Analysis of underground concrete pipelines subjected to seismic high-frequency loads

Abstract: Buried pipelines are tubular structures that are used for transportation of important liquid materials and gas in order to provide safety for human life. Such infrastructure systems crosses large areas with different geological conditions. During an earthquake, imposed loads from soil deformations on concrete pipelines may cause severe damages, possibly causing disturbance in vital systems, such as cooling of nuclear power facilities. The high level of safety has caused a demand for reliable seismic analyses, also for structures built in the regions that have not traditionally been considered as highly seismically active. The focus in this study is on areas with seismic and geological conditions corresponding to those in Sweden and Northern Europe. Earthquakes in Sweden are classified as intraplate events which for regions with hard rock may result in earthquakes dominated by high-frequency ground vibrations. Propagation of such high-frequency waves through the rock mass and soil medium affect underground structures such as pipelines.The aim of this project is investigating parameters that affect response of buried pipelines due to high-frequency seismic excitations. The main focus of the study is on reinforced concrete pipelines. Steel pipelines are also studied for comparison purposes. Two-dimensional finite element models are developed for dynamic analysis of pipelines loaded by seismic waves that propagate from the bedrock through the soil. The models describe both longitudinal and transverse cross-sections of pipelines. The interaction between pipelines and surrounding soils is accounted for, including a nonlinear behaviour. The pipelines studied are assumed to be surrounded by frictional soils with dense, medium and loose stiffness. The effects of water mass, burial depth, soil layer thickness and non-uniform ground thickness caused by inclined bedrock are studied. It is demonstrated how two-dimensional plane strain models can be used for seismic analysis of pipelines with circular cross-sections.The results are compared to those obtained for low-frequency earthquakes and the relationship between strong ground motion parameters and pipelines response is investigated. It is shown that the natural frequency of the models significantly depends on the soil type, soil layer thickness and non-uniformity of the ground. It is shown that, especially for high frequency earthquake excitations, non-uniform ground thickness due to inclined bedrock significantly increase stresses in the pipelines. For the conditions studied, it is clear that high-frequency seismic excitation is less likely to cause damage to buried concrete pipelines. However, the main conclusion is that seismic analysis is motivated also for pipelines in high-frequency earthquake areas since local variation in the ground conditions can have a significant effect on the safety.