Ok this is a tricky matter, and it will involve the following: tdoors, gravel/sand, and physics to be on.
First, you should make a wall that is 3 blocks thick so that gravel/sand traveling through it cannot be seen on either side. Now set up the tdoor so that whenever you click the button it opens a hatch on the bottom that flushes out the sand/gravel. You will need 2 buttons, one to close the portcullis( releases a door on the top that drops sand/gravel), and a door that opens the portcullis(opens a door on the bottom to flush the sand/gravel). You will have to refill the sand/gravel every now and then. Also, tdoor seems glitchy and may sometimes disappear, meaning that you will have to replace it. I will try to post pictures of it in the morning. Any questions ask me ^^.
If you are using beta, then simply use pistons
Edit: Please read bottom post!!! I have another much simpler way that doesn't need sand or gravel.
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