37.10. Post-Liquefaction Behavior of Soils

As excess pore pressures dissipate, reconsolidation occurs. Significant settlement can take place, particularly in loose, saturated fills.

After a soil layer experiences liquefaction, excess pore water pressure starts to dissipate, meaning that the water trapped between soil particles begins to escape. This process is known as reconsolidation. As the pore pressures decrease, the soil structure can settle, leading to a drop in surface elevation. This settlement is particularly significant in loose and saturated soil fills because those materials were already weak and held together primarily by pore water pressure. Once that pressure is gone, the soil can compact more tightly, leading to noticeable ground settlement.

Imagine a sponge that you have soaked in water. If you squeeze the sponge, water seeps out, and the sponge compresses and becomes smaller. Similarly, when the pore water pressure in soil reduces after liquefaction, the soil particles can pack together more closely, causing the surface to settle.

Flow slides and lateral spreads are common in post-liquefaction conditions. Structures built on such soils may tilt or displace horizontally.

In post-liquefaction scenarios, the soil not only settles but can also undergo lateral displacement. This happens when the soil loses its strength and begins to behave more like a fluid. Flow slides occur when soil moves downhill due to gravity, while lateral spreads refer to the sideways movement of soil layers. Structures that are anchored in these liquefied soils are at risk of tilting or shifting position. The displacement is often uneven, which can lead to structural damage or failure.

Think of a bowl of jelly where you have a maraschino cherry sitting on top. If you shake the bowl, the jelly moves and can cause the cherry to roll off or tilt. Similarly, when soil beneath a structure becomes liquefied and moves laterally, the buildings above can be affected, causing them to tilt or tip.

Soil may regain part of its strength through reconsolidation. But original structure/fabric is often lost, leading to different engineering properties.

After the pore water in liquefied soils escapes and the reconsolidation process occurs, the soil can regain some of its shear strength, which is the ability to resist sliding. However, it often does not return to its original state. This is because the original soil structure or fabric, which is crucial for its strength, can be altered, resulting in different physical characteristics. The newly settled soil may behave differently under load as compared to its pre-liquefaction state, which can impact the performance of structures founded on it.

Consider a clay sculpture that you’ve accidentally dropped and reshaped while picking up the pieces. When you put the pieces back together, the sculpture may still hold some of its original shape, but it won't be as strong or stable as it was before. Similarly, the soil may regrow some strength after liquefaction and reconsolidation, but its overall strength is compromised.

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