37.1. Soil Properties Affecting Dynamic Behavior

• Sands and silts with a uniform grain size are more susceptible to liquefaction.
• Well-graded soils offer more resistance due to tighter packing.
• Fine-grained soils (e.g., clays) typically do not liquefy unless they exhibit low plasticity.

The grain size distribution of soil significantly affects its behavior during seismic events. Soils with uniform grain sizes, such as many sandy or silty soils, are more likely to liquefy under stress. This is because their uniformity makes it difficult for the particles to interlock and provide strength. In contrast, well-graded soils, which consist of a mix of different grain sizes, can pack more tightly, offering greater resistance to liquefaction. Fine-grained soils, like clays, usually resist liquefaction unless they are low in plasticity, meaning they do not hold together well when wet.

Think of a bowl of marbles versus a bowl of sand. The marbles (uniform grain size) can easily move around each other, making the bowl feel loose under pressure. However, if you have a bowl filled with different-sized stones (well-graded), they pack tightly together and resist movement, much like how well-graded soil behaves under seismic loads.

• Loose sands (low relative density) are highly prone to liquefaction.
• Densification through compaction significantly increases resistance to liquefaction.

Relative density refers to the compactness of soil. Loose sands, which have a low relative density, are at high risk for liquefaction during an earthquake because they can easily collapse under stress and lose strength. On the other hand, densifying soil through compaction increases its density and improves its strength, making it much less likely to liquefy during seismic events.

Consider a jar filled with loosely packed marshmallows (loose sands). If you shake the jar (simulating an earthquake), the marshmallows shift and can collapse, causing a mess. However, if you compact them tightly (densification), they hold their shape better and resist collapsing when shaken.

• Influences the rate at which pore water pressures dissipate.
• Low-permeability soils trap water pressure, increasing liquefaction potential.

Permeability is a measure of how easily water can flow through soil. High permeability allows pore water pressures to dissipate quickly, reducing the risk of liquefaction. Conversely, low-permeability soils trap water, leading to increased pore pressure during seismic loading, thus enhancing the likelihood of liquefaction.

Imagine a sponge and a rock. The sponge can soak up water and release it quickly when squeezed (high permeability), whereas a solid rock doesn't let water pass through at all (low permeability). When you shake the sponge filled with water, it can release it. The rock, however, can buildup water pressure, which can lead to failure under stress.

• High void ratio indicates loose packing and high susceptibility to volume change under cyclic loading.
• Soils with high void ratios are more prone to pore pressure build-up, which triggers liquefaction.

The void ratio is the ratio of the volume of voids to the volume of solids in soil. A high void ratio means that the soil is loosely packed with many air spaces. Such soils can undergo significant changes in volume when subjected to cyclic loading, increasing the likelihood of pore pressure build-up and liquefaction. Thus, soils with a high void ratio are particularly vulnerable during seismic events.

Think of a pillow filled with air pockets (high void ratio). When you press down on it, it can compress significantly (volume change), which might eventually lead to a burst if too much pressure builds up. In contrast, a tightly packed cushion (low void ratio) won’t change shape easily when pressed.

• Clays with PI > 12 generally exhibit good resistance to liquefaction.
• Soils with low PI (non-plastic or slightly plastic silts) are more vulnerable.

The plasticity index measures the plasticity of fine-grained soils, indicating how well they can hold together when wet. Clays with a plasticity index greater than 12 are typically more resistant to liquefaction, as their cohesive properties help maintain integrity under stress. In contrast, soils with a low PI—like some silts—lack this cohesion, making them more susceptible to liquefaction during seismic activity.

Consider two types of dough: a strong, thick dough (high PI) that maintains its shape when rolled out, and a thin, crumbly dough (low PI) that falls apart easily. The strong dough can withstand pressure better, just as high plasticity clay maintains its structure better than loosely bonded soils during shaking.

• Full saturation (Sr ≈ 100%) is a critical requirement for liquefaction.
• Partial saturation allows drainage and reduces the risk of pore pressure build-up.

Saturation level refers to how much water fills the pores between soil particles. Complete saturation, where nearly all voids are filled with water, is essential for liquefaction to occur; it allows pore pressure to build up without any drainage. In contrast, partial saturation lets some water escape, helping to prevent excessive pore pressure and the risk of liquefaction.

Imagine a sponge underwater (full saturation), which can’t release water quickly when squeezed (high risk of liquefaction). Now, consider the same sponge on a surface where part of it is exposed to air (partial saturation). When pressure is applied, water can escape, reducing the risk of collapsing under pressure.

• Natural cementation or structured soils may initially resist liquefaction but may collapse suddenly once bonds are broken.
• Remolded soils are more susceptible.

Soil fabric refers to the arrangement and connectivity of soil particles, which can influence its stability during seismic events. Naturally cemented or well-structured soils can initially show resistance to liquefaction due to their strong bonding. However, once these bonds are broken—say during an earthquake—they can collapse suddenly. In contrast, soils that have been remolded or disturbed often lack this structural integrity, making them more susceptible to liquefaction.

Think about a set of Lego structures that are built with strong bonds (natural cementation); they may hold together well under pressure. However, if you shake them and some pieces break apart, the entire structure can fall apart quickly. On the other hand, if you mix those Legos loose (remolded), they won’t hold their shape at all when shaken.

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