A series of wave flume experiments are conducted to investigate the wave-induced pore pressure around a trenched pipeline in a silty seabed. The pipeline is half or full buried with three kinds of backfill sand (d50 =0.15mm, 0.3mm, 0.5mm), each is coarser than seabed soil (d50 =0.04mm). Results show that the buried soil reduces the excess pore pressure around pipeline and provides a degree of resistance to seabed liquefaction, and the excess pore pressure around the circumference of pipeline or beneath the pipeline is larger in the coarser backfill sand, and decreases with the increase of backfilled depth.
In submarine structure, pipeline is an important structure to transport oil and gas, and the wave-induced seabed dynamic response has received great concern among coastal engineers and researchers for protection or exploitation, as such, extensive investigations have been carried out in both theoretical and experimental approaches. The mechanism of wave-induced seabed response mainly embodies in the manner of pore pressure generation, it can be divided into two mechanisms as reported by Zen and Yamazaki (1990), one is the oscillatory mechanism, the pore pressure oscillates periodically with the wave loading, and accompanied with the amplitude damping and phase lag (Madsen, 1978; Jeng, 2013), the other one is the residual mechanism, the excess pore pressure accumulates to a certain degree caused by contraction of soil under the cyclic wave loading (Seed, 1978), when the accumulation of excess pore pressure is excessive, the seabed will act as liquid without any resistance to shear loads, leading to failure of seabed (Zen et al., 1998), and be harmful to the stability of pipelines. Therefore, in a liquescent seabed, evaluations of wave-induced seabed response are essential for the safety of pipelines.
In engineering practice, pipeline trenching is needed to decrease the effect of the uneven seabed surface, or to be a shelter against the trawl gear, anchors. Besides this, backfill is also a good way to enhance the safety of pipeline. Zhai (2018) conducted a series of experiments to study the backfill influence of a trenched pipeline in a sandy seabed, the result showed that there is no accumulation of excess pore pressure in the seabed and no occurrence of liquefaction phenomenon, as for the amplitude of oscillating pore pressure, increases with the increase of the backfill material grain size. For the numerical analyses, most studies are based on Biot's consolidation model (Biot, 1941). Jeng (2001) used a finite element model to examine the internal stresses within a pipeline, and was extended to investigate the interactions between nonlinear waves, a buried pipeline and a porous seabed by Gao (2003). In a trenched condition, Gao (2006) developed a two-dimensional finite element model to study the non-linear wave-induced response of soil around a trenched pipeline. It was shown that shear modulus and permeability of backfill soils significantly affect the excess pore pressures around trenched pipeline, and that the effect of wave nonlinearity becomes more pronounced and comparable with that of trench depth, especially at high wave steepness in shallow water. Zhao (2014) considered the preconsolidation due to the specific gravity of the pipeline, used a numerical simulation to study the development of liquefaction zone beneath the pipeline, found that section of seabed is more difficult to reach the residual liquefaction state with larger backfilled depth and specific pipeline gravity. Lin (2016) studied the stability of a trenched pipeline under wave condition, and results indicated that a definite buried depth can decrease the possibility of arising scour and the maximum transient liquefaction depth around the pipeline, in some cases, partial or full burial is unable to restrain the occurrence of liquefaction, but the thickness of cover layer above the seabed surface to a certain degree can support the pipeline to be away from the liquefaction zone. Duan (2017) proposed a 2D coupled model with current condition involved, and investigated the influence of the water depth, backfilled depth, seabed saturation, permeability and the parameters of wave and current.
