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Void Form vs Underslab Suspended Piping

Void Form vs Underslab Suspended Piping

Void Form vs Underslab Suspended Piping

I've been looking around for info on this subject but haven't found much. Here is the setup.

- We are working on a project, acting as Construction Management with design input. The design is not fully complete.
- Our building will have underslab radon collection, and will also have underslab plumbing/drainage piping.
- The building is being built over expansive soils, so the engineers want void-form under the slab. We are used to working with cardboard voidform, which requires contact with the moisture in the soil so that it can properly deteriorate over time.
- On previous jobs with suspended piping underneath slabs in expansive soil environments, the soil has been known to expand upwards and buckle/deform/shear the piping

Here is my dilemma.
- We can't use clear stone under the void form, otherwise there will be a capillary break and no moisture will reach the cardboard to deteriorate it.
- We still need to have some means of collecting the radon, and maybe that means weeping tile?
- In expansive soils, we have the problem of the soils expanding to put pressure on the piping system, but we can't control this by using clear stone or sand because that will cause a capillary break and jeopardize the void form.

Thoughts on a good approach here? How do we suspend the piping under the slab without risking it being damaged by expansive soils? How do we collect radon without jeopardizing the voidform?


RE: Void Form vs Underslab Suspended Piping

I'd check with an agriculture soil scientist and get their opinion on getting moisture to the cardboard forms. In a confined area that you have, I'd not believe that moisture transfer via vapor won't migrate through the stone.

RE: Void Form vs Underslab Suspended Piping

Install injection ports along the slab to introduce moisture to the void form after the slab has reached design strength.

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