Highway Embankment Support on Coastal Clay in Louisiana

Case Study 2: Highway Embankment Support on Coastal Clay in Louisiana

Scenario LA DOTD upgraded US-90 near Houma, requiring a 4.5-m-high MSE (mechanically stabilized earth) wall to retain a new embankment over soft, normally consolidated Gulf Coast clay. Due to rapid construction schedule and limited access, the design team used a temporary soldier pile–lagging system during construction, later transitioning to permanent MSE. Soil data from CPT and vane shear tests indicated low friction angle due to high water content; undrained analysis was inappropriate because the wall would be built in stages allowing partial consolidation. Thus, effective-stress Rankine analysis applied using consolidated-undrained (CU) triaxial φ′ values.

Given Data

• Angle of internal friction: 18° (low due to high plasticity index PI = 52)
• Unit weight of soil: 15.8 kN/m³ (saturated unit weight corrected for buoyancy not needed—groundwater table >10 m below surface)
• Depth below ground level: 4.5 m

Calculation

1. Active earth pressure coefficient: ( K_a = \tan^2\left(45^\circ - \frac{18}{2}\right) = \tan^2(36^\circ) \approx 0.529 \rightarrow \textbf{0.53} )

2. Passive earth pressure coefficient: ( K_p = \tan^2\left(45^\circ + \frac{18}{2}\right) = \tan^2(54^\circ) \approx 1.89 \rightarrow \textbf{1.89} )

3. Vertical effective stress at 4.5 m: ( \sigma'_v = 15.8 \times 4.5 = 71.1 , \text{kN/m}^2 )

4. Lateral earth pressure at depth: ( \sigma'_a = 0.529 \times 71.1 \approx 37.61 , \text{kN/m}^2 \rightarrow \textbf{37.61 kN/m²} )

Note: This unusually high Ka (vs. typical 0.3–0.35 for sands) reflects the low φ′—driving higher lateral loads and necessitating tighter reinforcement spacing in the MSE design.

Result and Decision The elevated lateral pressure (37.61 kN/m²) triggered redesign of reinforcement vertical spacing from 0.8 m to 0.6 m in the lower third of the wall and increased geogrid tensile strength from 25 kN/m to 42 kN/m. Settlement analysis confirmed acceptable long-term performance under combined embankment + traffic loading, validated via 6-month post-construction monitoring.

Lesson Low φ′ soils (e.g., plastic clays) produce disproportionately high Ka values—misapplying sand-based assumptions risks underdesign; always cross-check Ka against empirical correlations (e.g., from CPT-derived φ′) and validate with limit equilibrium software when Ka > 0.45.