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🕳️ Deep Underground Atrium
A sunken daylit atrium, held back by a stiff diaphragm wall. The wall is braced and barely moves, so the soil never relaxes into the active state — you get the full at-rest pressure. Then there's the groundwater, which pushes at full hydrostatic intensity with no friction-angle discount at all. Drop the water table and watch what happens to your bracing loads.
Ground
Excavation depth H = 12.0 m
γ_dry = 18.0, γ_sat = 20.0 kN/m³
Friction angle φ = 30°
Wall Capacity
Design capacity: 900 kN/m total lateral force
Live Result
—
Total Lateral Force (kN/m)
—
Base Moment (kNm/m)
Effective earth pressure force—
Hydrostatic water force—
Water share of total load
🎯 Goal: keep total lateral force under 900 kN/m. Try dragging the water table from 12 m up to the surface — the total load rises about 66%, far more than the change in soil weight alone explains.
Powered by at-rest (K0) earth pressure with proper
Terzaghi effective-stress treatment below the water table, plus separate full
hydrostatic pressure (
engine/deep-excavation.js). Verified before
publishing against independent closed forms at both limiting cases (fully dry:
K0·γ·H²/2; fully submerged: K0·γ'·H²/2 plus γ_w·H²/2), and K0 was checked to
agree exactly with the existing engine/earth-pressure.js so the two
implementations cannot silently diverge. Scope: cohesionless soil, no seepage,
no arching, no construction-sequence effects — a teaching model, not a
substitute for real excavation design.Method
Jaky (1944)K0 = 1 − sinφ — at-rest coefficient, appropriate for stiff braced walls that don't deflect enough to mobilize the active state.
TerzaghiEffective stress — below the water table σ'v accumulates with the buoyant unit weight γ' = γ_sat − γ_w; pore pressure adds separately at full intensity, with no K0 reduction.