Soil Spatial Variability and Structural Reliability of Buried Networks Subjected to Earthquakes

Abstract

Dysfunctions and failures of buried pipe networks like sewer networks are studied from the point of view of the heterogeneity of geotechnical conditions in the longitudinal direction and of the applied action (seismic action). Combined soil defects (differential settlements along the pipe, landslides, voids surrounding the pipe, etc.) and peak ground acceleration (PGA) induce stresses (which leads to an ultimate limit state ULS) and displacements (which constitute a violation of a serviceability limit state SLS). It is remarkable to note that the influence of the variability of the soil is not reflected in current European standards. A model has been developed which includes a description of the soil spatial variability, within the frame of geostatistics, where the correlation length of soil properties is the main parameter and a mechanical description of the soil–structure interaction of a set of buried pipes with flexible connections resting on the soil by a two parameter model (Pasternak model). Reliability analysis is performed on the sewer by using a Response Surface Model (RSM), with the reliability index calculated for two limit states: Serviceability limit state, corresponding to a too large “counterslope” in a given pipe, which can prevent the normal flow of fluids, and Ultimate limit state, corresponding to a too large bending moment, thus bending stress, which can cause cracks in the pipes. The response in time domain of a buried pipe subjected to natural ground motion records and by taking into account a longitudinal variability of the properties of the soil is modeled. Several conclusions are drawn: Soil heterogeneity induces effects (differential settlements, bending moments, stresses and possible cracking) that cannot be predicted if homogeneity is assumed and the magnitude of the induced stresses depends mainly on four factors: the soil-structure length ratio, which combines the soil fluctuation scale and a structural characteristic length (buried pipe length), a magnitude of the soil variability (i.e. its coefficient of variation), a soil-structure stiffness ratio, and a structure-connection stiffness ratio (relative flexibility).