The beauty of precast sometimes lies not in its aesthetics, but in its functionality and flexability. The precast used for line the "Chunnel" (the  Channel Tunnel that undersea rail tunnel linking the United Kingdom with northern France beneath the English Channel).

The geology of the site meant that two different systems were used to line the tunnel . . . although they are both curved precast panels. On the UK side, there are eight lining segments plus a key segment. One the French side, fice segments plus a key segment were used. First, the contractors used massive tunnel boring machines (starting on both sides and meeting in the middle). After the boring machines went through the earth, precast linings that used high-density concrete were set into place to shore up the tunnel walls and ensure that the sea wouldn't come crushing in. The loose material from the boring was sent out on conveyor belts.

There are many kinds of precast tunnel systems and linings. Each one may be specifically designed to meet the needs of a particular tunnel or project. The Chunnel precast is just one version of this technology, and done on such a large scale that it truly makes it one of my "Seven Wonders of Precast."