Design Pressure
Total design pressure is the sum of soil pressure, wheel loads, and vacuum. The thrust equation converts that pressure into a force per unit length in the wall.
P = PS + PW + PV
Wall Thrust
The thrust equation converts design pressure into force per unit length in the pipe wall.
T = P × DO / 24
Wall Area
The wall-area equation ensures the pipe cross-section can carry the thrust at the allowable stress for the plastic material.
A = T / σ
How It Works
Buried plastic pipes are checked for wall crushing by combining soil pressure, wheel loads, and vacuum into a total design pressure. The thrust equation converts that pressure into a force per unit length in the wall, and the wall-area equation ensures the pipe cross-section can carry it at the allowable stress for the plastic material.
Example Problem
A 24-inch HDPE pipe is buried with soil pressure of 300 lb/ft², wheel pressure of 150 lb/ft², and zero vacuum.
- Design pressure: P = 300 + 150 + 0 = 450 lb/ft²
- Thrust: T = 450 × 24 / 24 = 450 lb/ft
- Required wall area at 1,000 psi allowable stress: A = 450 / 1,000 = 0.45 in²/ft
When to Use Each Variable
- Solve for Design Pressure — when you know soil, wheel, and vacuum pressures and need the total external load, e.g., designing a buried HDPE drainage line.
- Solve for Soil Pressure — when you need to isolate the soil load from the total design pressure, e.g., analyzing the effect of changing burial depth.
- Solve for Wheel Pressure — when you need the surface traffic contribution, e.g., evaluating whether a plastic pipe can handle new equipment loads.
- Solve for Vacuum Pressure — when you need to determine internal vacuum from the total pressure and other loads.
- Solve for Wall Thrust — when you know design pressure and diameter and need the force per foot of wall, e.g., checking against the pipe's crushing capacity.
- Solve for Wall Area — when you know the thrust and allowable stress and need the minimum cross-sectional wall area, e.g., selecting a pipe wall profile that meets structural requirements.
Key Concepts
Buried plastic pipe design follows the same pressure-thrust-area approach as metal pipe but uses lower allowable stress values that account for plastic's creep behavior under sustained loads. The three-equation chain calculates total design pressure, converts it to wall thrust per unit length, and then checks whether the pipe's wall cross-section can carry that thrust at the material's long-term allowable stress.
Applications
- Municipal stormwater systems: designing HDPE and PVC storm drains under roads and parking lots
- Agricultural drainage: specifying buried plastic pipe for subsurface tile drainage systems
- Sanitary sewers: sizing gravity sewer mains in residential developments using plastic pipe
- Industrial effluent lines: routing chemical-resistant plastic pipe beneath plant roads and structures
Common Mistakes
- Using short-term burst stress instead of long-term creep-adjusted allowable stress — plastic pipes lose strength over time under sustained load; always use the manufacturer's long-term hydrostatic design stress
- Omitting vacuum pressure for pump-connected systems — rapid pump shutoff can create transient vacuum that adds to external loads and may collapse a thin-walled plastic pipe
- Ignoring temperature effects on allowable stress — elevated temperatures significantly reduce plastic pipe strength; HDPE at 60 degrees C may have half the allowable stress of the same pipe at 23 degrees C
Frequently Asked Questions
What types of plastic pipe does this apply to?
This calculator applies to HDPE, PVC, and other thermoplastic pipes that are checked for wall crushing under external loads per NRCS guidelines.
How is allowable stress determined for plastic pipe?
Manufacturers publish long-term hydrostatic design stress values that account for creep. For HDPE, typical values range from 800 to 1,600 psi depending on grade and temperature.
Why include vacuum pressure in the design?
If a pipe drains rapidly or is connected to a pump, internal vacuum adds to the external soil and wheel loads. Ignoring it can underestimate the total wall stress by 10–20%.
Reference: National Resources Conservation Service. National Engineering Handbook. 1995. USDA.
Related Calculators
- Buried Corrugated Pipe Calculator — design pressure and thrust for corrugated metal pipe.
- Buried Spiral Rib Pipe Calculator — pressure and thrust for spiral rib pipes.
- Pipe Vacuum Load Calculator — vacuum load per linear length of pipe.
- Pipe Wheel Loading Calculator — wheel load on buried pipes at various depths.
- Pressure Converter — convert between psi, kPa, bar, and other pressure units.
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National Resources Conservation Service. National Engineering Handbook. 1995. USDA.