Liquefaction

The ingenious technique you choose to make the base could be used to make a lid. Seal off a section of pipe at one end as you described. Turn it upside down have a large eyebolt embedded in the center top. Build a three pipe A-frame and use a chain hoist, block-en-tackle or cable puller and lift fit the thing in place. Build the unit such that the top is one foot or more below ground level. Tie a bright colored 8 feet or more polypropylene rope (florescent in UV if possible) to the eyelet on the lid. Cover the top with dirt and let the rope lay on the ground. The wind will blow it. It will float to the surface of the water. It will float to the surface of liquefied dirt. Hopefully some of the rope will be visible to discover where to dig. The area could get more dirt blown in or some dirt blown away.

My only comments have to do with needing to protect against possible liquefaction of the soil in the vicinity. In other words tie it all together so it doesn't separate when the strong shaking starts. Also, provide a way to minimize water damage. Expect the area to flood or be sometimes under water, water to get into the cracks and through the holes in the manhole cover. We will get near constant rain after the pole shift. After the pole shift, to get it open you can use the same A-frame and hoist or dig down beside it and pound a hole in the side with a sledge hammer. By the way I think these sections of pipe will need some way of holding them together other than slip joint friction. Pins or a strap over the joint or some other way of holding these joints needs to be provided for.

If the soil liquefies due to the shaking and the container and it's contents is lighter than the dirt it displaces, then it will float to the surface or above the surface of the ground. Like a cork in water. This may allow the unit to be blown apart from the wind or things hitting it. I recommend you store heavy things in this storage space or make the unit able to hold together if it pops up above the surface of the ground.

Offered by Mike.