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. When the mud surface is exposed to air, evaporation of pore water causes further water loss, shrinkage and desiccation.
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.
Consolidation affects sediment erodibility. As the mud becomes denser and develops greater effective strength, its resistance to erosion generally increases. Recently deposited surface mud can therefore be much more easily resuspended than the older, more consolidated sediment underneath.
Poro-elastic consolidation model
After deposition, a concentrated mud suspension may first form a fluid-mud layer. When the concentration becomes sufficiently high for a continuous particle network to develop, the sediment begins to consolidate under its own weight as pore water is gradually expelled. As consolidation proceeds, the mud becomes denser and the layer thickness decreases. When an exposed mud surface loses pore water by evaporation, further shrinkage and desiccation can occur.
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 the soil deformation is small, compaction can to a first approximation be described as a poro-elastic process[2]. In that case, the rate of consolidation of a thin mud layer depends on the dimensionless time
[math]t_v=\dfrac{c_v \, t}{h_0^2} \, , \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 soil properties are assumed to be homogeneous and the drainage process extends over a fixed drainage path. It has further been assumed that the consolidation coefficient remains constant during the compaction process. In reality, freshly deposited mud can undergo large strains during consolidation, which changes the permeability and compressibility. Finite-strain consolidation models are then more appropriate. Equation (2) means that for otherwise similar mud properties and drainage conditions, consolidation time scales approximately with the square of the layer thickness.
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]
Values of the consolidation coefficient of order [math] c_v \sim 10^{-8} \text{ to } 10^{-7} \; \text{m}^2/\text{s} [/math] are often reported for soft muds, but lower values may occur for highly organic or very soft estuarine muds. Fig. 1 then shows that the consolidation of a mud layer of 1 m thick may require months to years.
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). Consolidation behavior depends strongly on sediment composition and pore-water chemistry because these determine floc structure, permeability and compressibility.
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 (It is the approximate solution of Eq. (8) with [math]\partial p' /\partial t =0[/math], a time-independent total stress [math]\sigma(z)[/math] and boundary conditions for the pore pressure [math]p=0[/math] at the surface and [math]\partial p/\partial z=0[/math] at the bottom). 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 homogeneous and fully saturated. 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). Desiccation generally strengthens the remaining mud matrix, but cracking creates discontinuities that can influence infiltration and the erosion of the bed when it is subsequently inundated.
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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