Design Pressure
Total design pressure is the sum of soil weight, wheel loads from surface traffic, and internal vacuum. Once you know the design pressure you can calculate the thrust in the pipe wall.
P = PS + PW + PV
Wall Thrust
The thrust equation converts design pressure into force per unit length in the pipe wall to verify it does not exceed the corrugation’s crushing strength.
T = P × DO / 24
How It Works
Buried corrugated metal pipes must resist three external forces: soil weight, wheel loads from surface traffic, and internal vacuum. The total design pressure is the sum of all three. Once you know the design pressure you can calculate the thrust (force per unit length) in the pipe wall to verify it does not exceed the corrugation’s crushing strength.
Example Problem
A 36-inch corrugated pipe is buried under 4 ft of soil. The soil pressure is 480 lb/ft², wheel pressure is 100 lb/ft², and vacuum pressure is 0. What is the design pressure and wall thrust?
- P = 480 + 100 + 0 = 580 lb/ft²
- T = 580 × 36 / 24 = 870 lb/ft
When to Use Each Variable
- Solve for Design Pressure — when you know the individual soil, wheel, and vacuum pressures and need the total external load on the pipe, e.g., checking a culvert under a new road.
- Solve for Soil Pressure — when you know the total design pressure and the other components and need to isolate the soil contribution, e.g., back-calculating soil load from field measurements.
- Solve for Wheel Pressure — when you need to determine the surface traffic contribution to total pressure, e.g., evaluating the impact of heavier truck traffic on an existing installation.
- Solve for Vacuum Pressure — when you need to isolate the internal vacuum contribution, e.g., assessing whether a rapid-drain condition adds significant load.
- Solve for Wall Thrust — when you know the design pressure and pipe diameter and need the force per unit length in the wall, e.g., checking against the corrugation's rated crushing strength.
Key Concepts
Buried corrugated metal pipe design checks wall crushing by combining three external forces: soil weight (increases with burial depth), surface wheel loads (decreases with depth as load spreads), and internal vacuum. The total design pressure converts to wall thrust using the pipe's outside diameter. Wall thrust must stay below the corrugation profile's rated crushing strength, which depends on the metal gauge and corrugation geometry.
Applications
- Highway drainage: designing corrugated steel culverts under roads and driveways per NRCS guidelines
- Agricultural drainage: sizing farm drainage culverts under field roads and equipment crossings
- Stormwater management: specifying corrugated metal pipe for detention system outlets and cross-drains
- Mining and construction: temporary drainage conduits under haul roads carrying heavy equipment
Common Mistakes
- Ignoring vacuum pressure for gravity-drain installations — even a few feet of water draining rapidly can create significant internal vacuum that adds to wall loading
- Using inside diameter instead of outside diameter in the thrust formula — the outside diameter determines the load-bearing circumference; using ID underestimates thrust
- Assuming deeper burial is always safer — while wheel-load pressure decreases with depth, soil pressure increases; there is an optimal depth range that minimizes total design pressure
Frequently Asked Questions
What is wall crushing in corrugated pipe?
Wall crushing occurs when external pressure compresses the corrugation profile flat. The pipe fails when the thrust per unit length exceeds the corrugation’s rated crushing strength, typically provided by the manufacturer.
How deep should corrugated metal pipe be buried?
Minimum cover is usually 12 inches for driveway crossings and 24 inches under roads. Greater depth reduces wheel-load pressure but increases soil pressure, so both must be checked.
Does pipe diameter affect wall thrust?
Yes. Thrust is proportional to outside diameter. A 48-inch pipe under the same pressure as a 24-inch pipe experiences twice the wall thrust, so larger pipes need stronger corrugation profiles.
Reference: National Resources Conservation Service. National Engineering Handbook. 1995. United States Department of Agriculture.
Related Calculators
- Buried Plastic Pipe Calculator — pressure, thrust, and wall area for plastic pipes.
- Pipe Soil Pressure Calculator — soil weight and buoyancy calculations.
- Pipe Wheel Loading Calculator — wheel load on buried pipes at various depths.
- Buried Spiral Rib Pipe Calculator — pressure and thrust for spiral rib pipes.
- Pressure Converter — convert between psi, kPa, bar, and other pressure units.
Related Sites
- Dollars Per Hour — Weekly paycheck calculator with overtime
- Compare 2 Loans — Side-by-side loan comparison calculator
- Medical Equations — Clinical and medical calculators
- InfantChart — Baby and child growth percentile charts
- Temperature Tool — Temperature unit converter
- LoanChop — Loan prepayment and extra payment calculator
National Resources Conservation Service. National Engineering Handbook. 1995. United States Department of Agriculture.