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Holes: Is Underground Construction the Answer to Security Problems?

Some vexing security and safety challenges have a simple solution: Move the buildings underground.

Another problem with subterranean space is that the skills of making and marketing underground structures have little overlap with those required by surface buildings of the same volume. The structural analysis of a building that is surrounded and therefore supported by rock is totally different from that of a freestanding edifice. In any structure for which management of heat gradients is important (reactors, refineries), allowances will have to be made for the fact that rock is an insulator while air is not, and people will have to be shown why they should have confidence in those allowances. The same point can be made across a range of issues, from building integrity to fire prevention. All these problems have to be rethought and the pool of those competent to do so is small.

Both of these problems, high costs and small expertise pools, are affected by total market size; the larger the market, the more R&D capital and skilled labor flow into the sector. Fortunately, the list of applications for underground space has been expanding steadily during this century, beginning with utilities and transport to sidestepping the zero-sum games that surface structures must fight with the forces of preservation and conservation to city parking.

Tunnel Boring Machines

One consequence of this growth has been steady technical progress. The first of the two families of technologies employed in this work, drill and blast, was invented in the 14th century when Marco Polo brought black gunpowder back from China. The core procedure, which stayed basically unchanged for 600 years, was to drill holes in the working face, pack them full of explosives, run around the corner, touch off the charge, run back to the face (dealing with the fumes released by the explosion as best you could), try to get the roof supports shoved into place before the ceiling collapsed and then shovel out the fragments. It was a terrible job in every respect, not least because the system was impossible to scale. Only two or three people had room to work on a tunnel face at any one time, and conditions in chambers or caverns were not much better. Production was measured in a few linear feet a day.

In the 1950s an engineer named James Robbins hit on the idea of building a machine that would push cutter wheels or discs into the rock face and then rotate them in circles, like an immense glass cutter. Done with enough force, each of these rotations would shatter an inch or so of rock off the face. Improvements came slowly, but by the '90s tunnel boring machines, or TBMs, were handling the three key phases of the cycle: breaking the rock, picking up the pieces and dropping them on a disposal conveyor, and fabricating and installing a lining. A contemporary machine can excavate and line a tube with a diameter in the dozens of feet at an average rate of a hundred feet a day. (Actual rates vary widely.) The very largest TBMs have diameters approaching 50 feet, which is about what you need for a stack of two four-lane highways, one going in each direction.