Seawall Stability Calculator

• Rankine coefficients: Ka = tan²(45° − φ/2), Kp = tan²(45° + φ/2) (or manual).
• Active earth thrust: Pa = 0.5·Ka·γs·H² + Ka·q·H.
• Hydrostatic thrust: Pw = 0.5·γw·hw² (front water reduces net drive).
• Wall weight: W = γc·A, with trapezoid area A = t·H + (B−t)·H/2.
• Uplift: U = ru·0.5·γw·(hw_b + hw_f)·B.
• Sliding safety factor: FS = (μ·(W−U) + c_b·B + Pp + Pw_front_relief) / (Pa + Fw + Pw_back_net).
• Overturning safety factor: FS = M_stab / M_over about the toe, using standard force lever arms.
• Bearing: q_avg = V/B, q_max/min = q_avg·(1 ± 6e/B), with e = |B/2 − x_R| and middle-third limit e ≤ B/6.

1. Enter seawall geometry: height, base width, and top width.
2. Provide material and backfill parameters: unit weights and friction angle.
3. Set surcharge, water levels (front and back), and wave force if needed.
4. Define base resistance: friction, cohesion (optional), and embedment depth.
5. Choose passive coefficient mode (auto from φ or manual).
6. Set targets for sliding and overturning, plus allowable bearing pressure.
7. Press Calculate to view checks below the header.
8. Use Download CSV or Download PDF for records.

1) What this calculator checks

This tool runs screening checks for a gravity seawall per meter: sliding, overturning, and bearing. It combines earth pressure, surcharge, hydrostatic thrust, optional wave force, and uplift, then reports safety factors, toe/heel pressures, and eccentricity.

2) Typical input ranges used on projects

Backfill unit weight is often 18–20 kN/m³, concrete or masonry 20–24 kN/m³, and water 9.81 kN/m³. Backfill friction angles commonly range 28–38°. Base interface friction δ is frequently 0.67–0.80 of φ, depending on roughness.

3) Earth pressure assumptions and implications

Active pressure uses Rankine-type relationships (Ka from φ), which suits level backfill and simplified geometry. If you have sloping backfill, wall batter, or significant wall friction, re-check with a method consistent with your design basis. A quick sensitivity run using φ ± 3° is useful.

4) Water levels, net thrust, and uplift

Back-of-wall water increases driving and reduces effective stress in the backfill. Front water provides counter-thrust that may relieve net drive. Uplift is modeled as a triangular distribution scaled by r_u; use lower r_u when drains and filters are reliable, higher r_u when relief is uncertain.

5) Wave force as an equivalent horizontal load

For early sizing, enter wave action as an equivalent horizontal line load per meter and apply it at an assumed resultant height. A common screening location is 0.4H–0.7H above the base, depending on runup and geometry. Final design should use site-specific wave analyses.

6) Sliding resistance components

Base resistance is mainly friction: (W − U)·tan(δ). Optional cohesion can be added only if supported by geotechnical data and durability considerations. Embedment may mobilize passive resistance, P_p = 0.5·K_p·γ·d², but consider scour and construction tolerances before relying on it.

7) Bearing pressure distribution and eccentricity

Bearing uses a linear stress block from the vertical resultant. If eccentricity e exceeds B/6, part of the base goes into tension, which is typically unacceptable for soil foundations. Use q_max, q_min, and e to guide width changes, drainage improvements, or foundation upgrades.

8) Turning results into design actions

If sliding is low, increase base width, roughen the base, add a key, or reduce uplift with drainage. If overturning is low, increase weight or lever arm by widening the base or modifying geometry. If bearing exceeds allowable, widen the base, improve ground, or consider deep support.

1) What safety factors should I target?

Common preliminary targets are sliding ≥ 1.5 and overturning ≥ 2.0, with bearing pressures within a geotechnical allowable value. Your owner or code may require different targets for extreme storms, seismic cases, or temporary works.

2) Should I use drained or undrained soil parameters?

Use drained φ and γ for long-term conditions and typical granular backfill. For short-term clay behavior, use undrained strength methods as directed by your geotechnical engineer. Match parameters to the controlling design condition.

3) How do I estimate the wave force input?

For screening, convert expected wave action to an equivalent horizontal line load per meter of wall and apply it at an estimated resultant height. For final design, use project coastal criteria and hydrodynamic calculations from a specialist.

4) When should I include passive resistance?

Include passive resistance only when embedment is reliable and scour protection is confirmed. Reduce or omit it when scour is likely, soils are soft, or construction variability is high. Conservative designs often limit passive contribution significantly.

5) What does eccentricity mean in the bearing check?

Eccentricity is the offset between the base centerline and the resultant vertical load location. Large eccentricity increases toe pressure and can cause heel uplift or tension. Keeping e ≤ B/6 helps maintain compression across the base.

6) Can this model a stem wall with heel and toe?

Yes, approximately. Represent the overall gravity section using base width, top width, and wall unit weight so the area and centroid reflect your shape. For complex geometry, break the section into components and verify moments separately.

7) Is this suitable for final design submissions?

It is intended for preliminary sizing and checking. Final design should confirm soil stratigraphy, drainage, scour, wave/impact actions, and construction staging. Always align the assumptions with project specifications and local standards.