Everybody knows that C4 encapsulated in 1 feet of concrete lasts 33 years, but 1 more foot reduces that to 31 years, making your "theory" implausible.If you would read, you would learn that in the manufacturers package the shelf life is 10 years.
http://algoxy.com/psych/9-11scenario.html#anchor1154643
The manufacturere doesn't say how long it will last encapsulated in a couple of feet of concrete.