Abstract:Underground pipelines are the big arteries of present-day industry,agriculture,and city life.It is important to ensure the safety of pipelines in operation,especially under seismic loading.For underground pipelines,seismic damages can be classified as either wave-propagation damage or permanent ground-displacement damage.There have been some events where pipe damage has been due only to wave propagation.More typically,pipeline damage is due to a combination of hazards.However,the damage from large ground displacements typically occurs in isolated areas of ground failure and tends to be greater,whereas wave propagation tends to cause less damage.Large liquefaction-induced displacement (lateral displacement and settlement) is a potential source of major damage to underground pipelines during earthquakes.Therefore,soil liquefaction does major damage to underground pipelines during earthquakes.In order to analyze the damage to underground pipelines under a slope due to sand liquefaction,a three-dimensional nonlinear analysis was carried out to study the pipe characteristics damaged by liquefaction-induced large displacements using the FLAC finite-difference method and to analyze the displacement characteristics of the slope due to sand liquefaction and the pore water pressure buildup.A numerical model was established,which is similar to the real engineering project dimensions.The model consists of the saturated sand and dry sand layers,as well as the pipeline buried under the slope.The saturated sand on the foundation was modeled using a Mohr-Coulomb soil model coupled with a Finn model,which is the pore water pressure generation model.The dry sand of the slope was also modeled as a Mohr-Coulomb model without the pore water pressure generation model.The soil-pipe interaction was simulated by a bilinear elastic model,in which the elastic modulus before liquefaction is 103 times that after liquefaction.The base boundary was a rigid boundary.The calculation process is divided into two stages of static and dynamic analysis.In the initial static analysis,in order to compute the gravity stresses,the base boundary was fixed both horizontally and vertically,and the side boundaries were only fixed horizontally.In the dynamic analysis,free-field boundaries were used,and the sine waves were applied to the base boundary.After computing the static stress conditions,a time history dynamic analysis was carried out for sine wave velocities with different frequencies and amplitudes.It was shown that the occurrence of sand liquefaction and large displacement was caused by large sine waves.The displacement of the slope increased with time,which was different in the various parts of the slope.The displacement below the toe of the slope was bigger than that below the crest of the slope,and the sand above the slope had a trend of slipping into the foot of the foundation.The displacement of the pipe increased linearly in the first stage,and then increased nonlinearly with the increase in damage.The liquefaction-induced large displacement does damage to the buried pipe;the displacement of the pipe increases with an increase in the amplitude and frequency of applied sine waves.It is possible to use the nonlinear method to simulate the soil-structure interaction.It is necessary to find a simplified analysis method for predicting pipe damage.
