I'm not sure, just want to say to be mindful about where the water sheds (in torrential downpours, etc), especially if you're in very close proximity to a river.
Good luck.
I brainstormed more and the more I think about the viable options for a livable cache the more I lean towards a plastic corrogated 36" to 48" diameter culvert about 6 to 8 feet in length.
My idea is to cover one end with a tarp or seal it off. Dig about 4 feet below ground to get past the frost line. Drop it in, compact the soil above it, towards the other end dig down about 6 to 7 feet underground. Install a simple wooden ladder for entry. Climb into the culvert and sleep. This way during storms the water would divert down into the 6 to 7 foot pit. It'd have to fill with a few feet of water before getting to the opening. A simple filtration equation could figure out the time it would take or the type of storm it'd need to actually fill up with water. So if I solved for like a Q100 storm with the filtration rate of Colorado rainfall I could dig deep enough to prevent that from happening.
My only issue is the weight of the culvert. They are designed to not cave in from the forces of the lateral and vertical earth pressures pushing against them, which means it's probably a thicker plastic (i.e. has some weight to it). Not sure if I have the strength to carry this thing a mile in the woods and up a steep grade. But in theory this is the best option I came up with. It would waterproof it, give enough space to sleep, be below the frost line and you wouldn't have to worry about shit caving in and killing yourself. Plus I could always cover the opening with fallen down trees and be incognito from the world around me.
Does anyone in construction know what a 6 foot section of this plastic pipe would weigh with a 3 foot diameter? They are kind of expensive...a few hundred dollars, but shit if I can set this up, it beats paying rent each month.
