If you build the fire in the trench and the fill it in, you'll probably have to wait a while for the moisture in the soil to work its way out, unless you're working with dry sand.

"Save the firewood for later. Don't use until you get very cold."

But not too cold. Dying of hypothermia from not building a fire is just as dumb as dying of dehydration with water in your bottle. If you need it, use it.

Sue