SOIL GRAIN SHAPE J. Valdes SDSU Soil is a granular material. A granular material contains solids (the grains) and pores. The pores are filled with a fluid or multiple fluids. In a dry soil, the pores are filled with air. In a saturated soil, the pores are filled with water. In a moist soil, the pores are filled with air and water. So, by definition, a soil is defined as a material composed of grains and the fluids that are present in the pores. Perhaps the most important characteristic of a soil, as it pertains to the engineering behavior of the soil, is the sizes of the grains. There are four sizes: (1) clay particles are smaller than ~0.01 mm; (2) silt particles range between 0.01mm and 0.075mm; (3) sand grains range between 0.075mm and 4.75mm; and (4) gravel grains are larger than 4.75mm. A quartz sand grain with d � 0.7 mm is shown in the picture below (Figure 6). Figure 6. SEM image of a grain of Ottawa 20-30 sand. 11 The separation (0.075mm) between silt and sand is particularly important, as it highlights the relevance of different particle-level forces. You will learn more about this later. For now, know that water affects the strength and stiffness of soils composed of clay and/or silt particles (i.e., particles smaller than 0.075mm), whereas water does not affect the strength and stiffness of soils composed of sand and/or gravel particles (i.e., particles larger than 0.075mm). You will learn more about particle sizes later. This lab centers on the shape of the grains, which has a significant effect on the stiffness and strength of coarse-grained soils, i.e., soils composed of sand and/or gravel (i.e., particles larger than 0.075mm). Notice the shape of the sand grain in Figure 1. Although it has some contours, it can be reasonably described as spherical. What happens if we have grains with irregular shapes? Grain shape is evaluated with the use of microscopy, enabling the experimentalist to map observed grains on a sphericity (S) vs. roundness (R) map, as the one shown in Figure 7: 0.9 >- -a: f0 0.7 lLI :c 0. (/) 0.5 0.3 • • – • • • • • • • .. 0.1 0.3 0.5 0.7 0.9 ROUNDNESS Figure 7. Sphericity and Roundness map for particle shape characterization 12 Activity 1: Shape Evaluation 1. Visually inspect the two sands provided. One is an actual natural sand. The other is clearly a manufactured sand. 2. Remove the rubber cap from the top of the test tube that contains the natural sand, and place 10 to 12 grains under a microscope or utilize a magnifying glass. 3. Focus on one grain. First, look at the selected grain for 3 to 5 seconds and then at the map (Figure 7). Go back and forth 3 or 4 times, looking at the grain and then quickly at the map. Where does your grain land on the map? Write the value of Sphericity and the value of Roundness perceived visually for such grain. Repeat this procedure for 8 grains. This means that you will have 8 S values and 8 R values for the natural sand. 4. Do steps (1) and (2) above for the manufactured sand, but you only have to inspect 3 grains. This means that you will have 3 S values and 3 R values for the manufactured sand. Activity 2: Angle of Repose 1. Take the test tube that contains the natural sand (Figure 8a). Tilt the tube gently, to almost horizontal (do not let grains fall out). Observe the grains avalanche and spread out along the long side of the tube (Figure 8b). 2. Return the tube very gently to the vertical position and observe the grains avalanche back. The sand stays at a slope once the tube is vertical (Figure 8c). 3. Gently hold the tube vertically while keeping the slope stable. (Note: repeat steps if the slope is disturbed). Use a protractor to measure the angle between the horizontal plane and the slope. This angle is called the angle of repose. Figure 8. Procedure to measure the angle of repose. Note: tape is needed for a tube, but not for a test tube. 9 4. Repeat steps (1) to (3) above with the manufactured sand. Note: the manufactured sand grains may tend to stick to the sides of the cylinder due to electrical forces. If this happens, add water to the test tube so that the water level is one inch or so from the top of the tube. Place the rubber cap and do the experiment. The slope will be easier to keep stable when the entire soil mass is submerged. Important: once finished, place the sand and water in a bowl and remove the excess water (do not let any sand grains be discarded). Allow the sand to dry in the bowl (via water evaporation). This will take a couple of days. Report (Please note that these are minimum requirements) Sections: 1. Introduction. a. A short paragraph that describes the purpose of the lab. b. A paragraph on the importance of understanding how the shape of soil grains affects the engineering behavior of sands and gravels. Use textbooks, journal articles, and conference proceedings articles, to support your statements (list the references you cited). There are many textbooks and journals in the SDSU library (accessible online). Show the reader that you are knowledgeable on this particular concept of Geotechnical Engineering. 2. Results. a. An organized table with your values of R and S for each sand. Be thoughtful in terms of how you tabulate this information in an organized fashion. b. Use a computer to plot your S-R data: R in the x-axis and S in the y-axis (as in Figure 7). Should the shape of the plot be a square? Or is any shape ok? Pay attention to the min and max values of the axes in your graph (look at Figure 7). 10 c. A paragraph that describes the tube tilting procedure. Why is this experiment insightful? What does the angle of repose capture? Did the shape of the grains affect the angle of repose? Explain. Cite your references. 3. Additional features. Consider the two grains shown in the pictures below. The left one is a quartz sand grain and the right one is a carbonate grain. Both grains are about the same size (d ≈ 0.5 mm). Determine the S and R of each grain. Are they similar? What is a physical feature of the grains (other than overall shape) that you can observe in the pictures, and that can lead to variations in behavior? What type of behavior does this physical feature affect? Elaborate: one paragraph. SEM images of a grain of Ottawa sand (left) and a grain of carbonate sand (right). 4. References (list them). You should have more than 2. 5. Hand and spreadsheet calculations (if applicable). 11
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