why is it so expensive to build tunnels?

I assumed the answer was the digging. The giant machine, the depth, the engineering. Turns out that's the cheap part. The tunnel boring machine is only around 18 to 20% of construction cost, and construction itself is only about half the total program.

So where does the money actually go? Labor and risk. Labor alone is 40 to 50% of construction cost, and because a tunnel is a linear thing dug from one or two faces, you can't throw more crews at it to finish faster. Time is basically fixed, so labor cost is basically fixed.

Then there's how risk gets priced. The more risk an owner pushes onto the contractor, the bigger the contingency they bake into the bid. You pay upfront for every "what if we hit water" scenario, whether it happens or not.

But the part that actually reframed it for me is the country gap. Same physics, yet projects in places like Australia and North America run 3 to 10 times more than low cost countries. The difference isn't geology. It's slow regulatory review, gold plated design standards, and soft costs that can reach 45% of the total.

So tunnels aren't expensive because they're hard to build. They're expensive because of the decisions wrapped around building them.