6.4. Soil classification using group symbols

Coarse soils GW Well-graded GRAVEL GP Poorly-graded GRAVEL GM Silty GRAVEL GC Clayey GRAVEL SW Well-graded SAND SP Poorly-graded SAND SM Silty SAND SC Clayey SAND

This chunk outlines the classifications for coarse soils, which include various types of gravel and sand. The 'GW' symbol indicates well-graded gravel, which means it has a good distribution of particle sizes. 'GP' stands for poorly-graded gravel, indicating that the particle sizes are not well distributed. Silty and clayey variations of gravel are also specified by symbols 'GM' and 'GC', respectively. Similarly, symbols 'SW', 'SP', 'SM', and 'SC' are used for well-graded, poorly-graded, silty, and clayey sands. Understanding these symbols helps in quickly identifying the characteristics of coarse soils.

Think of coarse soils like a bag of mixed candies. Some bags have a variety of candy sizes (well-graded), while others may have mostly large candies with few small ones (poorly-graded). Each type of candy represents different characteristics that help you understand the overall quality of the bag.

Fine soils ML SILT of low plasticity MI SILT of intermediate plasticity MH SILT of high plasticity CL CLAY of low plasticity CI CLAY of intermediate plasticity CH CLAY of high plasticity OL Organic soil of low plasticity OI Organic soil of intermediate plasticity OH Organic soil of high plasticity Pt Peat

This chunk details the classifications for fine soils, including silts and clays. The 'ML' symbol refers to silts with low plasticity, while 'MI' and 'MH' refer to intermediate and high plasticity silts, respectively. For clays, the symbols 'CL', 'CI', and 'CH' indicate low, intermediate, and high plasticity clays. Organic soils also fall under distinct classifications, with 'OL' for low plasticity, 'OI' for intermediate plasticity, and 'OH' for high plasticity organic soils, alongside 'Pt' for peat. This classification is essential for understanding how these fine soils behave under varying conditions.

Imagine fine soils as different types of dough. Some doughs are easy to mold (low plasticity, like 'ML'), while others are stretchy and can be shaped in many ways (high plasticity, like 'MH'). Knowing what type of dough you have helps bakers decide how to use it effectively.

Activity: "Clayey soils" necessarily do not consist of 100% clay size particles. The proportion of clay mineral flakes (< 0.002 mm size) in a fine soil increases its tendency to swell and shrink with changes in water content. This is called the activity of the clayey soil, and it represents the degree of plasticity related to the clay content. Activity = (PIasticity index) /(% clay particles by weight) Classification as per activity is: Activity Classification < 0.75 Inactive 0.75 - 1.25 Normal

1.25 Active

This chunk explains the concept of soil activity, which relates to the clay content in soil and how it influences the soil's plasticity. Soil is not always made up of purely clay particles; therefore, the activity is calculated using the plasticity index divided by the percentage of clay particles by weight. The activity can be classified as inactive (less than 0.75), normal (between 0.75 and 1.25), and active (greater than 1.25). Soils with higher activity can expand and contract more significantly based on moisture levels.

Think of soil activity like a sponge. A sponge that absorbs a lot of water (active soil) will expand greatly, while a sponge that barely absorbs any water (inactive soil) stays much the same size. The more clay-like particles present, the more the 'sponge' expands and contracts.

Liquidity Index In fine soils, especially with clay size content, the existing state is dependent on the current water content (w) with respect to the consistency limits (or Atterberg limits). The liquidity index (LI) provides a quantitative measure of the present state. Classification as per liquidity index is: Liquidity index Classification

1 Liquid 0.75 - 1.00 Very soft 0.50 - 0.75 Soft 0.25 - 0. 50 Medium stiff 0 - 0.25 Stiff < 0 Semi-solid

This chunk discusses the liquidity index, which assesses the current state of fine soils based on their water content in relation to its consistency limits. The liquidity index is classified into several categories, ranging from liquid to semi-solid. This index is crucial for engineers and builders as it gives an understanding of how the soil will perform in different moisture conditions, which is important for construction projects.

Consider the liquidity index like the different states of a milkshake. If too much milk is added, it becomes liquid; if just the right amount is used, it may be thick and creamy (soft); and if there’s not enough milk, it becomes solid (stiff). Knowing the state helps you decide how best to drink or serve it!

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