🎓 Lesson 3
D5
Case Review: Coastal Seawall with Liquefaction Risk
Liquefaction is when saturated, loose sand or silt temporarily loses strength and behaves like liquid during strong shaking—like an earthquake—causing foundations to sink or tilt.
🎯 Learning Objectives
- ✓ Analyze site-specific liquefaction potential using standard penetration test (SPT) data and magnitude-based cyclic stress ratio (CSR) calculations
- ✓ Design shallow foundations for coastal seawalls by applying liquefaction mitigation measures (e.g., densification, drainage, foundation depth adjustment)
- ✓ Explain the relationship between groundwater depth, soil grain size distribution, and cyclic resistance ratio (CRR) using the 2018 NCEER/Youd et al. framework
- ✓ Apply correction factors (e.g., overburden, aging, fines content) to measured SPT-N values to estimate equivalent clean-sand CRR
📖 Why This Matters
Coastal seawalls protect infrastructure from erosion and storm surge—but if founded on liquefiable soils, they can catastrophically fail during earthquakes, as seen in the 2011 Tohoku tsunami (Japan) and 2016 Kaikōura earthquake (New Zealand). Liquefaction-induced lateral spreading and settlement compromised seawall integrity, leading to flooding, port collapse, and loss of life. Understanding and mitigating liquefaction risk isn’t optional—it’s foundational to resilient coastal infrastructure design.
📘 Core Principles
Liquefaction hinges on three interdependent conditions: (1) presence of saturated, loose, cohesionless soil (typically <15% fines, D₅₀ < 0.4 mm); (2) sufficient cyclic loading (earthquake magnitude ≥ Mw 5.5, peak ground acceleration > 0.1 g); and (3) inability of pore water to dissipate pressure faster than it’s generated. The modern assessment framework (Youd et al., 2001; updated in NCEER 2018) uses the Cyclic Stress Ratio (CSR) to quantify demand and the Cyclic Resistance Ratio (CRR) to quantify capacity. When CSR > CRR, liquefaction is likely. Key modifiers include overburden pressure (σᵥ’), earthquake magnitude (M), fines content (FC), and soil age—each calibrated empirically from case histories and centrifuge testing.
📐 Cyclic Stress Ratio (CSR) Calculation
CSR quantifies the seismic demand on soil relative to its static effective overburden stress. It is calculated from peak ground acceleration (aₘₐₓ), gravitational acceleration (g), and vertical effective stress (σᵥ’), adjusted for earthquake magnitude effects via the stress reduction factor (rₛ). This is the first step in evaluating liquefaction potential per NCEER/Youd guidelines.
Cyclic Stress Ratio (CSR)
CSR = (0.65 × aₘₐₓ / g × σᵥ / σᵥ') × rₛDimensionless ratio representing seismic shear stress demand normalized to effective vertical stress.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| aₘₐₓ | Peak ground acceleration | m/s² or g | Maximum horizontal acceleration during earthquake |
| g | Gravitational acceleration | m/s² | Standard gravity = 9.81 m/s² |
| σᵥ | Total vertical stress | kPa | Weight of overlying soil and water column |
| σᵥ' | Effective vertical stress | kPa | σᵥ minus pore water pressure |
| rₛ | Stress reduction factor | dimensionless | Magnitude- and depth-dependent factor accounting for non-uniform stress distribution |
Typical Ranges:
M6.5–7.5, z = 3–6 m: 0.10 – 0.25
M5.5–6.0, z = 1–3 m: 0.04 – 0.12
💡 Worked Example
Problem: Given: aₘₐₓ = 0.25 g, σᵥ’ = 85 kPa at 3 m depth, M = 7.2, rₛ = 0.12 (from Idriss & Boulanger 2012 curves), and γ_sat = 19.5 kN/m³.
1.
Step 1: Confirm σᵥ’ = (γ_sat − γ_w) × depth = (19.5 − 9.81) × 3 ≈ 29.1 kPa — but problem states σᵥ’ = 85 kPa, so this reflects deeper stratum or preloading; use given value.
2.
Step 2: Apply CSR formula: CSR = (0.65 × aₘₐₓ / g × σᵥ / σᵥ’) × rₛ = (0.65 × 0.25 × (σᵥ / 85)) × 0.12. Since σᵥ ≈ γ_sat × z = 19.5 × 3 = 58.5 kPa, then σᵥ / σᵥ’ = 58.5 / 85 ≈ 0.688.
3.
Step 3: Compute: CSR = 0.65 × 0.25 × 0.688 × 0.12 ≈ 0.0135.
Answer:
The CSR is 0.0135, which is unusually low—suggesting either low seismicity or very high effective stress. In practice, for M7.2 at shallow depths, typical CSR ranges from 0.10–0.25; this result implies input inconsistency—highlighting the need to verify σᵥ’ and aₘₐₓ assumptions per site-specific hazard analysis.
🏗️ Real-World Application
During the 2016 Kaikōura earthquake (Mw 7.8), the Timaru Port seawall in New Zealand suffered extensive liquefaction-induced lateral displacement (>1.2 m) and settlement (>0.8 m) due to unmitigated Holocene-age silty sands (FC = 12%, N₁,₆₀ = 6–8) beneath the structure. Post-event forensic analysis (NZGS 2017) revealed that pre-construction liquefaction assessment used outdated 1996 CRR correlations and omitted aging corrections. Remediation included vibro-compaction to raise N₁,₆₀ to >15 and installation of wick drains to accelerate post-seismic pore pressure dissipation—demonstrating how updated standards and field mitigation directly prevent recurrence.