Seismic analysis of large-diameter flexible underground pipes

A pseudostatic analysis method is proposed to identify the main causes of the transverse buckling of flexible circular underground structures, such as culverts and large-diameter corrugated metal pipes, during earthquakes. The method was initially developed to understand the collapse of a 2.4-m-diameter corrugated metal pipe in the Lower San Fernando Dam (LSFD), which was apparently caused by the intense near-field ground motion of the 1994 Northridge earthquake. However, a detailed field investigation revealed that this particular failure could not be attributed solely to either large ground accelerations or liquefaction of nearby hydraulic fills. The simplified analysis considers as factors leading to failure, static overburden pressure, peak ground acceleration, liquefaction-induced ground displacement, pore pressure buildup, and nonlinear soil response. The analysis reveals that the observed failure of the LSFD pipe was caused by the cyclic pore pressure build up in the embedding soils, and their resulting reduction in stiffness. It also points out that liquefaction was a contributing but not a necessary factor for failure. The proposed analysis, although developed for a particular case, is applicable for evaluating the response of flexible underground conduits subjected to earthquakes.