Frost Penetration Depth (Stefan / Modified Berggren)

Stefan X = sqrt(48 kf FI / L) balances the cold the air delivers against the latent heat of freezing the soil moisture: kf = frozen conductivity (BTU/hr-ft-F), FI = air-freezing index (F-days), L = 144 x dry density x water content/100 (144 BTU/lb is the heat of fusion). A soil at kf 1.0, 100 pcf, 15% water in a 2,000 F-day climate has L 2,160 and a Stefan depth of ~6.7 ft. Stefan over-predicts (it ignores soil heat capacity); the modified-Berggren lambda (~0.6-0.9, off the nomograph) cuts it to ~5.3 ft. CRITICAL: computes the PHYSICS, not the code frost line -- the footing depth is set by the locally adopted frost depth (IRC Table R301.2 / amendment), which this does not replace. A drier soil freezes deeper. A screen; the adopted requirement, the geotech report, and the AHJ govern.

L = 144 x dry_density x water_content/100 (BTU/ft^3); Stefan X = sqrt(48 x kf x FI / L) (ft); modified Berggren X_MB = lambda x X_Stefan, lambda ~0.6-0.9. FI in F-days, kf in BTU/hr-ft-F.

Stefan equation and the modified-Berggren frost-penetration method (US Army Corps of Engineers / FHWA pavement design), by name; the locally adopted frost-depth requirement, the geotechnical report, and the AHJ govern the footing depth.

The Stefan/Berggren heat-of-fusion relations are public (Army Corps / FHWA); the freezing index, conductivity, density, water content, and lambda are site and soil values the user supplies, and the code frost line is the adopted table.

Estimate. AHJ and licensed professional govern.

Field names used by the API: freezing_index_f_days, frozen_conductivity_btu, dry_density_pcf, water_content_pct, berggren_lambda, volumetric_latent_heat_btu_ft3, stefan_depth_ft, berggren_depth_ft

• Stefan / Berggren X = sqrt(48 kf FI / L); L = 144 x dry density x water content/100; X_MB = lambda x X_Stefan (lambda 0.6-0.9 from the nomograph)US Army Corps / FHWA
• Physics, not the code line Stefan over-predicts (ignores soil heat capacity); the jurisdictional frost line (IRC Table R301.2 / amendment), the geotech report, and the AHJ set the footing depth; drier soil freezes deeperadopted code / geotechnical practice