Parametric study of frost-induced bending moments in buried cast iron water pipes

Abstract

While the features of frost susceptible soils have been examined in various studies, the mechanisms by which volume changes due to ground freezing can influence cast iron water pipes buried below the frost line have not been explained, and the hypothesis that frost-induced ground deformations can induce ring fractures due to longitudinal bending of these pipes has not been proven. Therefore, a parametric study employing three dimensional finite element analysis is reported, where the soil–pipe interaction associated with a pipe crossing under an intersection of a major arterial road with a residential street are examined. The arterial road is modeled as having non-frost susceptible sub-base and the local street is represented as having a lower grade pavement with frost susceptible sub-base. One specific frost loading case featuring both isotropic pore water expansion and orthotropic ice lens formation is modeled.The analysis demonstrates how volume changes due to ground freezing in soil strata above the buried pipe can induce bending moments sufficient to cause ring fracture. Changes in the relative axial stiffness of the pipe were found to have only a small effect on pipe moments. Decreases in the relative flexural stiffness of the pipe resulting from reductions in pipe modulus also had only a small effect on pipe deflections and normalized moments. Changes due to soil modulus had a significant effect on deformations, but little influence on moments. Decreases in pavement stiffness decrease pipe deflections and moments. Trench backfill conditions greatly affect deflection and moment. Reduction in burial depth from 2m to 1.5m increases deflections, and increases moments beyond the failure capacity of the grey, cast iron pipe considered in the study, and this computational result is directly supported by field evidence.