Seismic Stability Assessment of a Mudmat on Liquefiable Seabed

Abstract

Abstract A mudmat supporting a pipeline end termination (PLET) sits on top of a silty sand deposit that could potentially liquefy during earthquake shaking. A conventional code-based bearing capacity analysis of the mudmat was initially performed that accounted for the effect of excess pore pressures generated during earthquake shaking on the shear strength of the sand. The excess pore pressures used in the code-based approach were estimated from one-dimensional free-field site response analyses. The conventional bearing capacity analysis predicted the mudmat would be unable to withstand the seismic design loads. As a result, two-dimensional (2D) non-linear seismic site response analyses (SRA) in a finite element program were performed using an advanced constitutive model. Excess pore pressures generated during the design earthquake for the free field case (i.e. no structure), and for the near-field case (with mudmat) were estimated. Then, a bearing capacity analysis was performed in the same finite element program using the generated excess pore pressure stress field for the near-field case. The 2D SRA results showed that liquefaction (excess pore pressure ratio equal or greater than 0.9) would only occur near the soil surface due to the low effective stresses. In general, the maximum excess pore pressure ratios (ru) were between 0.3 and 0.5 for the free-field case. The addition of the mudmat decreased the ru due to the added weight of the mudmat, which increased the effective stresses. In addition, the horizontal loading on the mudmat induces initial static shear stresses in the soil, which further decreases ru for the modelled relative density of the silty sand. The bearing capacity analyses predicted that the mudmat can tolerate considerably larger loads compared to the design loads during and following the design earthquake event. The case history presented in this paper provides an advanced design methodology for evaluating mudmat bearing capacity subjected to earthquake shaking on potentially liquefiable soil. In addition, it shows the benefit of performing advanced analyses with site-specific data, which reduces uncertainty and can provide increased reliability and significant cost savings to the overall project.