Collision Detection

Hello guys.

I been working on a prototipe for a game like Mr.Driller and this is what i got so far:

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As you can see, its not perfect, there some things in need of improvement, but sence its a prototipe, its k.

So question is:
How to make blocks drop when theres no block below?
i have worked with some code but cant seem to make it work.

p.s.: prototipe is for Android and uses Opengl ES

Thanks!