The sequence of sediment behaviour during wave‐induced liquefaction

Abstract

This paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves. The sediment was silty with d 50 ¼ 0.060 mm. Two kinds of measurements were carried out: pore-water pressure measurements (across the sediment depth), and water-surface elevation measurements. The process of liquefaction/ compaction was videotaped from the side simultaneously with the pressure and water-surface elevation measurements. The video records were then analysed to measure: (i) the time development of the mudline, (ii) the time development of liquefaction and compaction fronts in the sediment and (iii) the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment. The ranges of the various quantities in the tests were: wave height, H ¼ 9-17 cm, wave period, T ¼ 1.6 sec, water depth ¼ 42 cm, and the Shields parameter ¼ 0.34-0.59. The experiments reveal that, with the introduction of waves, excess pore pressure builds up, which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment, the sequence of processes known from a previous investigation. This sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples. The present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave-tank experiments. As for the final stage of the sequence of processes (formation of ripples), the ripple steepness (normalized with the angle of repose) for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history.

Experimental Study of the Development of Scour and Backfilling

“…The risk of liquefaction during the wave-only or wavecurrent runs has not been investigated experimentally in connection with the present tests and is assessed here with reference to the studies of Sumer et al [2006Sumer et al [ , 2007. They examined liquefaction of beds with grain sizes d 50 = 0.060 mm or d 50 = 0.147 mm.…”

“…Concerning the possible influence of liquefaction, Sumer et al [2006] found that liquefaction began in the upper bed layer with the smallest internal friction and led to compaction of the underlying soil layer until it eventually covered the entire depth and the liquefaction process had finished. A compaction process may still occur without liquefaction during weaker wave conditions but leads to comparably less compaction.…”

Wave-Induced Liquefaction and Floatation of a Pipeline in a Drum Centrifuge

“…Thus, the soil response observed in 1g wave tests cannot be properly extrapolated to field conditions. Sumer (2014) compared the buildup of residual pore pressure at shallow soil depth obtained from a 1g wave test on the bed of silt (Sumer et al 2006b) with that of centrifuge wave test on a bed of sand (Sassa and Sekiguchi 1999). Because the coefficient of consolidation [Eq.…”

“…(2)] was of the same order between them, the porepressure response of the 1g test was in good agreement with that of the centrifuge test. However, silt was used as the soil material instead of sand in the field (Sumer et al 2006b), as described previously.…”

“…When residual pore pressure u e reaches the level of initial mean effective stress level σ 0 m0 , mean effective stress does not reach zero (σ 0 m ave ≠ 0) in the seabed ground, because the principal stress ratio K increases in the laterally confined condition in association with increasing u e . The criteria for the onset of liquefaction by comparing σ 0 m0 with u e was adopted by several studies (e.g., Sumer et al 2006b;Jeng and Seymour 2007;Jeng 2013;Sumer 2014). In this criteria, however, there is room for further buildup of residual pore pressure after the residual pore pressure u e reaches σ 0 m0 and therefore, although marginal, some effective stress remains [Sumer (2014), with reference to ], and thus the soil is not in the state of liquefaction, as adopted in this study.…”