Earthquake inertia effects on shallow foundation bearing strength

Bearing strength surfaces, the locus of all combinations of vertical load, horizontal shear and moment that can induce shallow foundation bearing failure, can assist in envisaging the response of shallow foundations to general loading. Additionally, however, there is the question of the effect of earthquake loading on foundation bearing strength. Several researchers have investigated this from the perspective of the vertical capacity of the foundation. Not surprisingly, all show that the bearing strength under vertical load is reduced by inertia loads in the soil beneath the foundation. Somewhat surprisingly, these results are not directly relevant to the earthquake bearing strength calculations for shallow foundations, as the important role of the static actions sustained by the foundation prior to the earthquake may be overlooked. This technical note shows, using the bearing strength surfaces given in Part 5 of Eurocode 8, how it is that shallow foundation bearing strength for cohesive soils is not sensitive to the earthquake horizontal acceleration, but dry cohesionless soil may be more susceptible.