Soil Texture and Classification

Soil texture, a seemingly unassuming aspect of the Earth’s surface, holds profound significance in the construction industry. Beyond its humble appearance, soil texture plays a pivotal role in the success and safety of construction projects. In this comprehensive blog post, we’ll dig deep into the world of soil texture, exploring its nuances, the textural classification system, and its far-reaching implications for construction professionals.

The Essence of Soil Texture

Imagine soil as a complex blend of particles, each with its own size and shape. Soil texture is the art of deciphering this intricate blend. At its core, it’s a measure of the relative content of sand, silt, and clay within the soil matrix. These three components, in varying proportions, give soil its distinct texture.

Factors That Shape Soil Texture

Several factors come together to shape the texture of soil:

  1. Particle Size Distribution: The distribution of particle sizes within the soil is a primary determinant of texture. Sand particles range from a relatively coarse 0.05 to 2mm, silt particles are finer, spanning from 0.005 to 0.05mm, and clay particles are the finest, measuring less than 0.005mm.
  2. Particle Shape: The shape of these soil particles can vary significantly. Some soils contain angular, jagged particles, while others boast more rounded, smooth grains. These differences influence how particles interact and how tightly they pack together.
  3. Particle Gradation: Gradation refers to the arrangement of particles concerning size. A well-graded soil has a balanced distribution of particle sizes, while a poorly-graded soil might have an uneven distribution. Gradation impacts compaction and drainage characteristics.

The Textural Classification System

Now, let’s dive into the heart of soil texture classification. To make sense of this complexity, the United States Bureau of Public Roads employs a powerful tool—the textural classification system. This system employs an equilateral triangle to chart the percentages of sand, silt, and clay in a given soil sample.

This triangular diagram is divided into ten distinct zones, each corresponding to a particular type of soil. To determine the type of soil, a key is provided, indicating the direction in which lines should be drawn to locate the soil’s classification zone. For instance, if a point falls within the clay zone, the soil is classified as predominantly clayey.

Adapting for Coarser Particles

In some instances, soil samples might contain particles larger than 2mm, which poses a challenge to the standard classification. In such cases, a correction is applied to ensure the percentages of sand, silt, and clay sum up to 100%. For example, if a soil sample contains 20% of particles greater than 2mm, the corrected percentages become 15% sand, 30% silt, and 55% clay.

The Enigma of “Loam”

In casual conversation, you may have heard the term “loam” used to describe soil. Loam is a mixture of sand, silt, and clay in varying proportions, resulting in an ideal growing medium for plants. However, in the realm of soil engineering, the term loam is not commonly used. Instead, a modified triangular diagram, as proposed by the Mississippi River Commission, takes center stage.

The Right Triangle Chart

If you seek simplicity in soil texture classification, the Right Triangle Chart offers an alternative. It streamlines the process by representing clay and silt percentages on its two perpendicular sides. As the sum of clay and silt percentages invariably reaches 100%, plotting all three components becomes unnecessary. The percentage of sand particles can be calculated as 100% minus the sum of silt and clay percentages.

The lines where silt and clay intersect on the Right Triangle Chart provide the vital information needed for textural classification. This chart distinguishes itself by offering orthogonal lines, simplifying the task of precisely pinpointing the soil’s texture.

In Conclusion

Soil texture is the unsung hero of the construction world, silently influencing foundation stability, compaction efforts, and drainage systems. It’s a fundamental factor that construction professionals cannot afford to overlook. By wielding textural classification systems such as the triangular diagram and the Right Triangle Chart, experts in the field can make informed decisions, ensuring that construction projects stand strong on a solid foundation. Soil texture may be hidden beneath our feet, but its importance in construction is crystal clear.

Scroll to Top