This calculator was developed in accordance with the equations and procedures found in the Handbook for PVC Pipe Design and Construction (5th Edition). The user is encouraged to reference the document if more information on external load calculations is needed. Conservative assumptions have been used to provide a conservative design with appropriate safety factors. Designers are responsible for the proper use of this program for all projects.

The formula used to calculate deflection is called the Modified Iowa Equation and is as follows:

Iowa equation

Where:

  • ∆Y/D = Percent Deflection, %
  • KX = Bedding Constant
  • TL = Time Lag Factor
  • WE = Prism Earth Load, psi
  • WL = Live Load on the Pipe, psi
  • PS = Pipe Stiffness, psi
  • E’ = Modulus of Soil Reaction, psi

External Load Design

Pipe Diameter (D) 4 in.

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Pipe Stiffness (PS) 46 psi

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Unit Weight of Overburden (γS) 120 lb/ft3

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Burial Depth of Top of Pipe (H) 10 ft

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Bedding Constant (KX) 0.100

figure

Time Lag Factor (TL) 1.0

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Modulus of Soil Reaction of the Pipe-Zone Embedment (E’b) 1000 lbs/in2

Soil Type - Pipe Bedding Material
(Unified Classification System)
E' for Degree of
Compaction of Bedding, PSI
Dumped Slight, < 85% Proctor, < 40% relative density Moderate, 85%-95% Proctor, 40%-70% relative density High, >95% Proctor, >70% relative density
Fine-Grained Soils (LL > 50) Soils with medium to high plasticity, CH, MH, CH-MH No data available; consult a competent soils engineer; otherwise, use E' = 0 No data available; consult a competent soils engineer;
otherwise, use E' = 0
Fine-Grained Soils (LL < 50):, Soils with medium to no plasticity, CL, ML, ML-CL, with less than 25% coarse-grained particles
Fine-Grained Soils (LL < 50): Soils with medium to no plasticity, CL, ML, ML-CL, with more than 25% coarse-grained particles Coarse-Grained soils with fines: GM, GP, SW, SP, contain more than 12% fines
Coarse-Grained Soils with Little or No Fines: GW, GP, SW, SC, contain less than 12% fines
Crushed Rock
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Live Load (WL) None

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