Retaining Wall Design Calculator
Calculate active, passive, or at-rest lateral earth pressure on a retaining wall given soil properties, wall height, and surcharge loading. Use it when designing gravity walls, cantilever walls, or checking overturning stability.
About this calculator
Lateral earth pressure on a retaining wall depends on the wall's movement relative to the soil. Three cases are recognised: Active pressure (wall moves away from soil): Pa = 0.5·γ·H²·Ka + q·H·Ka, where Ka = tan²(45° − φ/2) is Rankine's active earth pressure coefficient. Passive pressure (wall pushed into soil): Pp = 0.5·γ·H²·Kp, where Kp = tan²(45° + φ/2). At-rest pressure (no wall movement): P₀ = 0.5·γ·H²·(1 − sin φ) + q·H. In all formulas, γ is soil unit weight (kN/m³), H is wall height (m), φ is soil friction angle (degrees), and q is surcharge load (kN/m²). Active pressure is the lowest and most common design case for free-standing walls that can deflect. Passive pressure resists sliding and overturning. At-rest pressure applies to rigid, unyielding structures such as basement walls. The resultant force acts at H/3 from the base for triangular pressure, and must be checked for overturning, sliding, and bearing capacity.
How to use
Design an active-case wall: H = 4 m, γ = 18 kN/m³, φ = 30°, q = 10 kN/m². Step 1: Ka = tan²(45 − 30/2) = tan²(30°) = (0.5774)² ≈ 0.333. Step 2: Soil pressure term = 0.5 × 18 × 4² × 0.333 = 0.5 × 18 × 16 × 0.333 = 47.95 kN/m. Step 3: Surcharge term = 10 × 4 × 0.333 = 13.33 kN/m. Step 4: Total active force Pa = 47.95 + 13.33 ≈ 61.3 kN per metre of wall length. This lateral force acts eccentrically and must be balanced by the wall's self-weight, toe resistance, and foundation friction in a full stability check.
Frequently asked questions
What is the difference between active and passive earth pressure in retaining wall design?
Active earth pressure develops when a retaining wall moves or deflects away from the retained soil, allowing the soil mass to expand slightly and mobilise its minimum resistance. It represents the lowest pressure the soil can exert and is the standard design case for most retaining walls. Passive earth pressure develops when the wall is pushed into the soil — the soil is compressed and mobilises its maximum resistance, which can be three to ten times greater than active pressure depending on friction angle. Passive resistance is relied upon to resist sliding at the toe of gravity and cantilever walls. At-rest pressure falls between the two and applies when the wall cannot move, such as a basement wall braced by floor slabs.
How does surcharge load affect lateral earth pressure on a retaining wall?
A surcharge load (q) is a uniform pressure applied at the ground surface behind the wall — from traffic, stored materials, or adjacent structures. In Rankine's theory, surcharge adds a rectangular pressure block of intensity q·Ka (active case) or q·K₀ (at-rest case) over the full wall height, unlike the triangular soil pressure which varies with depth. This additional uniform pressure increases the total lateral force and shifts its resultant upward, creating a larger overturning moment about the wall toe. Engineers must include realistic surcharge values; underestimating surcharge is a common cause of retaining wall failures, particularly near loading docks or roads.
What factors of safety are required for retaining wall stability against overturning and sliding?
Standard geotechnical practice requires a minimum Factor of Safety of 2.0 against overturning (ratio of stabilising moment to overturning moment about the toe) and 1.5 against sliding (ratio of horizontal resisting force to total lateral earth pressure). For walls retaining water-saturated soils or in seismic zones, these minimums are often increased to 2.5 and 1.75 respectively. Bearing capacity of the foundation soil must also be checked, ensuring the maximum toe pressure does not exceed the allowable bearing pressure with a safety factor of 3.0 on the ultimate bearing capacity. These checks should all be performed simultaneously, as improving one stability mode can sometimes worsen another.