When excavating below an existing footing, it is important to take provisions to prevent undermining the footing. Here in the city, this type of scenario occurs commonly when excavating for new additions next to existing structures. Class C & D soils are very common in DC. In soil types like this, an existing footing has an angle of repose that extends directly below the footing and outward to the sides.
Underpinning is the method to support the existing footing to avoid undermining the footing. Different methods of underpinning are available options, but common methods include needle beams, mass concrete, and mini-piled underpinning.