Urban Retaining Wall for Slope Stabilization in Seattle

Case Study 1: Urban Retaining Wall for Slope Stabilization in Seattle

Scenario A mixed-use development in Seattle’s Capitol Hill neighborhood required a 6-m-tall cantilever retaining wall to stabilize a steep, cut slope adjacent to an existing historic building. Site constraints included limited right-of-way (max 1.2 m excavation setback), strict settlement limits (<5 mm), and presence of glacial till overlain by weathered silt—requiring conservative soil parameters. Geotechnical investigation confirmed drained conditions (no groundwater at design depth), but surcharge from adjacent pedestrian walkway (15 kN/m²) was accounted separately per ASCE 7-16.

Given Data

• Angle of internal friction: 32°
• Unit weight of soil: 19.2 kN/m³
• Depth below ground level: 6.0 m

Calculation Using Rankine’s theory for cohesionless, level backfill (no wall friction or inclination):

1. Active earth pressure coefficient: ( K_a = \tan^2\left(45^\circ - \frac{\phi'}{2}\right) = \tan^2\left(45 - \frac{32}{2}\right) = \tan^2(29^\circ) \approx 0.306 \rightarrow \textbf{0.31} ) (rounded to 2 decimals)

2. Passive earth pressure coefficient: ( K_p = \tan^2\left(45^\circ + \frac{\phi'}{2}\right) = \tan^2\left(45 + 16\right) = \tan^2(61^\circ) \approx 3.27 \rightarrow \textbf{3.27} )

3. Vertical effective stress at 6.0 m: ( \sigma'_v = \gamma \cdot z = 19.2 \times 6.0 = 115.2 , \text{kN/m}^2 )

4. Lateral earth pressure at depth: ( \sigma'_a = K_a \cdot \sigma'_v = 0.306 \times 115.2 \approx 35.25 , \text{kN/m}^2 \rightarrow \textbf{35.25 kN/m²} )

Note: Total active lateral force per meter (for wall design) integrates this triangular distribution: ( P_a = \frac{1}{2} K_a \gamma H^2 = 105.8 , \text{kN/m} ); however, the tool reports point pressure at depth.

Result and Decision The calculated lateral pressure of 35.25 kN/m² at base level governed stem thickness and heel length. Combined with surcharge-increased moment, the design adopted a 0.45-m-thick reinforced concrete stem with 2.1-m-long toe and 1.8-m-long heel. A granular filter and weep holes were specified to maintain drained conditions—critical given Seattle’s high rainfall.

Lesson Even in drained, cohesionless soils, small increases in φ′ (e.g., 32° vs. default 30°) reduce Ka significantly—here by ~7%—directly lowering design loads; always use site-specific φ′ from consolidated-drained triaxial tests, not textbook defaults.