CEEN 545 - Lecture 23 - Soil Liquefaction (Part 1)

Summary: Soil Liquefaction, Part 1

The lecture introduces the mechanics of soil liquefaction, how engineers assess whether soils are susceptible, and the conditions under which liquefaction may begin.

1. Basic Mechanism

• Liquefaction is most readily explained using loose, saturated sand.
• When sheared, loose sand particles tend to rearrange into a more compact configuration.
• If drainage is possible, water escapes from the voids. The soil settles, and the particles regain contact and strength.
• If the soil is undrained, water cannot escape quickly enough. Excess pore-water pressure builds up, reducing effective stress and the soil’s shear strength.
• The loss of strength depends on the undrained loading condition. The lecture describes the particles as effectively “hydroplaning” on the water in the voids.

2. Two Forms of Liquefaction-Related Ground Deformation

• Flow liquefaction: A sudden, potentially catastrophic loss of strength. If the liquefied soil cannot support itself, it may flow downslope and produce large, rapid ground deformations.
• Cyclic mobility: Repeated earthquake shaking causes soil to deform progressively as pore pressure accumulates. It is generally less sudden than flow liquefaction but can still severely damage infrastructure. On slopes or near open channels, it can contribute to lateral spreading.

3. Three Questions Engineers Need to Answer

Liquefaction assessment is framed around three related questions:

1. Susceptibility: Is the soil the kind of soil that could liquefy?
2. Initiation: Given the soil’s condition, could the expected shaking trigger liquefaction?
3. Effects: If it liquefies, what ground deformations or damage might result?

A susceptible soil will not necessarily liquefy in every earthquake. Initiation also depends on the magnitude and duration of the loading.

4. Assessing Liquefaction Susceptibility

The lecture presents four broad types of criteria.

Historical Criteria

• Evidence that liquefaction has occurred at a site is a strong warning.
• Liquefaction tends to recur in locations that have liquefied before.

Geologic Criteria

• Deposits formed by water or wind—such as alluvial, fluvial, and aeolian deposits—are often more susceptible.
• Susceptibility is greater for young deposits, especially those formed within roughly the last 10,000 years.
• Shallow, saturated deposits are of particular concern. The lecture identifies depths under about 15 m, preferably under 10 m.
• Granular soils with few fines and relatively uniform particle sizes are commonly susceptible.

Compositional Criteria

• Poorly graded, cohesionless soils and gravel-to-coarse silt are identified as likely candidates.
• Some low-plasticity fine-grained soils can also be susceptible.
• The lecture contrasts two screening recommendations:
  • Bray and Sancio (2006): Soils with a plasticity index (PI) below 12 may be susceptible if the water content exceeds 85% of the liquid limit and the liquid limit is below 37%.
  • Idriss and Boulanger: A less conservative boundary is described as being near PI 7.
• For soils in the disputed PI range—roughly 5–7 to 12—the lecturer suggests considering laboratory testing.

State Criteria

• Whether a soil is loose or dense matters: loose soils tend to contract and may be susceptible, while dense soils tend to dilate.
• Critical void ratio: Casagrande proposed that soils tend toward a critical void ratio during shearing. This concept helped distinguish loose, contractive behavior from dense, dilative behavior.
• Steady-state line: Later undrained testing by Castro identified a steady-state line, which the lecture describes as a more practical boundary between contractive and dilative behavior under undrained shearing.
• In the lecture’s explanation, a soil state above the steady-state line is contractive and susceptible; a state below it is dilative.

5. Contractive and Dilative Soil Behavior

• Contractive soil: Tends to decrease in volume during shearing. Under undrained conditions, this tendency generates positive excess pore pressure, reduces effective stress, and can lead to loss of strength.
• Dilative soil: Tends to expand during shearing. Under undrained conditions, this can produce negative pore pressure (suction), increasing effective stress and strength.
• Intermediate behavior: Castro’s tests also showed soils that initially softened and then dilated. Casagrande called this “limited liquefaction”; the lecture notes that the terminology is not commonly used today.

6. Liquefaction Initiation and the Flow-Liquefaction Surface

• A soil may be susceptible without liquefying unless the loading drives its stress state far enough.
• In stress-path terms, the lecture describes a flow-liquefaction surface. When a soil’s stress path reaches this boundary, rapid strength loss begins and the path moves toward the soil’s steady-state strength.
• If the soil’s initial shear stress is greater than its steady-state undrained strength, sufficient loading can trigger flow liquefaction.

The lecture distinguishes two zones:

• Susceptible to flow liquefaction: The initial state is above the steady-state strength. Reaching the flow-liquefaction surface can cause a rapid, potentially catastrophic loss of strength.
• Susceptible to cyclic mobility: The initial state is below the steady-state strength. Repeated loading can still accumulate pore pressure and cause deformation, but it does not produce the same sudden strength drop.

7. Cyclic Mobility Scenarios and Controlling Factors

The lecture outlines three ways cyclic loading can lead to cyclic mobility:

• No stress reversal and peak stress remains below steady-state strength: Many cycles may be needed before the stress path reaches failure.
• Peak stress exceeds steady-state strength, but the initial state is below it: The stress path may touch the flow-liquefaction boundary during part of a cycle, increasing strain without producing catastrophic flow failure.
• Stress reversal occurs: The stress path crosses from positive to negative shear stress and back. The lecturer notes that this can accelerate excess pore-pressure generation and the onset of liquefaction.

The main factors governing initiation are:

• Whether the soil is contractive or dilative, as indicated by its position relative to the steady-state line.
• The duration of shaking and the number of loading cycles.
• The amplitude of shaking. Larger amplitudes, particularly those causing stress reversals, can accelerate pore-pressure buildup.

Speakers and Sources Mentioned

• Speaker: One unnamed lecturer; no other speakers appear in the subtitles.
• Researchers and technical sources cited by the lecturer:
  • Arthur Casagrande — critical void ratio concept.
  • Castro — undrained soil testing and steady-state behavior; the lecturer refers to work published in 1969 and 1976.
  • Bray and Sancio (2006) — susceptibility criteria for low-plasticity fine soils.
  • Idriss and Boulanger — alternative criteria for low-plasticity fine soils.
  • A plot is tentatively attributed by the lecturer to Boulanger and Idriss.