Earthquakes induce cyclic shear stresses in the soil strata as the stress waves propagate from the bed rock to the soil surface. Loose soils are known to suffer densification under shear stresses. During earthquakes there is no time for the volume change and consequently the pore pressure rises. These excess pore pressures in loose saturated sands cause full or partial liquefaction.
In Cambridge, many boundary value problems were investigated in which either full or partial liquefaction induces failure. From the soil dynamics point of view, it may be said that even when the soil is away from full liquefaction, the degradation in soil stiffnesscan bring a soil-structure system close to resonance. Experiments o tower structures have shown that the natural frequency of the tower-soil system suffers a sudden jump from its initially high value to earthquake driving frequency, there by resulting in resonance.
The new Stored Angular Momentum (SAM) actuator gives us the opportunity to investigate the near resonance behaviour of soil-structure systems as well as their dynamic behaviour far away from resonance.