Rigid Insulation under Digester Tank
Rigid Insulation under Digester Tank
(OP)
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?
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?





RE: Rigid Insulation under Digester Tank
RE: Rigid Insulation under Digester Tank
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.
RE: Rigid Insulation under Digester Tank
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.