 958 ft below sea level.


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