Q.: Soils in our area are stable enough for us to dig and form footings using the ground only. Is it permissible to build footings this way?
A.: Footings for residential and light commercial buildings are commonly formed this way in areas where the soils are cohesive enough to support a vertical face. However, Section 4.1.3 of Specifications for Structural Concrete (ACI 301-89) says that earth cuts shall not be used as forms for vertical surfaces unless required or permitted. Thus, on jobs with concrete specifications, you may need permission for earth forming from the engineer. You should also check local building codes.
The section on footings in Formwork for Concrete (ACI SP-4) says that when fabricated forms are omitted entirely and concrete is cast directly against the excavation, larger tolerances may apply. The book also suggests forming the top 4 inches when casting concrete directly against earth. This makes it easier to keep water and earth from washing into the excavation if it rains.
Concrete foundation contractor Buck Bartley prefers to use a combination of lumber forms and earth forming where the soil will hold an edge. His crews set 2x4 side forms then dig the remaining depth of the footing. Bartley says if crews use no wood forms at all it's much harder to build a level footing. And a level footing is the key to fast wall forming.