4.1.1. Dry Unit Weight Calculation

A soil's dry unit weight is an essential property that indicates how much a unit volume of soil weighs when it's completely dry.

Dry unit weight is calculated to understand how dense the soil is when there is no water content. This measurement is crucial for assessing the soil's stability and load-bearing capacity. It is calculated using the relationship between the bulk unit weight and the water content of the soil.

Think of dry unit weight like the weight of a sponge that's been fully wrung out. Just as the sponge's weight changes once it's dry, the soil's weight changes when moisture is removed.

The dry unit weight can be determined from bulk unit weight and water content. These inter-relations are vital for calculations in geotechnical engineering.

The relationships among the different soil properties allow engineers to derive one property from another. For example, knowing the bulk unit weight, which includes the weight of the water, and the water content, allows us to calculate the dry unit weight. This is important because direct measurement of dry unit weight may not always be possible.

Imagine two apples. One is a regular apple (bulk weight), and the other has been dipped in water (wet weight). If you know the weight of the wet apple and the amount of water it absorbed, you can figure out the weight of just the apple (dry weight). This principle of deriving one value from others applies to soil properties as well.

Example 1: A soil has a void ratio of 0.72, moisture content of 12%, and specific gravity (G) of 2.72. Determine its (a) dry unit weight, (b) moist unit weight, and (c) the amount of water to be added per m³ to make it saturated.

In this example, we are given three properties: void ratio, moisture content, and specific gravity. Using these properties and the equations related to unit weight, we can calculate the dry unit weight, moist unit weight, and how much water is needed to saturate the soil. Each calculation builds on the values provided. For instance, the dry unit weight is found to be 15.51 kN/m³, while the moist unit weight is 17.38 kN/m³, showing how water content affects weight.

Imagine filling a glass with a certain amount of water and then adding ice cubes. The total weight of the glass changes based on how much water and ice are in it, reflecting the concept of moist vs. dry weight in soils.

Example 2: The dry density of a sand with porosity of 0.387 is 1600 kg/m³. Find the void ratio of the soil and the specific gravity of the soil solids.

In this example, we calculate the void ratio from the given porosity and dry density. The void ratio is the ratio of the volume of voids to the volume of solids in the soil. This calculation helps in understanding how much empty space there is in the soil, which can affect its drainage and compaction properties.

Think of a container filled with balls and the spaces in between them. The void ratio tells you how much of the container's volume is empty space versus how much is occupied by the balls. This concept is similar to understanding soil structure where voids can hold water or air.

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