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. Bagged fill is commonly used to take up the volume between sloping ground and the underside of the foundation, thereby providing required support to the foundation slab.
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 usually tied into the top of the thick and rigid outside walls with rebar 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 interior beams under the slab. 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 a Bagged Fill foundation design, it supplements the existing design by creating a much more rigid floor slab that can resist settlement of underlying supporting fill; and it provides more closely spaced smaller beams that tie into the outside walls and the existing interior beams under the slab. The potential for de-lamination of the Wafflemat floor slab from the top of the outside walls is minimized.
Bagged Fill Foundation:
Wafflemat with Bagged Fill Foundation:
What happens when you use Wafflemat with Bagged Fill Foundation?
- Wafflemat creates a 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.)
- Beams formed by Wafflemat create a much stronger connection of the floor slab to outside walls, and resist de-lamination of the floor slab from the top of the walls. Primary reinforcement for the floor slab is in the beams formed by Wafflemat, not in the thin slab; and it ties the beams into the outside walls.
- Slightly less fill material is required.
And without Wafflemat?
- The thin flexible floor slab is more easily subject to cracking when bagged fill settles.
- Greater potential for de-lamination of the floor slab from the top of the outside walls.
- Slightly more fill material is required.