Introduction

This calculator was developed in accordance with the latest AWWA M23 manual and the Handbook for PVC Pipe Design and Construction. The user is encouraged to reference both document if more information on external load calculations are needed. 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
  • WS = Additional Surcharge Load on the Pipe, psi
  • PS = Pipe Stiffness, psi
  • E’ = Modulus of Soil Reaction, psi

The Modulus of Soil Reaction is a composite value derived from the modulus of soil reaction of the pipe-zone embedment and the modulus of soil reaction of the native soil.

External Load Design

Pipe Diameter (D) 4 in.

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Pipe Stiffness (PS) 129 psi

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Unit Weight of Overburden (γS) 120 lb/ft3

lbs/ft3 κμ Material
0.1650 Dry Sand
0.1650 Ordinary (damp) Sand
0.1650 Wet Sand
0.1300 Damp Clay
0.1100 Saturated Clay
0.1500 Saturated Topsoil
0.1650 Sand & Damp Topsoil
0.1924 Granular materials Without Cohesion
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Burial Depth of Top of Pipe (H) 10 ft

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Bedding Constant (KX) 0.100

figure
Bedding Angle (θ) Constant, K
0.110
0.108
0.103
0.100
0.096
0.089
0.085
0.083
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Trench Width at Pipe Springline (Bd)” 0 in.

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Time Lag Factor (T L) 1.0

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

UNCOMPACTED Mod >85% or >90% High >95% or >100%
1 Crushed Rock
2 GW GP SW SP
3 GC GM SC SM sCL sML gCL gML
4 CL ML

Source: Pipeline Installation 2.0 by Amster Howard, Lakewood, Colorado, 2015.

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Modulus of Soil Reaction of the Native Soil (E’n) 1500 lbs/in2

Unconfined Compressive
Strength, Tsf ASTM D 2166
Description E'n - lbs/in2
0 - 0.125 Very, very, soft
0.125 -0.25 Very soft
0.25 - 0.50 Soft
0.5 - 1.00 Medium
1.00 - 2.00 Stiff
2.00 - 4.00 Very stiff
4.00 - 6.00 Hard
> 6.00 Very hard

Source: Pipeline Installation 2.0 by Amster Howard, Lakewood, Colorado, 2015.

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Live Load (W L) None

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Additional Surcharge Load (Ws) 0 psi

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