What is Undercutting of a Footing?
In the feature photograph we see where an excavation was performed near a wall. The excavation is nearly alongside the wall and extends well below the bottom of the wall footing, which was about three feet below grade. This type of digging below the bottom of a nearby footing is called “undercutting.”
Assuming a stabile native soil, the general rule of thumb for excavating near an existing footing is to excavate a depth below the bottom of the footing no more than the distance between the edge of the excavation and the edge of the footing. For the more mathematically inclined, the excavation should penetrate below an imaginary 45 degree line extending down from the edge of the bottom of the footing. See sketch in Figure 1.
So, what happens when a footing is undercut? Sometimes nothing happens. The excavator gets lucky and the excavation is refilled with concrete, gravel or soil before anything happens. But, other times the weight of the structure pushes the soil at the edge of the excavation into the open area and we have a collapse. See Figure 2.
Even if there is not a collapse during the construction phase, that is, the excavation is filled and the contractor is long gone, we have seen the weight of the structure slowly push the soil at the edge of the excavation into the softer backfill over a period of time. This movement allows the footing to drop. Now we have settlement, settlement cracking, leaning walls and columns, pitching floors, etc. an unhappy building owner.
Most building codes specify that existing footings should not be undercut unless some type of protection is provided for the soil under the footing. A soils or geotechnical engineer can be very helpful in avoiding the problems associated with undercutting of footings.