More thinking out loud...

Where I live (South Dakota), any underground cache is going to be either innaccessible or extremely difficult to recover for 5-6 months out of the year. How many people have actually tried to excavate frozen soil? From personal experience, it's like chipping through concrete. It took a couple of us with 6 foot iron bars a couple of hours to 'dig' a 30cm/1 foot diameter hole through a foot of frozen soil.

Even if you reach the top of your PVC pipe, if it's frozen in place, you won't be able to retrieve the contents without the inner/outer tube method. How much bigger will your hole have to be to accomodate a strap/pipe wrench to open the outer pipe fitting? (Lots, which means even more work digging)

Also, while the cache location might be easily located in August, will you be able to relocate it under 2-4-8 foot deep snow drifts? For anyone who lives where the ground freezes, even for a short time, I don't think underground caches make a lot of sense.

Murphy says the cache will be frozen when you need it most.

Another thought - if things are so bad you need to access your cache, you're going to leave a lot of evidence (dark soil on white snow) that someone just recovered a bunch of hidden supplies - not a good idea.

Storing/hiding supplies in walls, with friends, family, 'in plain sight' - anywhere but underground would be a much better option in the snow belt.