Buried Flexible Pipe Deflection (Modified Iowa)
How much a plastic or thin-wall pipe ovals under its soil load, by the public-domain Modified Iowa formula: deflection % = DL x K x Wc / (0.149 PS + 0.061 E') x 100. A pipe under 12 ft of 120 pcf soil on good bedding (E' = 1,000) deflects 2.2% - fine - but drop the bedding support to E' = 200 and it jumps to 7.9%, past the 5% limit, on the same pipe. The compaction beside the pipe is the whole game; a mandrel test confirms it.
Formula and source
soil_load_psi = cover_ft x soil_density_pcf / 144; deflection_pct = DL x K x soil_load_psi / (0.149 x pipe_stiffness_psi + 0.061 x soil_modulus_psi) x 100.
Modified Iowa (Spangler) deflection formula by name (deflection = DL K Wc / (0.149 PS + 0.061 E')); public-domain (Spangler / Watkins).
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