5.2.3. Sedimentation Analysis

Sedimentation analysis is used only for the soil fraction finer than 75 microns. Soil particles are allowed to settle from a suspension. The decreasing density of the suspension is measured at various time intervals.

Sedimentation analysis focuses specifically on very fine soil particles that are smaller than 75 microns in diameter. This technique involves creating a suspension, which is a mixture where solid particles are dispersed in a liquid. As time progresses, these particles settle at different rates due to gravity, and the main objective is to measure how concentrated the particles remain in the suspension over time. By observing the changes in density at set time intervals, we can infer the behavior of these fine particles.

Imagine stirring a jar of sand mixed with water. Over time, if you stop stirring, you will notice that the heavier grains sink faster while finer particles remain suspended longer. Sedimentation analysis works similarly, but it uses precise measurements to determine how quickly and effectively these fine particles settle.

The procedure is based on the principle that in a suspension, the terminal velocity of a spherical particle is governed by the diameter of the particle and the properties of the suspension.

The terminal velocity principle states that a particle in a fluid will fall at a constant speed when the force of gravity pulling it down is equal to the resistance force of the fluid pushing against it. For sedimentation analysis, smaller particles will have lower terminal velocities. This means that their settling speeds are slower compared to larger particles, making it possible to study the sizes of the particles based on how quickly they settle.

Think of how a feather and a rock fall through the air. The feather drifts slowly due to air resistance, while the rock falls quickly. This difference in falling speed helps illustrate how sedimentation analysis determines particle sizes—it analyzes how quickly different-sized soil particles settle to understand their characteristics.

In this method, the soil is placed as a suspension in a jar filled with distilled water to which a deflocculating agent is added. The soil particles are then allowed to settle down. The concentration of particles remaining in the suspension at a particular level can be determined by using a hydrometer.

To carry out sedimentation analysis, soil samples are mixed with water and a deflocculating agent, which helps prevent clumping of particles. This mixture is placed in a container, and over time, the particles settle at varying speeds. A hydrometer is then used to measure how many particles remain suspended in the water at different levels, giving insights into the particle sizes based on their concentration in the solution.

Picture using a clear jar to observe candy corn sinking in water. You would notice that some pieces settle quickly while others float longer. By measuring how deep the candy corn sinks at various times, you could infer the sizes and weights of the candies. Similarly, sedimentation analysis allows scientists to deduce valuable information about soil particles by measuring what settles and how quickly.

Specific gravity readings of the solution at that same level at different time intervals provide information about the size of particles that have settled down and the mass of soil remaining in solution. The results are then plotted between % finer (passing) and log size.

After conducting the sedimentation analysis, the data collected from the hydrometer, like specific gravity readings, helps determine how much of the soil sample has settled and how much remains suspended. These measurements are crucial because they enable scientists to create a graph that illustrates the sizes of particles based on their settling patterns. This graph effectively portrays the distribution of particle sizes in the soil sample.

Imagine baking cookies and wanting to analyze the sizes of chocolate chips you used. If you put them in water, you could observe how many sink to the bottom after some time. By recording the amounts at different time intervals, you can determine the distribution of chip sizes. Similarly, sedimentation analysis helps reveal the complete profile of particle sizes in soil by using collected data to create informative graphs.

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