Mud consolidation and desiccation
A layer of freshly deposited mud will consolidate under its own weight as pore water is gradually expelled upward to the free surface (and sometimes downward into the underlying sediment, a case not considered here). As consolidation proceeds, the mud layer becomes denser and its thickness decreases. Desiccation of mud occurs when the expelled water evaporates into the air.
Consolidation of freshly deposited mud is a slow process. The consolidation time is approximately proportional to the square of the thickness of the mud layer. Thin layers thus consolidate much faster than thick layers.
Poro-elastic consolidation model
A theoretical curve for the consolidation of a freshly deposited unconsolidated mud layer is shown in Fig. 1.
The consolidation degree is expressed by the factor
[math]U \equiv \dfrac{\Delta h}{\Delta h_{\infty}} \, , \qquad (1)[/math]
which represents the ratio of the layer compaction [math]\Delta h = h_0-h[/math] at time [math]t[/math] to the final compaction [math]\Delta h_{\infty} = h_0 -h_{\infty}[/math] after a very long time. The initial layer thickness is [math]h_0[/math] and the thickness at time [math]t[/math] is [math]h[/math].
If it is assumed that consolidation can be described as a poro-elastic process[2], the rate of consolidation of a thin mud layer depends on the dimensionless time
[math]t_v=\dfrac{c_v \, t}{h^2} \, , \qquad (2)[/math]
where [math]c_v=\dfrac{K}{\rho g (m_v+n \beta)}[/math] is the consolidation coefficient[1], [math]\rho=[/math] pore water density, [math]g=[/math] gravitational acceleration, [math]K=[/math] mud permeability (hydraulic conductivity), [math]m_v=[/math] coefficient of mud volume compressibility, [math]n=[/math] mud layer porosity and [math]\beta=[/math] pore water compressibility. The pore water compressibility is very small and can usually be neglected.
The consolidation curve of Fig. 1 can to a good approximation be represented by
[math]t_v\lt 0.6 \; : \; t_v=\dfrac{\pi}{4} U^2 \quad \text{and} \quad t_v\gt 0.6 \, : \; t_v=-0.085-0.405 \, \ln(1-U) \; . \qquad (3)[/math]
Common values of the mud consolidation coefficient are in the range [math] c_v \sim 10^{-8} \text{ to } 10^{-7} \; \text{m}^2/\text{s} [/math] although lower values may occur for highly organic or very soft estuarine muds. Fig. 1 then indicates that the consolidation of a mud layer of 1 m thick will generally take more than a year.
The value of the coefficient of volume compressibility [math]m_v[/math] for a particular mud sample is usually determined in the laboratory by means of an oedometer test. The coefficient of consolidation, [math]c_v[/math], can likewise be determined from the same test by observing the time-dependent settlement of the sample and interpreting the degree of consolidation using Eqs. (1) and (2).
The theoretical consolidation curve shown in Fig 1 is based on the simple poro-elastic model described in the appendix of the article Wave-induced seabed liquefaction. A more elaborate model of mud consolidation, taking account of the variation of the coefficients of permeability and compressibility as consolidation proceeds, was proposed by Gibson et al. (1981[3]).
Desiccation of mud
The poro-elastic consolidation model assumes that the mud deposit is fully saturated and horizontally uniform. However, when the mud surface is exposed to air, evaporation occurs not only from the expelled pore water at the surface, but also from the pore water within the upper layer of the deposit. As a result, this surface layer becomes unsaturated and starts to desiccate.
Desiccation generates matric suction in the unsaturated layer, which draws pore water upward from the underlying saturated mud by capillary forces. The increase in matric suction raises the effective stress and causes shrinkage of the mud skeleton[4].
Because this shrinkage is generally non-uniform and is partly restrained by the underlying material, tensile stresses develop within the surface layer. When these stresses exceed the tensile strength of the mud, desiccation cracks form. These cracks allow air to enter the soil and further accelerate drying.
This process produces the characteristic polygonal cracks on the surface of dried mud deposits [5] (see Fig. 2).
Related articles
References
- ↑ 1.0 1.1 Verruijt, A. 2001. Soil Mechanics. Lecture notes Technical University Delft
- ↑ Terzaghi, K. 1925. Erdbaumechanik auf Bodenphysikalischer Grundlage. Franz Deuticke
- ↑ Gibson, R. E., Schiffman, R. L., & Cargill, K. W. (1981). The theory of one-dimensional consolidation of saturated clays: II. Finite non-linear consolidation of thick homogeneous layers. Can. Geotech. J., 18:280–293
- ↑ Fredlund, D.G. and Rahardjo, H. 1993. Soil Mechanics for Unsaturated Soils. Wiley & Sons
- ↑ Kodikara, J.K., Barbour, S.L. and Fredlund, D.G. 2000. Desiccation Cracking of Soil Layers. Procs. Asian Conf. in Unsaturated Soils, UNSAT ASIA 2000. Balkema, pp. 693-698
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