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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
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%.
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.
National Resources Conservation Service. National Engineering Handbook. 1995. USDA.