Uplift Tests And Numerical Analysis On A Full-scale Pipe Segment In Lumpy Soft Clay Backfill

Upheaval buckling of pipelines caused by thermal-and pressure-induced loading is an important issue in pipeline design. Uplift bearing capacity of pipelines is determined by the pipe-soil interaction during pipeline upheaval in soil. Pipelines to be installed on land or in the offshore soft clay seabed are usually placed into trenches and then backfilled.In this paper, a set of test devices were developed, and full-scale model tests were carried out on a pipe segment buried in lumpy soft clay backfill, including backfilling tests, load-controlled uplift tests and a displacement-controlled test. Eight total pressure transducers were embedded in the wall of the pipe segment to measure soil pressures on the pipe segment in all tests, and5LVDTs were arranged to record the vertical displacement of the pipe segment and the surface of soft clay ground.The stabilizing force keeping the pipe segment in place during backfilling process was found to fit a nearly liner relationship with the dimensionless undrained shear strength of soft clay. Variation of soil pressures on the pipe segment during uplift loading was significantly affected by the buried depth of the pipe segment and the undrained shear strength of soil. For all present load-controlled tests in lumpy soft clay backfill, the test ultimate uplift resistances were only about19-81%of the results calculated by DNV (2007) approach. Mainly due to the creep of lumpy soft clay backfill, the difference between initial and stable displacements in a loading stage for a load-controlled test or initial and stable loads in a displacement level for a displacement-controlled test is remarkably. An upper-bound and a lower-bound uplift resistances are corresponding to the short-term and the long-term behaviours of the pipe segment, respectively. Their calculated approaches were commended according to the test results.In numerical analysis, the paper focused the impact of the presence of soft clay blocks on the uplift bearing capacity of the pipeline. It was obvious that soft clay blocks make a great reduction on the uplift bearing capacity of the pipeline, compared with the homogeneous soft clay. The reduction of the uplift bearing capacity could be reflected by a reduction factor which was found to fit a nearly linear relationship with the size of the soft clay blocks. And the linear formula was given for test S2T。...