You're looking at the business end of the Battery Townsley shock tube. In 1967, the Office of Civil Defense contracted with URS Corporation to develop, evaluate and operate this shock tube to conduct full-scale tests of loading, response, and debris characteristics of structural elements.
The tunnel built in the central corridor of Battery Townsley, a WWII 16-inch gun battery located at Fort Cronkhite in Marin County. Primacord was strung on an X-frame inside the 163 foot long metal tunnel. When detonated it produced a shock wave similar to the shock wave produced by a megaton-range weapon.
Wall panels of various materials were secured in front of the shock tube and subjected to blasts. Instruments measured how the materials responded to the blasts.
The work seems to have stopped around 1975, although it appears that there was some thought of reopening the facility in the 1980s.
This was not the first time Battery Townsley was used as a test bed. In 1941-1942, one of Battery Townsley's 16-inch guns was used to test concrete slabs, apparently for the Special Engineering Department of the Panama Canal. Four large concrete slabs were built near the battery. Each of them had a different concrete mix and rebar arrangement. One of the slabs was 42 feet by 23 feet and 13 feet thick. A 16-inch gun was then used to shoot at them to determine how the concrete would hold up.
The National Park Service is preparing Battery Townsley for public tours - perhaps sometime in 2007.
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