Specific Gravity

November 11, 2017 | Author: Christine Joy Casas Agquiz | Category: Soil Mechanics, Soil, Density, Applied And Interdisciplinary Physics, Physical Quantities
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ME 420L/ME 506L: Soil Mechanics Laboratory Lab 2: Specific Gravity of Soil Solids ASTM D-854

Introduction The specific gravity of a substance, designated as Gs, is defined as the ratio of the density of that substance to the density of distilled water at a specified temperature. Since it is a ratio, the value of Gs does not depend on the system of units used and is a numerical value having no units. In soil mechanics, the specific gravity of soil solids is an important parameter and is a factor in many equations involving weight-volume relationships. Remember that the specific gravity of soil solids refers only to the solid phase of the three phase soil system, it does not include the water and air phases present in the void space. For soil solids, Gs may be written as:

𝐺𝐺𝑠𝑠 =

𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 π‘œπ‘œπ‘œπ‘œ π‘‘π‘‘β„Žπ‘’π‘’ π‘ π‘ π‘œπ‘œπ‘œπ‘œπ‘œπ‘œ 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘œπ‘œπ‘œπ‘œ 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 π‘œπ‘œπ‘œπ‘œ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘œπ‘œπ‘œπ‘œ π‘Žπ‘Žπ‘Žπ‘Ž 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 π‘œπ‘œπ‘œπ‘œ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

The procedure which will be used in this lab is applicable for soils composed of particles which are less than 4.75 mm in size, i.e. will pass through a number 4 U.S. sieve.

Procedure 1. 2. 3. 4.

5.

6.

7. 8.

Fill the volumetric flask to the 500 ml mark with de-aired, distilled water. Using the scale, determine the mass of the flask and water filled to the 500 ml mark, M1. Insert a thermometer into the flask to determine the temperature of the water, T1 Β°C. Place the appropriate amount of air-dried soil into an evaporating dish. The lab TA will tell you the amount of soil to weigh out; the amount is based on the type of soil which is being tested. Soil types will be discussed later on in the semester, for now don’t worry about its meaning. If the soil you are working with is cohesive, e.g. sticks together when moist such as a clayey soil, then add enough water to form a smooth paste. Keep it soaked for one-half to one hour. This is an important step to ensure that there will be no entrapped air present in the soil sample. If your soil is granular, e.g. sandy, this step is not necessary. Transfer the soil into a clean volumetric flask and then add distilled water until the flask is approximately two-thirds full. To remove any air from the soil-water mixture, place the flask onto a hot plate and bring to a gentle boil for 15 to 20 minutes while agitating. After boiling is completed, allow the flask to cool to room temperature, i.e. the temperature which was measured in step 3. Add de-aired, distilled water to the flask until the 500 ml mark is reached. 1

ME 420L/ME 506L: Soil Mechanics Laboratory Lab 2: Specific Gravity of Soil Solids ASTM D-854 9. Determine the combined mass of the flask plus soil-water mixture, M2. 10. Pour the soil-water mixture into an evaporating dish, rinsing out the flask to ensure all

soil particles are removed. Place the evaporating dish in the oven and dry to a constant weight. This will be the mass of the soil; Ms. Data may be recorded in Table 1 on the following page.

Calculations The volume of soil added to the flask in step 6 displaces an equal volume of water. The mass of the displaced water, Mw, equal in volume to that of the soil, is

𝐺𝐺𝑠𝑠 =

π‘΄π‘΄π’˜π’˜ = (π‘΄π‘΄πŸπŸ + 𝑴𝑴𝒔𝒔 ) βˆ’ π‘΄π‘΄πŸπŸ

π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘œπ‘œπ‘œπ‘œ 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑴𝑴𝑠𝑠 𝑴𝑴𝒔𝒔 = = π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘œπ‘œπ‘œπ‘œ π‘Žπ‘Žπ‘Žπ‘Ž 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 π‘œπ‘œπ‘œπ‘œ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 (𝑴𝑴1 + 𝑴𝑴𝑠𝑠 ) βˆ’ 𝑴𝑴2 π‘΄π‘΄π’˜π’˜ 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 π‘Žπ‘Žπ‘Žπ‘Ž 𝑻𝑻1

The density of water varies with temperature. The correction factor 𝐴𝐴 = 𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀𝑀

𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 π‘Žπ‘Žπ‘‘π‘‘ 20°𝐢𝐢

can be used to find Gs at 20Β°C by the equation 𝐺𝐺𝑠𝑠 π‘Žπ‘Žπ‘Žπ‘Ž 20°𝐢𝐢 = 𝐺𝐺𝑠𝑠 π‘Žπ‘Žπ‘Žπ‘Ž 𝑇𝑇1 (𝐴𝐴), see Table 2.

Table 1. Recorded data and calculations for the specific gravity of soil solids

Description of soil Water temperature T1

Date Correction factor A

1 Mass of flask filled with water only, M1 (g) Mass of flask filled with soil-water mixture, M2 (g) Mass of dry soil, Ms (g) Mass of equal volume of water, Mw (g) Gs at T1 Gs corrected to 20Β°C Average of Gs corrected to 20Β°C

2

Test 2

3

ME 420L/ME 506L: Soil Mechanics Laboratory Lab 2: Specific Gravity of Soil Solids ASTM D-854

Table 2. Value of the correction factor A at various temperatures Temperature (T1 Β°C) 16 17 18 19 20

A 1.0007 1.0006 1.0004 1.0002 1.0000

Temperature (T1 Β°C) 21 22 23 24 25

A 0.9998 0.9996 0.9993 0.9991 0.9988

Temperature (T1 Β°C) 26 27 28 29 30

A 0.9986 0.9983 0.9980 0.9977 0.9974

Comments Entrapped air is the most common source of error in this procedure, be sure to pay close attention to step 6 to minimize this error. When writing your report, consider what other possible sources of error may be encountered, also compare your value for the corrected Gs to published values for various soils and for some common minerals.

Note Your lab report should include: 1. A complete description of the soil 2. Proof of the equation used to find specific gravity 3. A comparison of the obtained results with the typical values 4. Type of the soil and minerals based on the obtained specific gravity 5. Sources of errors and the effect of them on specific gravity 6. Applications of specific gravity in geotechnical engineering 7. The method used to find specific gravity of cement

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