Red eared slider, might be pregnant

Question
Hi, I have 2 red eared slider turtles and they have grown up together in a 50 gallon tank and are about 3-3.5 years old and each about 5.5 inches. I they have been acting different in the last 2.5 months. The female has been digging a lot! Basking a lot! And even eating some of the pebbles or at least try to. Also, i saw the male doing the mating dance to the female. Also the female has been gaining weight. More so around the hind legs and tail. I have put a container of dirt in there in case she wanted to lay eggs. Also i was wondering if she is pregnant? If so how do i let her lay eggs? Please provide me with an exact way to set up an egg laying environment. Thank you Andrew.

Answer
Honestly, these guys have more detail than I would be able to give:
http://www.austinsturtlepage.com/Care/caresheet-red_ear_slider.htm

Remove the pebbles if there is any risk that they will be eaten.  A tank without any substrate is much easier to keep clean, anyhow.  Pebbles could cause an intestinal obstruction, and surgery to remove them would not be a simple procedure!

There is more information on creating a proper nesting site, and incubating eggs, here:
http://www.redearslider.com/reproduction.html