Uplift tests on full-scale pipe segment in lumpy soft clay backfill

Abstract

Upheaval buckling of pipelines caused by thermal- and pressure-induced loading is an important issue in pipeline design. The uplift capacity of pipelines is determined by the pipe–soil interaction during pipeline upheaval in soil. Pipelines to be installed in soft clay are usually placed into trenches and then backfilled. In this paper, a set of test devices were developed and a series of 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, and five linear variable differential displacement transducers (LVDTs) were arranged to record the vertical displacement of the pipe segment and the surface of the soft clay ground. The stabilizing force keeping the pipe segment in place during the backfilling process was found to fit a nearly linear relationship with the dimensionless undrained shear strength of soft clay. The 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 the soil. For all present load-controlled tests in lumpy soft clay backfill, the test ultimate uplift resistances were only about 19%–81% of the results calculated by the Det Norske Veritas approach. Mainly due to the voids’ compression, shearing and strain softening of lumpy soft clay backfill, the difference between initial and stable displacements in a loading step for a load-controlled test or initial and stable loads in a displacement step for a displacement-controlled test is remarkable. The limits of uplift resistances are recommended for the instant and sustaining behaviors of the pipe segment, respectively.