Gabion terrace wall — retaining wall stability

The problem: an MM Local build — structural gabion retaining walls for terraced garden beds at home, max height 2 ft, gravel backfill (dirt on top for growing surface), no heavy equipment or vehicle loading. Goal: a defensible spec (basket size) for sign-off purposes.

Context: most jurisdictions require a PE stamp on retaining walls above roughly 3–4 ft — worth confirming locally, but a 2 ft garden wall with no vehicle loading typically sits under that trigger. This chapter builds real engineering literacy for specifying and sanity-checking these walls; MM Local's structural engineer partner still stamps anything that needs it as the business scales.

Concepts pulled in — new territory, soil mechanics, not just statics: Rankine active earth pressure theory. Soil doesn't push on a wall uniformly — pressure increases with depth in a triangular distribution, because it's the weight of soil above a given point that drives the push.

Assumed: gravel backfill, friction angle φ = 35° (standard for compacted crushed stone), backfill unit weight γ = 125 pcf, gabion stone-fill effective unit weight ≈ 100 pcf (accounts for voids between rocks), base friction coefficient μ = 0.5 (stone-on-soil).

Calculation

• Active pressure coefficient: Ka = (1−sinφ)/(1+sinφ) = (1−0.574)/(1+0.574) ≈ 0.27
• Max lateral pressure at base (h = 2 ft): p = Ka × γ × h = 0.27 × 125 × 2 ≈ 67.5 psf
• Total lateral thrust per linear ft of wall: F = ½ × p × h = ½ × 67.5 × 2 ≈ 67.5 lb/ft, acting 0.67 ft above the base

Stability check by basket width (front-to-back depth)

• 1.0 ft basket: wall weight/ft ≈ 200 lb, overturning FS 2.22, sliding FS 1.48
• 1.5 ft basket: wall weight/ft ≈ 300 lb, overturning FS 5.0, sliding FS 2.22
• Overturning FS = restoring moment (weight × arm to centroid) ÷ overturning moment (thrust × 0.67 ft). Sliding FS = base friction resistance (μ × weight) ÷ lateral thrust.

Answer

A 1.0 ft basket falls just under the typical 1.5 minimum safety factor for sliding — too tight to spec with confidence. A 1.5 ft basket depth clears both checks with real margin (sliding FS 2.22, overturning FS 5.0) and is still a standard gabion product size. Sign-off spec: 1.5 ft minimum basket depth for a 2 ft gabion wall on gravel backfill under garden loading only.

As-built spec (Zach's choice)

Going with a 2×2 ft basket — deliberate over-build, not a correction to the 1.5 ft minimum. The 1.5 ft code-minimum-plus spec (300 lb/ft, overturning FS 5.0, sliding FS 2.22) already clears standard safety factors with real margin; the 2.0 ft as-built spec (400 lb/ft, overturning FS 8.9, sliding FS 2.96) is roughly a third more stone and wire for a wall that's essentially immovable under these forces — intentional extra margin for permanence and peace of mind, not a fix for an unsafe design.

A working rule this chapter taught: FS ≈ 1.5 is code-minimum, already carrying margin for real-world imperfection. FS 2–3 is deliberate, reasoned extra margin. FS 5+ is where "over-engineered" becomes an honest label rather than caution — useful to build on purpose sometimes (a wall you never want to think about again), costly to do by default everywhere.

Open items for a later pass

Bearing pressure at the base (a quick check with these numbers is well under typical allowable soil bearing, but a real check should use site-specific soil data), and how the numbers change if any tier ever exceeds 2 ft — lateral thrust scales with height squared, so a jump to 3 ft roughly doubles the force, not adds 50%.