Digging tunnels is an engineering marvel that has come a long way since tunneling projects required huge teams of workers armed with picks and shovels slowly chipping their way through countless tons of rock and dirt over the course of many years. As cities continue to grow in size and population, it has become necessary to go underground for continued expansion, and that necessity has given rise to new methods of tunneling which are safer, cheaper and more efficient than ever before.

Eighty feet below central London, the Royal Mail tunnel was used to carry mail throughout the city from 1927-2003, according to ConstructConnect. Though it’s no longer used to carry post, University of Cambridge engineers are using it as a laboratory of sorts, embedding it with hundreds of sensors to monitor any unsafe changes as a massive new tunnel is being dug just beneath it for Crossrail, a new commuter train line. Running parallel for more than 300 feet, it’s the first time ever two tunnels have been dug in London so close together and over such a distance.

The Crossrail tunnel project presents considerable engineering challenges, according to Robert Mair, head of civil engineering for Smart Infrastructure and Construction at Cambridge. Among the most pressing, he says, “is how you excavate large tunnels underneath urban infrastructure without causing any distress to buildings or other tunnels.”

Tunnel boring machines (TBMs) have been around since the 1970s, and are still considered one of the cheapest and most efficient ways to dig underground. 

Tunnel boring machines (TBMs) have been around since the 1970s, and are still considered one of the cheapest and most efficient ways to dig underground. The machines automatically position pre-cast concrete support lining as they dig, whether through rock or under water, utilize chemicals to loosen or harden soil as needed during the process, and are outfitted with electronic sensors that can automatically avoid above-ground structures while digging, according to Construction Dive.

Sometimes the earth itself is the engineering challenge when it comes to tunneling. Kuala Lumpur, Malaysia has a high groundwater table and much of the earth below it is Karstic limestone, dotted with cavities, sinkholes and is weakened from years of mining tin. The so-called SMART tunnel (Stormwater Management And Road Tunnel) is a multi-use six-mile storm drainage and roadway, the second longest in Southeast Asia. During flash floods, the tunnel is closed off to traffic with water-tight gates, serving as a diversion tunnel for water. When the flood waters have receded, the roadway reopens for vehicular traffic. Were it not for TBM technology, such a project would not have been possible.

The future of tunneling, of which development is currently underway, involves high-speed capsule-based transport that can move passengers at speeds of 760 miles per hour (or 35 minutes to traverse the 350 miles between Los Angeles and San Francisco.) At the tip of the spear is Tesla and SpaceX founder Elon Musk, who believes the proposed transport system, called Hyperloop, is the future of underground transportation. To dig these massive underground would-be transit tunnels, Musk started a new venture, The Boring Company.

“To solve the problem of soul-destroying traffic, roads must go 3D, which means either flying cars or tunnels." 

“To solve the problem of soul-destroying traffic, roads must go 3D, which means either flying cars or tunnels. Unlike flying cars, tunnels are weatherproof, out of sight and won't fall on your head. A large network of tunnels many levels deep would fix congestion in any city, no matter how large it grew (just keep adding levels). The key to making this work is increasing tunneling speed and dropping costs by a factor of 10 or more – this is the goal of The Boring Company,” according to The Boring Company FAQ.

Musk’s The Boring Company believes TBM machines are inefficient, and the company is working on improving the technology to dramatically lower the cost and increase the speed of tunnel digging. The company says the pace at which a snail digs is 14 times faster than TBM, and proposes methods like tripling TBM’s power output, using electric vehicles and automating the process to improve the technology.

Tunneling has been around for centuries, and will only continue to evolve as cities expand and more infrastructure simply has no place to go but underground.