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
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"Live as if you were going to die tomorrow. Learn as if you were going to live forever" Mahatma Gandhi
Texture, size and shape of soil particles
The solid part of a soil mass consists mainly of mineral particles and organic matter. The texture, size and shape of these particles, play a fundamental role in the behavior of the soil mass. Continue reading this post...
Texture of soils
The texture of a soil is related to its appearance and the way it "feels", and depends on the relative sizes and shapes of the particles that compose it, as well as their size distribution (Holtz et al, 2011).
That is why coarse-grained soils, such as gravels and sands, obviously have a coarse texture; whereas fine-grained soils, such as silts and clays, have a completely different fine texture. Figure 1 illustrates soils with these textures.
Figure 1 Texture of different soils: coarse soil at left and fine soil at right (Source: tomado de https://www.google.com/?hl=es).
The texture of soils, especially in the case of coarse soils, has a certain relationship with their engineering behavior. That is why most of the soil classification systems are based on this property.
As for fine soils, the presence of water significantly affects their engineering response, since it influences the interaction between mineral grains, generating two main effects: 1) it alters their plasticity (generally defined as the soil's capacity to be molded); and 2) it changes their cohesiveness (their capacity to hold together).
The separation between coarse and fine soils has been conveniently established as the limit at which individual particles can be observed with the naked eye, which corresponds to a size of approximately 0.075 mm, which is the aperture of the #200 sieve.
Soil particle sizes
As mentioned above, the size of soil particles, especially in the case of granular soils, has a certain effect on their engineering behavior. Accordingly, for classification purposes, it is important to consider not only the particle sizes present in the soil, but also the distribution of those sizes.
The range of possible sizes in soils is enormous. Soils can range from pebbles several centimeters in diameter to colloids, which are ultrafine materials whose interactions are governed more by electrostatic forces than by gravitational effects. This range is so wide that its variation is on the order of 108!
Figure 2 shows the divisions between various textural sizes, according to the USCS and the AASHTO System, usually applied in practice, as we have seen in previous posts.
Figure 2 Soil size ranges according to USCS and AASHTO soil classification systems (Fuente: modified from Holtz et al, 2011).
An important clarification: in Geotechnical Engineering, for the sake of simplicity, the term clay is used interchangeably to refer to: 1) specific minerals called clay minerals; and 2) soils containing these clay minerals. Since the behavior of some materials is strongly influenced by the presence of clay minerals, the term clay to refer to these soils should be replaced by soils containing sufficient clay minerals to affect their engineering behavior.
The bar graph shown in Figure 3 may help to understand the above. This graph is an analysis of the distribution of particles passing through the #200 sieve carried out on 220 samples. It is clearly observed that, in the cases of soils that classify as clays, the colloid content (particles smaller than 2μ) varies between 20% and 60%. The rest is silt.
Figure 3 Analysis of the distribution of particles passing the #200 sieve for different types of soils (Source: Boiero, 2019).
Based on the above, the simplification of the terminology clay used in practice must be properly understood, in order to avoid confusion in understanding soil behavior.
Importance of soil particle shape
The shape of individual particles is as important as the size distribution, particularly as it affects the behavior of granular soils.
While there are certain rules for quantifying the particle shape of coarse soils, developed by experts in petrology and sedimentology, in practical Geotechnical Engineering it is common (and sufficient) to describe particle shape qualitatively as part of the visual identification of soils.
Thus, coarse soils are usually classified according to the shapes shown in Figure 4.
Figure 4 Tupical shapes of coarse soil particles (Source: Youd, 1973).
On the other hand, as Cho et al (2006) mentioned, the size and shape of soil particles reflect the history of grain formation, while the macro-scale behavior of a soil mass is the result of interactions at the particle level, which are obviously affected by their shape.
These authors, by analyzing experimental data and the results of studies published up to 2006 on particle shape, confirmed that increasing particle angularity leads to an increase in the maximum and minimum void ratios (emax and emin). They also observed that the increase in particle irregularity leads to a decrease in stiffness and a consequent increase in compressibility under load.
As pointed out in this post, a better understanding of the texture, size distribution, and shape of soil particles will allow a better understanding of the behavior of the soil mass under external loads generated by civil projects.
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.
- Cho, G.; Dodds, J. & Santamarina, C. (2006) “Particle Shape Effects on Paccking Density, Stiffness and Strength: Natural and Crushed Sands”. DOI: 10.1061/(ASCE)1090-0241(2006)132:5(591).
- Holtz, R.; Kovacs, W. & Sheahan, T. (2011) “An Introduction to Geotechnical Engineering”. Second Edition. Prentice Hall. New Jersey, USA.
- Youd, T. (1973) “Factors Controlling Maximum and Minimum Densities on Sands”. Evaluation of Relative Density and its Role in Geotechnical Projects involving Cohesionless Soils. ASTM – STP 523. Philadelphia, USA.
