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My understanding is that sand is sometimes used to lower the resonant frequency of a pier. Having said that though, if a pier is solid in itself, and solidly fixed to the concrete foundation, I can't see any benefit at all in lowering its resonant frequency. Whether imaging or observing, we want the pier to stop vibrating as quickly as possible, which generally won't happen if it's vibrating slower rather than faster.
If filling a pier with sand makes the pier a lot heavier, then a given force imparted to the pier may produce less movement than it would have if the pier were not sand-filled. In this - I can see a benefit.
I'm not sure if there is any benefit thought to be gained from having a loose filling (like sand), in as much as it can 'slosh' around a bit help the damping.
Chris
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