I want to thank JIR for his article and the efforts he went through showing us how to construct and supply underground caches. I just wanted to suggest an alternative to the custom made containers by using a 300 gallon spherical below ground septic tank. They are made of watertight plastic with a o-ring sealed lid and weigh around 110 pounds. (See the Tank Depot web site.) The rough size of the tank is 54″ in diameter and 51 inches tall with a 20 inch manhole cover. You would also only dig 118 cubic feet for a 5 foot diameter x 6 foot deep hole versus 280 cubic ft for a 5 foot wide x 4 foot tall x 14 foot long trench. The price for one of these tanks (without delivery) is under $400 ea. – DWJ
Its a little more expensive possibly but you can use molded water/chemical tanks for your cache. They would be much easier to use than building a culvert. These ones have a 16″ diameter opening at the top which would make it much easier to load and retrieve items and come in a variety of sizes. These tanks are very heavy duty and of seamless construction.
To line the pit used for any tank or cache I would use heavy pond liner. You can get this online up to 45 mils in thickness. Home Depot sell 15′ x 15′ pond liner in 20 mil thickness. Just line the hole with the appropriate size, add you cache container and wrap. I would then place a sheet on top sloping away from the cache that I was burying to direct seeping rainwater away. – Ken L.