7-3/4 in. x 10-3/4 in. x 10-3/4 in. Concrete Deck Block

The simple answer is no. The center of the block is made for a 4x4 set in vertically. The slots are made for a 2x4, 2x6, 2x8 and so on, where the the 2" width fits into the slot.

This is one of many websites that offer free advise on using deck blocks for sheds and floating decks. https://plasticinehouse.com/building-shed-base-with-deck-blocks/ The guidance from this website is: "Generally speaking, you want support every 3-4′ for a normal use building and 4-6′ for lighter situations. An average 12′ x12′ can use around 18 blocks, but more is certainly ok, just space evenly."

Since it's made of concrete, the deck block is quite durable. I've used the deck block to support three sheds in my back yard. The oldest shed was built over twenty five years ago and the deck blocks that support it show no sign of wear or deterioration.

MemoRex, All mine have been on the ground and on a solid gravel base. IMO, you should be able to put them below ground. since they are solid concrete. I would make sure the base they sit on is sold and level, not just dirt. And of course, your wood posts should be treated for outdoor weather. Hope this helps.

These footers should work, but you'll need to check local building codes to determine maximum ground pressure allowed for construction. Each footer generally covers one square foot, so if 200 pounds per square foot (PSF) is the maximum allowed, then you'll need no less than 15 footers (3000 divided by 200) evenly spaced over the area supporting the tub (in this example I'd recommend 16 footers evenly spaced in a 4x4 pattern). Be sure to check the soil type as well, as you may have to reinforce the ground with gravel to aid in both drainage and minimize any settling.

This site will be very helpful: https://plasticinehouse.com/building-shed-base-with-deck-blocks/#:~:text=How%20big%20are%20deck%20blocks%3F%20The%20block%20size,square%20and%207″-8”%20high.%20Weight%20–%2048%20lbs.

The grooves (slots) are approximately 1 1/2" deep. If the block is set even with the ground, the bottom of the 2x6 would be approximately 6 1/4" above the ground.