This article was created due to the number of inquiries regarding the Maximum Allowable Combined Stress for Class 3 Design Areas. The research by M. G. Spangler, Battelle and industry allowed this information to be a standard practice today with Wheel and Track Load analysis today.
The M. G. Spangler work on overburden with vehicle loads on buried pipe includes Battelle research that is incorporated. Vehicle weight is the primary factor on the pipe to support this load; whereas, the soil is the secondary factor. There are other factors; however, this research focuses on these two loadings. Frictional forces between the trench wall and backfill are also considered in the PLTB as coefficients for various soils.
Because the pipe supports the entire load of a vehicle, the soil redistributes a great deal of the wheel or track load. Considering small segments of pipe wall thickness as beams allows a calculation of the bending stresses to be included. Between Spangler’s work on one layer (soil) and Battelle’s work on types of pavement or other bedding materials allowed these calculations to be more refined. Combining these calculations with longitudinal bending stresses due to the pipe settlement is also considered due to the deformation of soil in the bottom of the trench.
These calculations are based on a K factor equal to 20 psi and a length equal to 24 feet using a Diameter to thickness ratio equal 96. Transcontinental Gas Pipeline (Transco) now Williams Gas Pipeline Transco using the Battelle Petroleum Technology Report on “Evaluations of Buried Pipe Encroachments” and the work of M.G. Spangler on overburden and vehicle loads on buried pipes came up with the following table for Maximum Allowable Combined Stress.
It should be noted that the default value in the program is 13.9 psi. If a different loading is known to be the maximum, then it should be used as with the Williams Gas Pipe Line case.
