Foundations

OBC requires at least a foot of undisturbed compacted soil between your footing and the water table. Load bearing capacity diminishes if the soil is wet. The water table should remain at 5 feet below the soils adjacent to the foundation, more is definitely prefered.

Nearby persistant standing water should be 6 feet or more, below where the soil meets the back of a walkout home. When the body of water is out of reach a small hole at the perimeter of the property can be used.

 

The Sump Pit Indicator (Walk-outs)  If the bottom of the sump pit is physically higher than the lowest footings, the sump system cannot drain the soil around those footings. (Lstiburek, Joseph. Understanding Foundations. Building Science Press.) It is the norm for a walk-out to have lower footings which do not drain to the internal sump pit. A late 90’s walk out home’s lowest footings, are most likely to drain through weeping tiles.

Weeping tiles cannot work effectively if at or near the water table.

                                                    

Because walk out basement slabs sit 4 to 5 feet above the footings, immersed footings 4 feet underground can go un-noticed. This is because drainage for the lower footings is often handled independently from the sump, as these footings are at lower levels. The upper French drain feeds to the sump pump  which protects only the front footings. Concern arises when the Rear footings have French drains, intended to percolate to the water table. This is only effective if reasonably above the water table. If not, an additional sump system is needed to mitigate risks.