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Response of buried steel pipelines subjected to longitudinal and transverse ground movement Karimian, Seyed Abdolhamid

Abstract

The performance of buried pipeline systems in areas subjected to ground deformations is an important engineering consideration, and there is a need for further research to advance the current fundamental understanding of this problem. A new full-scale physical modeling facility was developed to investigate soil-pipe interaction of relatively large diameter steel pipelines. The facility comprises a large soil box with the capacity to impose axial and lateral displacements on buried pipelines while simulating desired backfill and native soil configurations. Methods were developed to measure normal soil stresses on the pipe surface and observe movement of sand particles during testing. Numerical models, with parameters derived from geotechnical element testing and validated by physical modeling results, were used to conduct parametric studies of pipeline pullout response. The measured axial soil loads on pipes in loose sand were comparable with those from the commonly used ASCE-equation. Pipes in dense sand exhibited axial soil loads several-fold higher than those estimated from the same equation. The increase in normal soil stress on the pipe surface due to constrained dilation of sand in the shear zone was found to be the key reason for these high soil loads. For dilative soils, the equivalent lateral earth pressure coefficient (K) representing average normal stress distribution on the pipe is a more appropriate parameter for use in the ASCE-equation than the "at rest" lateral earth pressure coefficient (K₀). Based on numerical modeling, a series of charts and formulae were developed to obtain the value of K. The commonly used methods of wrapping pipelines with geosynthetic-layers were found to be generally effective in reducing axial soil loads. Measured lateral soil loads were generally lower than those estimated from existing guidelines. The outcomes from physical and numerical modeling were used to develop approaches to account for the effects of geometric, material, and interface parameters on the lateral soil loads. When in hard native soil, pipe buried in sand in a suitably wide trench with adequate horizontal distance from the trench boundary may effectively reduce the lateral soil resistance. In dual-geotextile-lined trench configurations, in addition to interface frictional characteristics, the relative stiffness of the native soil in comparison to the backfill and the ability of the backfill to move as a "cohesive block" become critical in reducing the lateral soil loads.

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