I would urge you to focus on saftey. I am glad to hear your grandpa is helping that is very cool of him. You need to make sure you have the proper supports to hold up the walls and keep dirt from falling in on you. You need a ceiling strong enough to hold the dirt on top as well as anything that might walk or drive over it. Also do not build in an area where it may flood.
Have you ever considred a tree house? That would be safer.