Cyclically Loaded Foundations and Pipelines in Cohesive Soils

Abstract

This research addresses three thrust areas associated with cyclically loaded foundations and pipelines in cohesive soils. The first is thixotropy and sensitivity of Gulf of Mexico (GoM) clay. The second thrust area investigates the cumulative displacements due to nonuniform cyclic amplitudes using finite element method. The third studies seabed-riser interaction at the Touchdown Zone (TDZ) of pipelines/risers subjected to vertical, lateral, and zig-zag monotonic and cyclic loading. The first thrust area measures the remolded strength and the thixotropic strength recovery in GoM clay. To fill the existing gap in long-term measurements of thixotropic strength recovery in this clay, two techniques are used: the full-flow penetrometer (T-bar) and the miniature vane shear test. The second thrust area presents two parts 1) experimental results and 2) finite element (FE) investigations. The experimental tests investigate the degradation of vertical and lateral resistance and calibrate a constitutive model. The FEA investigates the cumulative deformations for different aspect ratios (L/D = 4, 5, and 6) of caissons subjected to inclined cyclic loading and different load angles. The third thrust area provides data for developing a model for 1) vertical cyclic loading tests of risers-soil interaction at TDZ, 2) lateral cyclic loading tests of pipelines/riser-soil interaction at TDZ and/or at seabed, and 3) zig-zag cyclic loading tests of pipelines/riser-soil interaction at TDZ and/or at seabed. The vertical cyclic loading tests evaluate the effects of cyclic loading, loading rate, load amplitude, riser embedment depth, and consolidation on the vertical stiffness of the soil supporting the riser in the TDZ. The lateral and zigzag cyclic loading tests of pipelines/risers evaluate the fundamental mechanism of the pipeline/riser-soil interaction in the lateral direction subjected to monotonic and cyclic loading, the evolution of lateral resistance with different (small to large) displacement amplitudes, the degradation of lateral resistance, and the recovery of the soil strength with time.