| Wall height (m) | Rate (฿ / m run) |
|---|
Active pressure (Rankine): Ka = tan²(45 − φ/2), adjusted for backfill slope β. Thrust acts on the vertical plane through the heel over full height Hw = H + t_base.
Pa = ½·Ka·γ·Hw² at Hw/3 · surcharge Ka·q·Hw at Hw/2.
Overturning FoS = ΣM_resist / ΣM_overturn (target ≥ 2.0). Sliding FoS = (μ·N + Pp) / ΣH (target ≥ 1.5). Bearing: eccentricity e = B/2 − (Mr−Mo)/N; if e ≤ B/6, q = N/B·(1 ± 6e/B); q_max must stay ≤ allowable.
Passive Pp = ½·Kp·γ·(Df+dk)² over the front embedment plus shear-key depth, counted only when the box is ticked. (Conservative — it ignores the extra soil-on-soil friction the key mobilises.)
Counterfort mode: the stem spans horizontally between ribs, so its governing moment drops to about p·s²/10 instead of the tall cantilever's ∝ H³. Overturning, sliding and bearing are unchanged — ribs redistribute the stem's load, not the wall's overall equilibrium.
Built as an interactive engineering aid — verify all output against a licensed engineer's stamped design.