Rigid Insulation under Digester Tank

[pat2]

I have a client that wants to place rigid insulation around the walls and base of a buried digester reinforced concrete tank. There are 20 tanks side by side with each tank measuring 50'x 200' with 20' of sludge. The client does not want to place the tank on piles, so the tank will bear on the rigid isolation. I attached a section for clarification.

My question is has anyone experience designing a reinforced concrete tank similar to this? Has anyone created specs for this type of construction? One can pick a rigid insulation based on crushing strength of the insulation required to resist the bearing pressures and a subgrade modulus that corresponds to the properties of the rigid insulation, but what else would you consider?

 

[Ltwine]

Not clear on purpose of the insulation, very difficult to comment on. However, as a starter/teaser, can you raise the tank slab a few inches (similar to suspended slab )to accommodate the insulation, but let the wall extends to the base mat. So the wall and insulation will share the slab dead weight and weight of the sludge.

 

[JedClampett]

Digester sludge needs warm temperatures for the bugs to do a good job eating the sludge.
I've never heard of insulating the base slab or buried walls, but if the owner wants to pay for it, I don't see a huge issue. The walls should be easy. Apply the insulation like any other exterior insulation.
For the base, I'd pour a lean concrete pad on the subgrade, place styrofoam or other rigid insulation on the pad, place your reinforcing and then pour your slab on that. Keep the pour rate slow enough so that the foam doesn't get crushed.

 

[aeoliantexan]

Liquified ammonia tanks usually have about 4 inches of expanded polystyrene insulation under the steel plate floor. They place a sand fill, strike it smooth, lay a vapor barrier, two layers of rigid insulation, building paper, and a thin layer of oiled sand. I don't think you need the vapor barrier because the heat flow is the opposite direction.

The steel floor doesn't mind a little deflection due to nonuniform contact between the insulation and the sand fill, but a concrete floor is not so forgiving. If you can't get really good contact between the insulation and the sand, you can cut the insulation into smaller squares.

The insulation may not be necessary under the floor if the walls are buried and the digesters are wide, as you described. The heat flow path from the floor down, around and up to the ground surface will be very long, so the heat flow will be slow. I seem to remember a rule of thumb that about 8 feet of soil has the insulation equivalent of an inch of foam. A little research could save your client quite a bit of money.

However, if the soil is subject to shrinkage, you may need to minimize heating of the soil by using the insulation.