query on how to prevent burning of dum
Keep the heat low. "Dum"ming the biryani is steaming it rather than cooking it with high heat. You'll also need to seal the container. 
Burning happens because of the lack of liquid and the high heat. Mainly because the water evaporates while cooking. So yes, a little water will indeed help, but make sure you keep the heat low. Don't worry if the rice at the bottom becomes sticky. It's called "kurchan" and some people find it very tasty. Another alternative is to prepare the biryani in an oven (when you start with uncooked meat) in a very thick, covered oven pan with no foiling at 200 degree Celsius for approximately thirty minutes.