When homes are built on hilly and other sloping sites in Texas, such as in north San Antonio, one common solution is to use a bagged fill type of foundation.
The bagged fill can be crushed rock or soil, with plastic sheet used at the edges to hold back the fill and maintain vertical faces on the fill until the outside concrete walls are poured. Widely spaced beams are used to provide some stability for the floor slab in the same manner as they are used for the ribbed slab foundation design.
The thin floor slab is tied into the top of the thick and rigid outside walls and reinforced with post-tensioned cables. It is important to remember that bagged fill cannot always be fully compacted, even if it is crushed rock. Bagged fill will settle over time, even if only in small amounts. This settlement will reduce the supporting effect of the fill on the underside of the thin floor slab.
The result is that the thin floor slab can become suspended between the outside walls and the beams. Also, with the use of post-tensioned cables in the thin slab, de-lamination of the floor slab at its joint with the top of the rigid outside walls may occur.
When Wafflemat is used in this type of foundation design, it supplements the existing design by creating a much more rigid floor slab that can resist settlement of underlying fill; and it provides more closely spaced beams that tie into the existing beams and the outside walls. The potential for de-lamination of the floor slab from the top of the outside walls is minimized.
Using Wafflemat with Bagged Fill Foundation
|With Wafflemat:||Without Wafflemat:|
|Much more rigid floor slab that resists settlement of the supporting fill material. (Remember that even crushed rock that is placed without any compaction will settle over time, even if only in small amounts. It only takes a small vertical amount of settlement to suspend a thin concrete slab between widely spaced beams.)||Thin flexible floor slab, more easily subject to cracking.|
|Beams formed by the Wafflemat™ System create a much stronger connection of floor slab to outside walls to resist de-lamination of the slab from the top of the walls. Reinforcement is in the deep beams, not the thin slab.||Greater potential for de-lamination of the floor slab from the top of the outside walls.|
|Slightly less fill material is required||Slightly more fill material is required|
|Expansive soil from the site can be reused for fill, where appropriate to the design||More off-haul of expansive soil|