Geotechnical engineers will sometimes require a deep soil cut-off wall to be used around the perimeter of the foundation. The cut-off wall is used to block the migration of water from outside the foundation to under the foundation, thereby reducing soil expansion under the foundation. Soil cut-off walls can be designed as an integral part of the foundation and thereby add rigidity to the perimeter of the foundation.
The Wafflemat system is ideally suited to work with foundation perimeter cut-off walls. Geotechnical Engineers sometimes require perimeter cut-off walls to be used in highly expansive soils, in order to reduce the amount of Potential Vertical Rise (PVR) in the soil under the foundation. The Wafflemat system, when used under the foundation slab, will provide voids to absorb expansive soil volume change within the perimeter cut-off wall. The following drawings demonstrate the two-part construction sequence that can be successfully used with cut-off walls:
The first concrete pour is only for the perimeter cut-off wall. The amount and placement location of rebar in the cut-off wall, including the use of any post tensioned (PT) cables and plastic sheeting, are determined by the engineer. Vertical “L” shaped rebar is used to tie the foundation slab to the cut-off wall, as required by the engineer. Doing so makes the perimeter of the foundation very rigid and adds enormous stiffness to the entire foundation.
Depending on soil conditions and foundation loads, some engineers may determine that the Wafflemat foundation slab only needs to rest on top of the cut-off wall with light interconnecting vertical reinforcement, if any. The engineers may also determine that the cut-off wall should have post-tensioned cables incorporated in the horizontal reinforcement. The second concrete pour is made after formwork, plastic sheeting, Waffleboxes, and steel reinforcement are placed.
The following two drawings are provided to show different examples of how rebar and/or post-tensioned reinforcement can be used with the Wafflemat system. Note that Wafflemat can be designed to work with either type of reinforcement.
The above drawings are only examples of how the Wafflemat foundation slab can be used with perimeter cut-off walls. The width and depth of the cut-off wall will vary due to site soil conditions, as determined by the Geotechnical Engineer. And, the use of rebar and/or post-tensioned reinforcement in the cut-off wall and foundation slab will be determined by the engineer doing the foundation design.
Using Wafflemat With Soil Cut-Off Wall
|With Wafflemat:||Without Wafflemat:|
|Absorbs volume change of |
expansive soil inside the
perimeter cut-off wall, thereby reducing uplift forces
|Expansive soil pushes the foundation upward
|Much more rigid floor slab||Thin flexible floor slab, more easily subject to cracking
|Non-expansive fill is usually |
|Non-expansive fill is used to attempt to reduce uplift forces of expansive soil