37.2. Stress-Strain Behavior of Soils under Cyclic Loading

Cyclic stress-strain curves show energy dissipation. Area inside the loop represents damping capacity. Loose soils show larger loops (more damping) but lower strength.

Hysteresis loops in cyclic stress-strain curves illustrate how soils respond to repeated loading. When soil is cyclically loaded, it doesn't return to its original position after unloading. Instead, it forms a loop on the stress-strain graph, representing energy loss (dissipation). The area inside this loop indicates the damping capacity of the soil. Loose soils, which are less compacted, generally exhibit larger loops because they can dissipate more energy, but they have lower overall strength compared to denser soils.

Think of a spring with a loose coil compared to a tightly wound one. When you stretch and release the loose spring, it wobbles back and forth, losing energy each time, similar to how loose soil behaves under cyclic loading. The larger the wobble, the more energy is lost in each cycle.

Shear modulus (G) decreases with increased cyclic strain. Dynamic modulus curves (G/G_max vs. shear strain) are used to model this.

The shear modulus (G) is a measure of a soil's ability to resist shear deformation. Under cyclic loading, as the strain increases, the ability of the soil to resist deformation declines, leading to a reduction in shear modulus. This relationship can be modeled using dynamic modulus curves, where the ratio of the current shear modulus to the maximum shear modulus (G/G_max) is plotted against the shear strain. As cyclic loading continues, soil becomes weaker, indicating that it is less capable of supporting structures above it.

Imagine a rubber band that can stretch a lot without breaking initially. However, after a while of constant stretching, it loses its ability to go back to its original size and becomes weaker. This is similar to how soils degrade under repetitive loading; they may start strong but weaken significantly with continued stress.

Occurs due to cyclic loading in undrained conditions. Leads to progressive reduction in effective stress, potentially to zero (liquefaction).

When soils are subjected to cyclic loading without allowing for drainage (such as during an earthquake), pore water pressures in the soil can increase excessively. This process is known as the build-up of excess pore water pressure. As pore water pressure rises, it reduces the effective stress within the soil, which is the stress that actually contributes to soil strength. If this effective stress approaches zero, the soil can undergo liquefaction, behaving more like a liquid than a solid, leading to significant instability.

Consider a sponge submerged in water. When squeezed (loaded) underwater and then released, it retains some water inside and may become very soft. If enough pressure builds up in the sponge (excess pore pressure), it can no longer hold its shape and starts to feel like a liquid. This is akin to soils losing their strength due to excess pore water pressure during seismic activity.

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