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Calculating Loads on Buried Culverts Based on Pipe Hoop Stiffness

Evaluating the hoop compression capacity of buried pipe, whether for total stress, local buckling capacity, or general buckling capacity, requires an accurate design model to compute the compressive thrust in the pipe wall. Flexible pipe has traditionally been designed based on the assumption that vertical soil load is the weight of soil directly over the pipe, known as the “soil prism load.” Field experience and research have shown that some pipe with low cross-sectional area and low modulus of elasticity can be buried at depths much greater than calculated by using the soil prism load, indicating that this load assumption is too conservative under some conditions. Investigation using the Burns and Richard elasticity solution for a circular tube embedded in an elastic medium shows that the ratio of the soil stiffness to the pipe hoop stiffness (EA/R) is often the controlling factor in determining the load on a buried pipe instead of the flexural stiffness of the pipe. The significance of the hoop stiffness factor for determining load on flexible and rigid buried pipe is explored here and development of a simplified design model for predicting loads based on the Burns and Richard theory is presented. The proposed equations are consistent with past practice and with tests showing that pipe with low hoop stiffness can carry far greater depths of fill than predicted by past practice.