Wave-induced soil liquefaction

Definition of Soil liquefaction: When a water-saturated soil starts behaving as a fluid, losing stiffness and bearing capacity. This is the common definition for Soil liquefaction, other definitions can be discussed in the article

Submerged soils which are not well consolidated are prone to liquefaction when subjected to strong shaking. Soil liquefaction can be produced by earthquakes, but here we concentrate on submarine soils which are subjected to cyclic loading by waves. Structures built on freshly deposited submarine soils can collapse or sink when the seabed is liquified.

Two types of soil liquefaction

Two types of wave-induced soil liquefaction may be distinguished: Transient liquefaction (also called instantaneous or momentary liquefaction) and residual liquefaction. Both types can occur in poorly drained loosely packed soils.

Fig. 1. Principle of transient soil liquefaction. After De Groot et al. 2006

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Transient liquefaction

The upper part of the seabed can be liquefied if the pore pressure near the bed surface is higher than the pressure exerted by the overlying water mass. This occurs when the pore pressure in the seabed follows with delay (depending on the soil permeability) the oscillating water pressure exerted by surface waves. The soil top layer then experiences an upward force during the passage of the wave trough, which lifts sediment particles near the seabed surface out of the soil skeleton, see Fig. 1. The soil top layer becomes liquefied over a short period of time during which this condition prevails.

Residual liquefaction

We consider a loosely packed water-saturated soil with slightly compressible pore water due to the presence of a small air fraction. The soil consists of fine sediments – fine sand, silt and clay – with poor drainage capacity (low hydraulic conductivity). When such a soil is subjected to cyclic wave-induced loading and associated shear deformations, the soil grains tend to rearrange such that the soil skeleton is progressively compressed. (The reverse happens for a densely compacted soil.) The resulting reduction of the pore volume is associated with an increase of the pore water pressure. When the pore pressure exceeds the pressure exerted by the load of the overlying water and soil, the contact friction points between soil grains are broken and the soil skeleton collapses. The soil grains become unbound and completely free, carried by the pore water (Fig. 2). The soil begins to act like a liquid, losing its stiffness and bearing capacity. Any structure built on this soil will sink or collapse.

Fig. 2. Residual liquefaction-compaction process of a silt-clay soil. Silt grains are yellow and clay particles are red. Left panel: Skeleton of initial loosely packed silt-clay soil. Middle panel: Grain contact points are broken due to wave-induced grain rearrangement and pore pressure increase, the soil is liquified. Right panel: Final compacted soil after pore water evacuation.

The onset of liquefaction occurs first at the surface of the bed, and rapidly spreads out across the soil depth, causing the entire soil to behave like a liquid. When the liquefaction reaches the impermeable base, the soil begins to compact. The pores between particles become gradually larger, allowing pore water drainage and wave-induced compaction. Soil grains fall out of the liquid state, settle through the pore water until they come into contact with each other. The compaction gradually progresses from the impermeable base in the upward direction, and the entire sequence of the liquefaction/compaction process comes to an end when the compaction reaches the soil surface. After compaction, the soil will be much less susceptible to renewed liquefaction. The liquefaction-compaction process is depicted schematically in Fig. 3.

Fig. 3. Time variation of pore water pressure at a certain depth. After Sui et al. 2020.

Influence of soil composition

Flume experiments show that the liquefaction potential strongly depends on the soil composition. Experiments by Gratchev et al. (2006) revealed the significant influence of plasticity on soil liquefaction resistance. Soil vulnerable to liquefaction had an open microfabric in which clay aggregates mainly gathered at the silt particle contact points, forming low-strength 'clay bridges' that were easily destroyed during cyclic loading (Fig. 2). The contact friction between particles that maintained the soil skeleton structure was reduced by the clay aggregates, so grains could easily slip and rotate once the seabed was subjected to wave loading. Clay content thus enhances the plasticity and compressibility of the clay-silt seabed and increases the sensitivity of the seabed to liquefaction.

Experiments with kaolinite-fine sand and illite-fine sand showed that the susceptibility of a soil to liquefaction increases with increasing clay content if the clay content is low (0.5-5%), but decreases with a higher clay content. At a clay content of more than 10-30%, depending on the type of clay, the silt-clay mixture was not susceptible to liquefaction. This can be explained by the fact that in case of high clay content the silt grains are completely encapsulated in the clay matrix and therefore cannot rearrange under cyclic shear stresses, leading to resistance to liquefaction. Experiments by Kirca et al. (2014) showed that the sensitivity of a sand-clay mixture to wave-induced liquefaction not only depends on the clay content but also on the sand grainsize. Liquefaction occurred at a much larger clay content for medium sand than for fine sand or silt. The presence of shell fragments also decreases the susceptibility of silt to liquefaction. Experiments for a medium-sand-clay (kaolinite) seabed supporting a submerged breakwater showed that wave-induced liquefaction required a clay content greater than 40%. These experiment also showed that the initial water content in the soil is relevant to the liquefaction potential as it affects the pressure build-up process. Higher pore water fractions allow the residual pore pressure to accumulate faster.

The soil in an actual field situation usually has a long history of wave action, and hence it normally is a consolidated stiff soil, with low sensitivity to wave-induced liquefaction. This does not hold for freshly deposited soils. This may be the case, for example, where a pipeline is laid in a trench, and the trench is then backfilled.

Fluid mud can be considered liquified soil. However, cohesive soil is not liquefied by the process of pore pressure build-up. Fluid mud is formed when a high suspended concentration of fine cohesive particles settles on the sea floor and forms a dense colloidal suspension. See Fluid mud.

Preventing soil liquefaction

The susceptibility of a soil to liquefaction can be checked through Standard Penetration Tests or Cone Penetration Tests. Liquefaction risks can be reduced by measures such as replacing soft soil (expensive) or by consolidation measures. Soil consolidation can be done for example by vibro compaction or by preloading the soft bed with coarse sediment. Monitoring of the consolidation process should ensure that a stable situation is reached.

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References

The main author of this article is Job Dronkers Please note that others may also have edited the contents of this article. Citation: Job Dronkers (2023): Wave-induced soil liquefaction. Available from http://www.coastalwiki.org/wiki/Wave-induced_soil_liquefaction [accessed on 23-11-2024] For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Dronkers J