Presented here are overall observations on the response of residential foundations to the Loma Prieta earthquake. These observations are based on field inspections across the entire damage area affected by the quake, review of numerous engineering investigations and repair recommendations, and many detailed interviews conducted with geotechnical and structural engineers with extensive experience in the EQ damage to the residential structures.

Foundation damage from the Loma Prieta Earthquake was observed as the result of induced ground movement as well as from literally shaking. EQ-induced ground movements which most commonly resulted in residential foundation damage were mainly the result of landsliding, soil densification, and liquefaction. The occurrence of these different phenomena mainly depended on geologic setting, geotechnical properties of the ground, and the magnitude of the shake. Overall, perimeter foundation systems were found to be the most sensitive to damage. 

Although the shaking damage was less significant than the ground induced damage, there were certain more prominent damage scenarios including tensile cracking of concrete stem walls and footings, floor sliding on foundation, racking of cripple walls, and racking post-and-pier foundations. In the Loma Prieta earthquake some houses were knocked completely off the foundation (see Figure 3).

Hillside homes resting on post-and-pier foundations present a structural situation where shaking is accentuated. This condition results because the entire residential structure is top heavy, likened to a heavy weight on a stick that is shook at its base. Consequently, because the posts are typically not sufficiently braced nor fixed into the ground, they become significantly racked and displaced as the house shook. Figure 4 is a photograph of this type of racking damage.