There are various reasons for foundation underpinning.
Some of these reasons are:
· To support the additional loads of a second-story addition.
· The existing foundation has settled or rotated at some areas.
· Removal of load bearing walls.
· Addition of strong shear walls.
Major foundation underpinning is typically done in the A-B slot method. For example every other 5' section of the foundation is underpinned (A), and subsequently the other 5' sections (B) are underpinned. This method is used to eliminate or minimize the need for shoring.
Each foundation underpinning retrofit is a unique situation and has to be designed by a professional engineer or an architect.
TYPICAL UNDERPIN RETROFIT STEPS:
CLICK ON PHOTOS FOR LARGER IMAGES.
1. The portions of the house being underpinned may be shored.
2. A trench is then dug UNDER the existing footing. This underpin may be as little as 12" deeper than the original footing or up to 5' or more in depth. (Photo A)
3. Steel reinforcements (rebars) are then installed in the trench and are elevated above the soil with concrete spacers. The new rebars may be dowelled into the original footing with high strength epoxy. (Photos A, B, C)
4. Ready-mix concrete, delivered to the job site by concrete trucks, is pumped into the trench. (Not shown in photos)