Yes, heat rises but its going to drop also.

Heat does not rise. This is a myth that comes from the fact that hot air rises. Heat goes to cold. I would not expect airflow inside a pile of sand, and so I do not believe that gravity is a factor for your application.

For your consideration:

8' x 8' x 2' = 128 cubic feet = 4.74 cubic yards

1 cubic yard of dry sand weighs

2619 pounds. So you are talking about 12,416 pounds of sand. (4.74 x 2619 = 12,414)

The specific heat of sand is 830.

The specific heat of water is 4182.
So a pound of sand will hold about 20 percent what a pound of water will hold. (830/4182 = .198 = 19.8%)

A btu is defined as the amount of energy required to raise 1 pound of water 1 degree Fahrenheit. This tells us that to store one btu of heat, you would need to raise 5 pounds of sand by 1 degree Fahrenheit.

So, for each degree that you raise your 12,414 pounds of sand, you will be storing 2483 btus. (12,414/5 = 2483)

A single gallon of propane contains 91,500 btus. So, you would need to raise the temperature of your box of sand by 36 degrees to store the equivalent heat energy contained in a gallon of propane.