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Specific Gravity

# Specific Gravity - University of Missouri-Columbia Civil...

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University of Missouri-Columbia Civil and Environment Engineering MEASUREMENT OF SPECIFIC GRAVITY PURPOSE The purpose of this laboratory is to teach you how to measure the specific gravity of soil particles. The specific gravity of solids (G s ) is defined as the ratio of the weight in air, of a given volume of soil particles, at a stated temperature to the weight in air, of an equal volume of distilled water, at the same temperature. The specific gravity of solids is a parameter that is widely used in geotechnical engineering for weight-volume calculations. Although many engineers prefer to assume a value for G s rather than measure it, there are some applications where it is necessary to measure the specific gravity of solids directly, e.g., with organic soils (whose specific gravities may be less than 2.0) and with soils that contain heavy substances, such as iron, which may cause G s to exceed 3.0. BACKGROUND The specific gravities of many of the minerals that form soil have been measured and are known. Typical values for several soil-forming minerals are summarized in Table 1. The range in specific gravities is relatively narrow (2.54 to 2.86 for the data in Table 1), which is one reason why many engineers choose to guess specific gravity rather than to measure it. It is usually not possible to determine the specific gravity of soil solids from known specific gravities of various minerals because most soils are composed of an indeterminate mix of a number of minerals. One possible exception might be clean quartz sand, which is composed almost entirely of quartz with a specific gravity of approximately 2.65. Some engineers make it a practice to assume that the specific gravity of soils composed predominantly of sand-sized particles is 2.65 whereas G s for clays may be closer to 2.70 to 2.80. In order to measure the specific gravity of solids, we need some means for determining the weight of a particular sample of soil particles and the volume occupied by the particles. The weight is relatively easy to determine; we simply, weigh out a certain amount of soil and measure its water content so that the dry weight can be determined. To find out how much volume is occupied by the soil particles, we immerse them in water and determine the volume of water that is displaced, which must equal the volume occupied by the individual soil grains. EQUIPMENT AND SUPPLIES The volume of water displaced by a sample of soil particles will be measured in a glass container. Two types of containers are frequently used: Table 1 -- Specific Gravities of Several Soil-Forming Minerals (from Lambe and Whitman 1967). Mineral Name Specific Gravity Quartz 2.65 K-Feldspar 2.54 - 2.67 Na- or Ca-Feldspar 2.62 - 2.76 Calcite 2.72 Dolomite 2.85 Kaolonite 2.64 Illite 2.60 - 2.86 Montmorillonite 2.75 - 2.78 1. A volumetric flask (Fig. la) with a typical capacity in the range of 100 to 500 ml. An etch mark on the neck is used for control of volume. 2.

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Specific Gravity - University of Missouri-Columbia Civil...

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