Sizing a retaining wall by "look" instead of a stability check? That's how walls tip over.
A retaining wall doesn't fail because the concrete is weak — it fails because the soil behind it pushes harder than the wall's own weight and footing can resist. NSCP 2015 sets the minimums — Section 206.6 requires a factor of safety of 1.5 against sliding and overturning and a minimum soil pressure, and Section 209 / Table 209-1 gives minimum lateral soil loads — while the calculation itself runs on classical soil mechanics: Rankine's earth pressure theory plus three stability checks any engineer runs by hand or by software. Here's exactly how that works, and a calculator that runs the numbers for you.
Soil behind a wall exerts lateral (sideways) pressure that increases with depth, like water against a dam. A wall resists that push with its own weight, the soil weight over its footing heel, and friction/passive resistance at its base. Three checks confirm it's enough:
The lateral soil push creates a moment trying to rotate the wall forward about its toe. The wall + footing + heel-soil weight creates an opposing moment. Resisting moment must exceed overturning moment by the required factor of safety.
The same lateral push tries to slide the whole wall forward along its base. Friction between the footing and soil (plus passive resistance from soil in front of the toe, often ignored for a conservative check) must resist that force with margin.
The resultant of all vertical loads must land within the middle third of the footing base (so the whole footing stays in compression), and the maximum pressure at the toe must not exceed the soil's allowable bearing capacity.
AEDO designs retaining structures, cut-and-fill slopes, and foundation systems using site-specific soil data — not assumed values.
Talk to a licensed engineerThe most common method for computing lateral soil pressure on a retaining wall is Rankine's theory, which assumes a smooth, vertical wall back and a horizontal soil surface (real walls often deviate from this, which is why more advanced projects use Coulomb's theory or software instead — but Rankine is the standard hand-calculation starting point):
φ (phi) is the soil's angle of internal friction — a measure of how much it resists shearing. Denser, coarser, well-compacted soils have higher φ (25°–35° or more) and push less; soft, saturated clays have low φ and push significantly more. γ (gamma) is the soil's unit weight, typically 16–20 kN/m³ for common Philippine backfill soils. Both values should come from an actual soil investigation, not a guess — see the section below on why this matters.
Enter your wall geometry and soil parameters to check overturning, sliding, bearing pressure, and eccentricity.
NSCP 2015 sets the two stability minimums directly. Section 206.6 requires retaining walls to resist sliding by at least 1.5 times the lateral force and overturning by at least 1.5 times the overturning moment, using allowable stress design (unfactored) loads. The same section sets a floor on the soil push: walls retaining drained soil with a level surface must be designed for an equivalent fluid pressure of at least 4.7 kPa per metre of depth, plus any surcharge. Section 209.1 and Table 209-1 then give minimum design lateral soil loads by backfill type — active pressure from 5 kPa/m for clean gravels and sands up to 10 kPa/m for clayey sands and lean clays — unless a soil report approved by the building official specifies otherwise. The remaining rows below are engineering practice, not Code values:
| Check | Minimum | What It Protects Against |
|---|---|---|
| Overturning | 1.5 — NSCP §206.6 (some engineers use 2.0 for higher-risk walls; practice, not Code) | Wall rotating forward about the toe |
| Sliding | 1.5 — NSCP §206.6 | Wall sliding forward along its base |
| Bearing capacity | qa from the soil report; FS of 2.5–3.0 on ultimate capacity is common practice (NSCP §304.1 only requires the report to state the FS used) | Soil crushing/settling unevenly under the footing |
| Eccentricity | e ≤ B/6 (resultant within middle third) — practice | Uplift at one edge — soil cannot resist tension |
Allowable bearing capacity qa normally already carries a safety factor (commonly 2.5–3.0 in practice) applied by whoever ran the soil investigation, who must state it in the report (NSCP §304.1) — you check your wall's maximum pressure directly against that allowable value, not the ultimate one. For sliding without a soil report, NSCP Table 304-1 gives presumptive base friction coefficients of 0.35 for gravels (GW, GP) and 0.25 for sands and silty/clayey gravels (SW, SP, SM, SC, GM, GC), multiplied by the dead load. For clay, sandy clay, silty clay and clayey silt it gives a sliding resistance of 7 kPa × contact area instead, and in no case may lateral sliding resistance exceed one-half the dead load.
Using textbook soil values instead of an actual site investigation. Assuming γ = 18 kN/m³ and φ = 30° for "typical" backfill is a reasonable first pass, but real Philippine sites vary enormously — reclaimed/fill areas, weathered volcanic soils, and typhoon-saturated clay can have dramatically lower φ (meaning much higher earth pressure) and lower bearing capacity than assumed. A wall that "passes" on assumed values can fail on real ground.
Because Ka = tan²(45° − φ/2), the relationship between friction angle and pressure is not linear. Dropping φ from 30° to 20° — a realistic difference between compacted granular fill and a soft, poorly-drained clay — increases Ka from about 0.33 to about 0.49, a roughly 47% increase in active pressure for the same wall height. That's the difference between a wall that comfortably passes its stability checks and one that fails overturning outright. This is exactly why a geotechnical investigation (test pits or borings, with lab or field shear tests) matters more for retaining walls than almost any other structural element — the input parameters swing the answer this much.
AEDO's engineers check overturning, sliding, bearing, and eccentricity against your actual soil data — with sealed structural plans your LGU will accept.
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A retaining wall that "looks thick enough" is a guess dressed up as engineering. Running the overturning, sliding, and bearing checks against your real soil data tells you exactly what footing width and toe/heel split your wall actually needs — before you pour a single meter of concrete.
NSCP 2015 Section 206.6 requires at least 1.5 against sliding and at least 1.5 against overturning, using allowable stress design loads, plus a minimum equivalent fluid pressure of 4.7 kPa per metre of depth. Section 209.1 / Table 209-1 gives minimum lateral soil loads by backfill type, and bearing pressure must stay within the soil's allowable bearing capacity. A 2.0 overturning factor used by some engineers is practice, not Code.
Rankine's theory: Ka = tan²(45° − φ/2). Total active force per meter of wall is Pa = 0.5 × Ka × γ × H², applied at H/3 above the base.
Overturning about the toe, sliding along the base, or bearing failure under the footing. A fourth mode — deep global/slope failure — requires a full slope-stability analysis beyond a single wall's stability check.
For anything beyond a low garden wall (roughly under 1.2m), yes. Assumed soil values instead of tested site values are one of the most common causes of under-designed retaining walls in the Philippines, especially on fill slopes or after typhoon-saturated soil.
