No / It is important to consider the potential instability of the seabed due to the accumulation of wave-induced pore pressure in the design of a composite breakwater as the pore pressure within the seabed can considerably build-up under waves loading and eventually leads to a sharp decrease of the effective stress. Due to the importance in practical engineering, many theoretical models have been developed to evaluate the magnitude and distribution of the residual pore pressure. However, most of these studies treat the soil skeleton as an invariant medium, which ignores the damping of the soil strength due to the reduction of the effective stress. In this study, a two-dimensional poro-elastoplastic model, in which the influence of the reduction of the effective stress on the soil strength has been considered, is proposed to investigate the accumulation of pore water pressure around a composite breakwater and its effect on the soil characteristics. The simulation results show that the liquefaction is likely to occur around the toe of the breakwater due to the accumulation of pore water pressure there. The liquefaction leads to the decrease of soil resistance, which has great effect on the development of the residual pore pressure. Analysis shows that the development of residual pore pressure is also greatly affected by both the wave height and soil permeability. The simulation demonstrates that if the decrease of soil resistance is not considered, the soil liquefaction depth will be overestimated. / National Natural Science Foundation of China (Grant No. 51479053), the 111 Project (Grant No. B12032), the marine renewable energy research project of State Oceanic Administration (GHME2015GC01), the Natural Science Foundation of Jiangsu province (Grant No. BK20150804), Colleges and Universities in Jiangsu Province Plans for Graduate Research and Innovation Projects (Grant No. B1504708) and the Distinguished Visiting Fellowship from the Royal Academy of Engineering

Yes / Most previous studies on seabed liquefaction around offshore pipelines focused on investigating the wave-induced pore pressure variation in sandy seabed, while limited studies have been conducted for silty seabed. In this study, laboratory experiments are conducted to investigate wave-induced pore pressure within silty bed around the buried or partially/fully backfilled pipeline. Results show that residual pore pressure is the dominant factor that causes the liquefaction in silty soil. For buried pipeline, liquefaction first occurs at the pipeline bottom, then propagates upwards and downwards vertically. Comparing with the buried pipeline, the liquefaction potential is reduced when the pipeline is placed in a trench. To protect pipeline from liquefaction, backfill is recommended. Experiments show that the residual pore pressure significantly decreases as backfill depth increases. Fully backfilled pipeline is the best choice for silty seabed. Furthermore, backfill material with coarser particle size than native soil provides better protection for pipeline. In this study, there is no residual pore pressure around the pipeline periphery for three backfill soils (d50 = 0.15 mm; 0.3 mm; and 0.5 mm) tested. Results indicate that for the range of this experimental study, d50 = 0.15 mm is the best backfill material that provides the most protection for the underneath pipeline. / National Postdoctoral Program for Innovative Talents granted by China Postdoctoral Science Foundation (Grant No. BX20190105) and the Fundamental Research Funds for the Central Universities (Grant No. B200202062).