The effect of water in slopes
Water is normally associated with most of the failure mechanisms that a slope may experience. From various types of loading that increase the stresses acting on the slope, to the decrease in the shear strength of the soil, water appears as a fundamental factor to be considered in any stability analysis.
Contents
The water
When we deal with soil-related problems in general, we constantly mention water. But, what is water? What are its properties?
Water is, above all, a chemical compound, which has an atomic structure such that the hydrogen atoms form an angle of approximately 105° between the straight lines joining their centers with the oxygen atom, which gives it the property of bipolarity. Figure 1 shows a schematic of the molecule of this compound.
Figure 1 Water molecule (Source: modified from https://www.renovablesverdes.com/molecula-de-agua/).
The fact that water is a bipolar molecule implies that it is similar to a small magnet that can orient itself according to the magnetic field; and therefore, it is the one that allows it to react with other chemical elements, generating, for example, the expansion of clays.
Given its physical properties, water is a broad-spectrum solvent, in addition to being the most abundant substance in the earth's crust, and the only one found in the solid, liquid and gaseous states. The water cycle (which we described in the post "Where does water come from? The hydrological cycle") describes the presence and movement of water on Earth. It may be useful to take a look at it.
The solid as porous media
What condition must soil meet in order to interact with water? For a solid to contain water it must necessarily be porous, that is, it must contain spaces that can be occupied by water.
Porosity is usually defined as the ratio of "empty" (i.e., non-solid) spaces to total volume. However, this concept can be misleading, as can be seen from the analysis in Figure 2.
Figure 2 Diferent types of porosity. In the figure: roca vacuolar = vacuolar rock; roca con fracturas comunicadas = rock with communicating fractures; suelo granular = granular soil; arcilla = clay; roca con fracturas no comunicadas = rock with non-communicating fractures (Source: compiled from https://www.google.com/).
Five (5) materials with similar porosity are shown in the figure above. In the case of vacuolar rock (in which each pore is independent of another), if it contains any fluid, the fluid must have formed at the same time as the rock. In the case of granular soils, the pores are interconnected, and it follows that they formed at the same time as the solid phase. Fractured rock, on the other hand, may show interconnected fractures, or pores that are not connected, since they formed later than the rock. Finally, clays do not have pores per se, even though porosity is measured by the amount of water absorbed per unit volume. In this case, water, by means of a physical-chemical process and because it is a dielectric, opens up space to penetrate between the laminated clay crystals, which is why all clay is expansive in the presence of water.
According to the above, the existence of several types of porosity is evident, the most important for soil-water interaction being the one related to the circulation of water between the material, i.e., the so-called effective porosity.
Main effects of water in slopes
Most slope failures are related, in one way or another, to water, since water plays a very important role in most of the processes that generate either a decrease in the shear strength of the soil or an increase in stresses that affect the state of stresses in the soil mass.
Therefore, the soil-water interaction must be adequately understood, since it is one of the most important factors to be considered in slope stability analysis. The main effects of water as a trigger for slope failure, are as following:
Figure 3 Main effects of water as a trigger for a slope failure.
Let's take a look at each factor:
- Increase in soil weight, since the soil is a porous medium, it may eventually become saturated (due to the action of very heavy rains, for example, or by infiltration of water associated with anthropic effects), and thus considerably increase its unit weight, which favors the movement of the land mass.
- Decrease in resistance due to adsorbed water is particularly important in the case of clay materials, in which water is easily absorbed, adhering to the edges and faces of the solid particles, thus generating a decrease in the shear resistance between them
- In certain materials (such as some residual soils, or potentially collapsible soils), the flow of water through the soil pores generates the dissolution of the smaller mineral particles, which serve as bonds between the larger ones, causing the collapse of the soil structure
- Internal erosion, is associated with the flow of water between the pores of the ground, which can generate caverns, thus inducing the failure of the soil and the formation of a slope fault.
- The saturation of the soil causes the pores to fill with water, and if for some external reason an increase in pore pressure is generated (as may occur due to heavy rains, or infiltration of water from anthropic processes), the shear strength of the soil decreases, according to the well-known Law of Effective Stresses developed by Terzaghi.
Of course, all soils are affected by these processes. However, the nature of the materials will affect the speed with which these phenomena occur. In general, it can be said that in highly permeable materials, these effects can occur rapidly, in a matter of minutes, while in the case of low permeable soils, the changes are slower.
So, be careful with the effects of water! And consider them properly in slope stability analyses!
References
- Abramson, L; Lee, T; Sharma, S. & Boyce, G. (1996) “Slope Stabilization and Stabilization Methods”. John Wiley & Sons, INC. USA.
- Sancio, R. (1995) “Elementos de Hidrología e Hidrogeología”. I Curso Panamericano de Movimientos de Masas. Barquisimeto, Venezuela.
