4.1 Footing depth and spacing If the soil is a sand one cannot excavate to a depth greater than that of the existing foundation. The excavation depth of a o-c soil is limited by the equation: 2c 9o 2 (SF)yK (SF)Y4.1 Footing depth and spacing ▪ If the soil is a sand one cannot excavate to a depth greater than that of the existing foundation. ▪ The excavation depth of a φ-c soil is limited by the equation: ( ) (SF) q SF K c zf 2 0 = −