Geotechnical Engineers sometimes require perimeter moisture cut-off walls to be used in highly expansive soils under the perimeters of foundations. This is done in order to intercept horizontal migration of water in the soil from outside to under the foundation, thereby reducing the amount of potential vertical rise in the soil under the foundation.
Wafflemat is ideally suited to work with foundation perimeter moisture cut-off walls. Wafflemat will immediately provide voids under the foundation to absorb soil volume change due to expanding soil within the perimeter moisture cut-off wall boundary.
Moisture cut-off walls can be poured at the same time as the Wafflemat foundation, or they can be poured separately in two pours. Depending on soil conditions and foundation loads, some foundation design engineers may determine that the Wafflemat foundation slab only needs to rest on top of the moisture cut-off wall, and not be tied to it.
The following drawings demonstrate the two-pour construction sequence, with the moisture cut-off wall tied to the foundation, that can be successfully used with a Wafflemat foundation:
The first concrete pour is for the moisture cut-off wall. The amount and placement location of rebar in the moisture cut-off wall, including the use of any post tensioned cables and plastic sheeting, are determined by the foundation design engineer. Vertical “L” shaped rebar is used to tie the foundation slab to the moisture cut-off wall, as required by the foundation design engineer. Connecting the moisture cut-off wall to the foundation slab makes the perimeter of the foundation very rigid and adds enormous stiffness to the entire foundation.
The second concrete pour is made after form work, plastic sheeting, Waffleboxes, and steel reinforcement are placed for the foundation.
The following two drawings provide examples of how rebar or post-tensioned primary reinforcement can be used with the Wafflemat system. Note that Wafflemat can be easily designed to work with either type of reinforcement.
The above drawings are only examples of how the Wafflemat foundation design can be used with perimeter moisture cut-off walls. The width and depth of the moisture cut-off wall will vary due to site soil conditions, as determined by the Geotechnical Engineer. The use of rebar or post-tensioned primary reinforcement in the moisture cut-off wall and foundation slab will be determined by the foundation design engineer.
What happens when you use Soil Moisture Cut-Off Wall with Wafflemat?
- Wafflemat foundation absorbs some of the volume change of expansive soil inside the perimeter moisture cut-off wall, thereby reducing uplift forces.
- Wafflemat foundation creates a much more rigid floor slab.
- Non-expansive fill may not be required.
And without Wafflemat?
- Expansive soil inside the perimeter moisture cut-off wall pushes the foundation slab upward with its full force.
- The thin flexible floor slab is more easily subject to cracking.
- Non-expansive fill may not adequately or economically reduce uplift forces of expansive soil.