Mad about Science: Boring machines

By Brenden Bobby
Reader Columnist

One of my high-school teachers told me something that would shape my worldview:

“Only boring people get bored.”

This was a masterfully diabolical dig at an entitled student (yours truly), who expected to be entertained at any given time. Sure enough, this got me thinking about what he meant in detail. Sitting idly and waiting for entertainment is a fool’s errand. Any situation you’re stuck in can relieve you of boredom, so long as you’re willing to put in a little effort and seek out enjoyment.

That being said, this article has nothing to do with a lack of entertainment and everything to do with boring machines.

Courtesy photo.

We explored boring machines a little in last week’s article. They’re big machines that bore holes through the earth to create tunnels for travel. This becomes necessary in places like The Alps, where vehicular travel is virtually impossible without tunnels, or places like New York, which can utilize subterranean tunnels and commuter trains to alleviate traffic congestion in a cramped place like Manhattan.

Boring machines are engineered to perform a specific task based on the job they will be used for. There is no single boring machine that will work in every situation, but there are principles that can be used and shared between different machines. As an example, a machine digging through sandy soils may use pressurized water to break apart sediment, while a machine tunneling through rock may need to utilize explosive charges to break apart the stone.

Using blasting for drilling is a bit rarer nowadays. The bulk of tunneling is done using a rotating head that delivers a cutting action and a forward driving force powered either by electricity or hydraulic jacks. These machines are huge, often weighing upward of 6,000 tons and up to 300 feet long. The bulk of the machine’s length comes from the support systems attached to the back of it, which we will cover later.

Examining the cutter head of the boring machine, you could likely draw parallels with a cheese grater. Applying forward pressure, the cutting head spins to grind and cut away rock and dirt. How loose debris is handled varies from machine to machine; but, generally, they spray pressurized water to help reduce heat caused by friction, act as lubrication and help contain debris by transforming it into a slurry that falls into the excavation chamber behind the cutter head. 

A screw conveyor (also known as “Archimedes’ screw,” after the ancient Greek mathematician and inventor) transports the slurry debris to a conveyor belt that runs the length of the machine and out the back. In a way, the boring machine looks a bit like one of the worms from Dune, eating up rock and pooping it out the other side, where it’s carried away by other vehicles.

You may be wondering why debris doesn’t simply fall onto the boring machine from above. This is a good thing to wonder about, and it’s something engineers thought about ahead of time by placing a curved metal shield over the top of the device. This helps to keep loose debris from falling into the guts of the machine, while also providing support to the tunnel before it is reinforced by concrete.

The process of how engineers place the concrete support structure of the tunnel is an act of genius. In days pays, shotcrete was utilized by spraying semi-solid concrete out of a pressurized hose to coat walls and stick to formed rebar. Workers would then have to smoothen out the concrete to keep it looking uniform. The modern method is performed completely differently and far more intelligently than the utilization of shotcrete.

A segment erector works by taking premade sections of a concrete cylinder and placing them like bricks to form a ring of concrete, with each segment offset from the one behind it to help improve structural integrity of the tunnel. In some instances, the final piece of concrete placed at the top is smaller than the rest of the segments. This is called the key segment, and it plays a very important role you’d have to ask your high-school math teacher about, because it’s quite honestly over my head. 

Once the ring is complete, hydraulic jacks press against the exposed side of the concrete tunnel to push the rest of the machine forward and continue digging.

A tunnel machine is able to make about 30 feet of progress per day. Due to their unique nature, boring machines are specially manufactured and carry a hefty price tag, ranging anywhere from $5 million to $30 million each. That’s a massive expenditure for an item you may use only a single time.

Getting such a massive object to turn is no small feat, either. There is a reason that subway tunnels are often straight, with only gentle curves and absolutely no 90-degree turns. A curve can be achieved while tunneling by applying more cutting pressure to one region of the surface than the other. This is similar to how a tank will turn left or right. In order for a tank to turn, one set of tracks will stop moving while the other continues to move, pivoting the tank. Essentially, the boring machine is applying more pressure to one set of directional jacks pushing it forward than the other, causing it to drill in a curve. Engineers will have to account for this angular change when placing the concrete segments.

As you can see, boring machines are anything but boring. They’re a fascinating accomplishment of engineering.

Stay curious, 7B.