The blog of Alvaro Boiero
"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi
The blog of Alvaro Boiero
geo
"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi
Atterberg limits
Atterberg limits are fundamental to predict the engineering behavior of fine-grained soils, so their use is practically mandatory in any geotechnical project. But, what is their relationship with plasticity, how are they defined, and how do they help to understand the engineering behavior of soils? If you want to know the answers to these questions, continue reading this post...
Content
Water and fine-grained soils
As we have already mentioned in several posts, the presence of water in the voids of fine-grained soils significantly affects their engineering behavior.
Another very important characteristic to consider for this type of soils is plasticity, the physical characteristic that best defines the behavior of clayey soils. In fact, through simple manual tests, it is possible to distinguish between fine plastic soils (i.e. clays) and fine soils of low or no plasticity (i.e. silts). In addition, plasticity is also used to distinguish between different types of clays, depending on the degree of plasticity they possess.
Figure 1 Different soil particles and their relative sizes. Boxed: fine-grained particles (Source: modified from https://www.pinterest.es/pin/553942822895820606/).
An important aspect to take into account is that the plasticity of a clay depends on its moisture content. At this point, the so-called Atterberg Limits take on special importance, since these limits correspond to specific, unique moisture contents, which serve as a reference to estimate the expected engineering behavior of the soil.
Accordingly, in addition to moisture content, Atterberg limits are the most important parameters for describing fine-grained soils. That is why they are used in different soil classification systems (such as the Unified Soil Classification System and the AASTHO System, which we analyzed in this blog), as well as in numerous correlations to estimate parameters associated with the properties and engineering behavior of this type of materials.
Atterberg limits
Atterberg limits were developed in the early 20th century by a Swedish scientist named Albert Atterberg. This scientist conducted an extensive research on the consistency properties of fine-grained remolded soils, which is the basis of our current understanding of how water influences the plasticity of these types of materials.
Atterberg defined several behavioral limits for fine-grained soils, and developed manual tests to define them. The most commonly used in practice are shown in Figure 2
Figure 2 Atterberg límites (Source: modified from https://www.slideshare.net/1mirfan/geotechnical-engineeringi-lec-9-atterberg-limits).
During his experiments, Atterberg deduced that at least two parameters are required to define the plasticity of clays: the upper and lower plasticity limits. At the same time, he defined the plasticity index (PI), a parameter that represents the range of moisture content in which the soil has plastic behavior, and suggested for the first time that the PI could be used to classify soils.
One of the main practical problems related to the limits proposed by Atterberg was associated with the fact that they were arbitrary, and the tests to determine them were not easily reproducible in any laboratory.
In view of the above, in the 1920s, Terzaghi and Casagrande, working for the U.S. Bureau of Public Roads, standardized the Atterberg limits so that they could be used to classify soils.
Currently, in practical Geotechnical Engineering we routinely use the liquid limit (LL) and plastic limit (LP), not only to classify fine-grained soils (and the fine portion of coarse soils), but also to estimate numerous parameters from correlations. Also, when the presence of soils susceptible to expansion is suspected, the shrinkage limit (SL) is used, a parameter that provides a preliminary estimate of the expansive potential of this type of soils.
Atterberg limits and soil behavior
Since the Atterberg limits are specific moisture contents, at which the behavior of the soil changes, it is possible to plot these limits as a function of moisture content, as shown in Figure 3. This figure shows the general stress-strain response corresponding to the states derived from the variation of the moisture content of a fine-grained soil sample, and its proximity to the consistency limits.
Figure 3 Atterberg limits and the stress-strain response corresponding to each consistency state. (Source: available at https://link.springer.com/article/10.1007/s10064-019-01668-y).
From the figure above, it is evident that the soil increases in stiffness as the moisture content decreases, which is particularly visible for samples with moistures below the shrinkage limit.
In the case of the plastic behavior range of the soil (between PL and LL), the soil may present a variable response, depending on whether the natural moisture content is closer to PL (higher stiffness) or LL (lower stiffness, high compressibility).
In the case of moisture contents higher than LL, the soil is practically a viscous liquid, and therefore lacks rigidity for practical purposes.
With this in mind, it is possible to estimate (and, better yet, understand) the expected behavior of a soil sample being tested, simply by knowing its natural in situ moisture content and consistency limits. Something invaluable to "get the hang of" the soil in terms of its engineering behavior in any project.
With this we close this post. We already have a compilation of general information about the Atterberg limits. In future posts, we will be reviewing in more detail each of these limits, how to determine them, and their use in practical Geotechnical Engineering.
References
- Boiero, A. (2019) “Caracterización Geotécnica de Suelos”. Jornadas Especiales de Geotecnia 2019. Facultad de Ingeniería: Centro de Investigación y Desarrollo de la Ingeniería, UCAB. Caracas, Venezuela.
- Holtz, R.; Kovacs, W. & Sheahan, T. (2011) “An Introduction to Geotechnical Engineering”. Second Edition. Prentice Hall. New Jersey, USA.
