Seismic Stability Performance of a Coastline Slope Beneath a Pipeline: A Case History of Onshore Observations for Offshore Application

Abstract

Under the Pacific Ocean off the south coast of Peru the Nazca tectonic plate subducts beneath the South American plate, creating a high seismic hazard along this coast. Onshore site development and preparation for LNG plant and export terminal construction in this area required a significant volume of excavation throughout the process area and storage area, which are located on a plateau cut to elevation 125 m, separated from the marine loading terminal by coastal cliffs. Steep cuts were incised into the plateau to build a 3.5H:1V sleeper way to allow the LNG cryogenic pipeline system to traverse down the cliff onto a marine jetty to reach the loading terminal. Approximately 230 thousand cubic meters of spoils resulting from the excavation of the sleeper way were pushed over the cliff onto the undercliff area. Additionally, about 630 thousand cubic meters of the stripped material from the process and tank area was pushed over the cliff onto existing colluvial materials in the undercliff. Soon thereafter a crack measuring about 0.3 m in width with a vertical displacement of 0.4 m, and sympathetic cracking occurred sub-parallel to the shoreline at numerous locations across the undercliff area. A translational mass movement with a volume of approximately 18 million cubic meters was identified following the installation of inclinometers. Regrading of the spoil piles was designed and implemented causing the mass movement to stop. However, simplified and detailed slope stability analyses indicated that seismically-induced permanent slope displacements should be expected during significant earthquake events. A cumulative distribution function (CDF) that incorporated slope parameters and ground motion uncertainty was developed for slope displacements using the Newmark procedure. CDFs were developed conditional to the occurrence of the design earthquake events. Design displacements were selected based on owner risk tolerance and then incorporated into the design of the pipeline through: (a) the installation of a thicker wall at the location where displacements were expected; and (b) installation of an additional extensional loop on the pipeline. Shortly after completion of regrading to stabilize the mass movement, the slope was subjected to an M = 8.0 earthquake that caused permanent slope displacements in the range of 10 to 80 mm across the sliding plane. This paper describes slope performance from inclinometer and surface movement marker data following the M = 8.0 earthquake, comparison to the results of a probabilistic prediction of seismicallyinduced permanent slope displacements using simplified procedures, and discussion for the analysis and design of slopes subject to earthquake induced ground motions and incorporation into subsea design basis.