Tunnel boring machines all share one habit. However hard the rock, the hole comes out round, since a spinning cutterhead traces a circle the same way a drill bit does. Round works fine for subways and sewers. It works a lot less well in a mine, where the miners want to drive trucks through each new tunnel the moment the machine moves on, and a circle’s floor is curved, which a loaded truck on rubber tires does not appreciate.<\/p>\n
So Mexico’s oldest silver mine has been running a machine that cuts a different shape entirely. The MDM5000, a 303-ton rig (275 metric tons) built by TBM maker Robbins, carves a rectangular tunnel through the Fresnillo mine, 16.4 feet wide and 14.8 feet tall (5.0 by 4.5 meters), with a floor flat enough that rubber-tired trucks can drive straight in behind it. Nobody has to come back later and pour a concrete roadway inside a round bore. That’s the entire sales pitch, and frankly it’s a good one.<\/p>\n
It’s also a machine with a story. It flooded, spent 14 months in recovery, backed itself 1,640 feet away from the trouble, and started cutting a second tunnel. We’ll get to that.<\/p>\n
Nothing, if you’re building a subway. There’s a second reason everybody cuts circles, which is that a round tunnel spreads the squeeze of the surrounding rock evenly around the ring, and once the boring machine is through, you build a flat deck inside and lay your track on top. The curved floor disappears under the finished structure and no passenger ever thinks about it.<\/p>\n
A mine can’t work that way. Miners cut fresh tunnel constantly to chase the ore, they want trucks rolling through each new stretch right away, and building a roadway behind every bore adds time and cost before the first load moves. That’s why most mine tunnels are still blasted out with explosives instead, a method that leaves a usable floor but only advances as fast as you can drill, load, clear out and detonate. Mining companies keep testing ways around that cycle, like the electric rock cutter that spent ten months chipping through a Canadian mine<\/a>.<\/p>\n The MDM5000 still uses disc cutters, 26 across the face plus 4 more working the edges, each 17 inches in diameter. They’re the same kind of steel rollers a circular TBM presses against the rock face until the rock cracks and flakes off. The difference is the motion. Instead of spinning, the whole cutterhead swings up and down through the face, and Robbins<\/a> says each downward stroke does double duty, breaking rock and sweeping the broken pieces onto a conveyor that carries them out the back. Pads on the machine’s sides press against the tunnel walls to brace it, since something has to soak up the force of shoving 30 steel discs through hard rock.<\/p>\n And this is hard rock. Robbins rates the machine for rock up to about 29,000 psi (200 megapascals), while the concrete in a typical driveway is rated somewhere around 4,000. The rock in this mine is andesite and shale shot through with quartz, about 2,300 feet (700 meters) down.<\/p>\n The speed is the reason anyone bothers. In its first campaign, announced by Robbins in 2021<\/a>, the machine put away more than 5,500 feet of tunnel, hitting 171 feet in its best week and 627 feet in its best month, which the company says is roughly double the pace of drill-and-blast. Those figures are from 2021, and I’d usually skip the qualifier, but here the date’s doing real work.<\/p>\n Fresnillo is a wet mine at depth, and somewhere along that first drive the MDM5000 ran into a fault and took on water. A technical paper published in June 2026<\/a> by engineers from TOPO Machinery, Robbins and Dean Brox Consulting describes the machine being partly flooded 2,300 feet underground, and it says getting everything back in order took 14 months.<\/p>\n So what do you do with a 303-ton machine sitting at the bottom of a flooded mine?<\/p>\n You pump the water out, rebuild what it ruined, and then, in this case, you put the thing in reverse. The paper says the MDM5000 drove itself 1,640 feet (500 meters) backward, away from the fault, before starting a second tunnel in 2024. Backing up isn’t a standard TBM talent, since most of them only move forward and usually don’t come out at all; they get buried or cut apart when the job ends. We covered another exception recently, a boring machine in Lesotho that reversed nearly a mile through an access tunnel<\/a> to reach its starting point.<\/p>\n One caveat from the same paper: water eroded parts of that flat floor over time, so the surface still needs upkeep like any mine road. The machine spares the mine the step of building a road in the first place, and that’s the saving.<\/p>\n What I can’t give you is a current speed. Neither Robbins nor the June paper publishes an average advance rate for the second drive, and I’m not going to stretch a 2021 press release to cover 2026. The honest version is that the machine’s cutting rock at Fresnillo again, and the numbers that made it famous are from before the flood.<\/p>\n Still, the experiment survived something that kills plenty of tunneling projects outright. The second drive has been underway since 2024, and the June 2026 paper puts the MDM5000 back at work at the same 2,300-foot depth where it flooded, cutting flat-floored tunnel through the oldest silver mine in Mexico.<\/p>\n","protected":false},"excerpt":{"rendered":" Tunnel boring machines all share one habit. However hard the rock, the hole comes out round, since a spinning cutterhead … <\/p>\nHow do you cut a rectangle with round discs?<\/h2>\n
Then the tunnel flooded<\/h2>\n