PLASTICITY I J

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PLASTICITY I J. Valdes SDSU l. Consider a bucket of sand. Add water. The result is simply wet sand. Now consider a bucket with flour. Add water. The result is not wet flour; the result can range from a cookie dough-like material, to a paste, to a fluid (depending on h?w much water is added). 2. Clearly, a chunk of cookie dough, a paste, and a fluid have different strengths (a fluid has no static strength, because by definition it flows when forced to carry a shear stress). A soil is said to be plastic when its strength depends on the relative amount of water in its pores, i.e., the water content, defined as the mass of water divided by the mass of solids (w = mw/m s). Dry sand and saturated sand have the same static strength; therefore, sand is non-plastic. Baby powder (which actually contains clay-sized particles) is plastic. 3. There are various ways to evaluate the plasticity of a soil, but all are based on the following water content spectrum: brittle solid wc = LL a. If the water content of the soil is the liquid limit (LL) or higher, then the soil behaves like a fluid; that is, it flows when shear is imposed. b. If the water content is the plastic limit (PL) or lower, then the soil behaves like a brittle solid; that is, it cracks easily when shear is imposed. c. If the water content is between the PL and the LL, then the soil behaves like a plastic solid; that is, it does not crack easily when shear is imposed. d. The plasticity index Pl is the range of water contents within which the soil exhibits the behavior of a plastic solid. Plastic soils with low Pl tend to be silty, whereas soils with high Pl tend to be clays or clayey. e. The consistency and strength of plastic soils with low Pl changes significantly if the water content changes by a small amount. In contrast, a small change in water content does not alter the consistency and strength of plastic soils with high Pl. 28 f. Although the Pl value revea ls t he susceptibility of the soil strength to changes in water content (see item e above), t he LL is actually used to classify a fine grained soil or the finer fractio n of a coarse soil as having high plasticity or low plasticity. If the LL is above 50%, then the soil is deemed to be a high plasticity clay (CH ) or a high plasticity silt (M H). If the LL is below 50%, then the soil is deemed t o be a low plasticity clay (CL), or a low plasticity silt (ML), or a " CL-ML" (i.e., a soil with a behavior that lies somew here between a CL and an ML). The uses plasticity chart (below) is used for classification purposes. MIi CL-1\IL 10 ., i\lL...___ _ _ _ _ _ _ __ o4 ~ _ _.___ _ _ __ II Ill l h ~ti .Iii 41J 511 l.i4uiJ I 611 711 XII 111 1 _ l•HI 1111 111111. 11 1 4. The degree to which a soil exhibits plasticity is at the heart of soil classification, which in turn, ultimately captu res t he force balance on each particle. Generally, fines (clay and silt) are plastic and sa nd and gravel are non-plastic (see Figure 11 below). I FINE Clay Silt COARSE Sand Gravel Behavior controlled by Behavior controlled by water content and water chemistry grain size distribution particle shapes 29 Activity: Hands-on LL evaluat ion Steps: 1. Read the entirety of this document before proceeding. 2. Place 2 cups of dry clay in a large bowl. Determine the mass of the dry clay in the bowl (ms), 3. Compute the mass of water needed to bring the clay in the bowl from a zero water content (w = 0) tow= 15%. 4. Add the computed amount of water to the bowl with the clay and mix thoroughly with a spoon or spatula. Take pictures. Touch the soil. Press it with a finger. Write down qualitative observations; for example, is the soil clumping? Does the soil look like cookie dough, toothpaste? Is it hard t o mix? Destroy clumps by smearing them along the side of the bowl with the spoon or spatula. S. Compute the mass of water needed to bring the clay in the bowl from w = 15% to w = 20%. Add the water to the clay and mix. Again, take pictures, touch the soil, and write observations. 6. Compute the mass of water needed to bring the clay in the bowl from w = 20% to w = 25%. Add the water to the clay and mix. Again, take pictures, touch the soil, and write observatio ns. 7. Continue adding water in w = 5% increments, taking pictures and writing observations along t he way. Look out for the state of the soil when the consistency is that of toothpaste. The water content in this state is approximately the LL. Finish the experiment when w = 80%. 8. Put you r soil in a trash can and then clean your bowl and tools with dry and/or wet paper t owel. Do not pour clay into the sink! 30 Due: 8/4/2021 | 11:59 PM (PST) Plasticity I (Group) I have broken down the minimum requirements of information needed: Introduction. a. A short paragraph that describes the purpose of the lab. b. A detailed paragraph that describes the "Casagrande device", and how it is used [routinely] to determine the LL of a soil. Do not plagiarize. c. A paragraph on the importance of understanding how soil (in general) responds to the presence water. Show the reader that you are knowledgeable on this particular concept of Geotechnical Engineering. Material a. A short paragraph that describes your soil when it is dry. Procedures a. A detailed paragraph that describes the procedure conducted. Results a. Produce a detailed water-content history plot, as shown in Figure 12, below. b. Denote the toothpaste consistency and water content on your water-content history plot. Label it as LL. Provide at least three references. Yes, you may use the notes provided by Dr. Valdes, above, as one. Since you will be documenting the experiment through photographs, no video is required for this experiment, but all members should produce data results and calculations should be explained in report. You may choose the best history plot of your group or attempt to incorporate all the plots. In either case, you will be incorporating all members' data. JC GO --??,... G ' ••??,__,Soll lNl1__, etc Figure 12. Schematic of the role of water content on the perceived plasticity of a soil. a. Denote the toothpaste consistency and water content on your water-content history plot.