Coefficient of curvature

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Cc means the coefficient of curvature of the gradation curve. Sometimes the symbol for coefficient of gradation) is used instead of formula for determining or  is as follows:

D30 is the grain size, in millimeters, indicated by the gradation curve at the 30-percent passing level. In figure 16-4, it is (for FT-P1-1) about 0.35. Therefore, is 0.352 divided by (0.11 x 1.25), or about 0.89.

FT-P1-1 is obviously a sand, since more than half of its coarse fraction passes the No. 4 sieve. It is a clean sand, since less than 5 percent of it (see table AV-1, appendix V) passes the No. 200 sieve. However, it is not a well-graded sand (SW), because although its CM is greater than 6 (prescribed for SW in appendix V), its Cc is less than 1, the minimum prescribed for SW.

Therefore, it is in the SP (poorly graded sands, gravelly sands, little or no fines) category.


The following soil classification problems are presented to show you how the soil classification chart (table AV-1, appendix V) is used to classify soils.

Sample Problem 1. From a sieve analysis, a soil shows a CMof 20 and a Cc of 1.3 and contains 12-percent gravel, 88-percent sand, and no fines (smaller than No. 200). When you are classifying this soil, the first question is whether the soil is coarse-grained or fine-grained. To be Coarse-grained, a soil must have less than 50-percent fines. This soil contains no fines; therefore, it is a coerse-grained soil with the first letter in the symbol either G (gravel) or S (sand). Since it contains more sand (88 percent) than gravel (12 percent), the first letter in the symbol must be S. The next task is to determine the second letter in the symbol. Since the soil contains no fines, it has no plasticity characteristics; therefore, the second letter of the symbol must be either W (well graded) or P (poorly graded). Since the soil has a Cu greater than 6 and a Cc between 1 and 3, it must be well-graded. Therefore, the symbol for the soil is SW, meaning "well-graded sand."

Sample Problem 2. A sieve analysis shows that a soil contains (50-Percent gravel, 20-percent sand, and 20-percent fines. Plasticity tests show that the portion passing the No. 40 sieve has an LL of 35 and a PI of 8. Since the soil contains less than 50-percent fines, it is a coarse-grained soil. The first letter is therefore either G (gravel) or S (sand). Since gravel predominates over sand, the first letter is G.

The next questions are (1) does the soil contain less than 12-percent fines and (2) is it nonplastic? The answer to both questions is negative, since the sieve analysis shows 20-percent fines, and an LL and PI have been obtained. It follows that the second letter in the symbol must be either C (clay) or M (silt). If you plot LL 35 and PI 8 on the plasticity chart (fig. 16-3), you will find that the plotted point lies below the A-line. Therefore, the complete symbol is GM, meaning "silty gravel."

Sample Problem 3. A sieve analysis shows that a soil contains 10-percent sand and 75-percent fines. Plasticity tests show that the portion passing the No. 40 sieve has an LL of 40 and a PI of 20. Since the soil contains more than 50-percent fines, it is a fine-grained soil; therefore, the first letter in the symbol is either O (organic), M (silt), or C (clay). Assume that the soil shows no indication of being organic (principal indications are black color and musty odor); it follows that the first letter must be either M or C.

If you plot an LL of 40 and a PI of 20 on the plasticity chart, you find that the plotted point lies above the A-line; therefore, the first letter in the symbol is C. Since the liquid limit is less than 50 (which brings the plotted point to the left of the B-line), the second letter of the symbol is L (low plasticity or compressibility). The complete symbol is CL, meaning "clay with low compressibility."


Sometimes the lack of time and facilities makes laboratory soil testing impossible in military construction. Even when laboratory tests are to follow, field identification tests must be made during the soil exploration. Soil types need to be identified so that duplicate samples for laboratory testing are held to a minimum. Several simple tests used in field identification are described in this section. Each test may be performed with a minimum of time and equipment. However, the classification derived from these tests should be considered an approximation. The number of tests used depends on the type of soil and the experience of the individual using them. Experience is the greatest asset in field identification; learning the technique from an experienced technician is the best method of acquiring the skill. If assistance is not available, you can gain experience by getting the "feel" of the soil during laboratory testing. An approximate identification can be made by spreading a dry sample on a flat surface and examining it. All lumps should be pulverized until individual grains are exposed but not broken; breaking changes the grain size and the character of the soil. A rubber-faced or wooden pestle are recommended. For an approximate identification, however, you can mash a sample underfoot on a smooth surface.

Field tests may be performed with little or no equipment other than a small amount of water. However, accuracy and uniformity of results is greatly increased by the proper use of certain items of equipment. For testing purposes, the following equipment or accessories may be used:

. SIEVES. A No. 40 U.S. standard sieve is perhaps the most useful item of equipment. Any screen with about 40 openings per lineal inch could be used. An approximate separation may be made by sorting the materials by hand. Generally, No. 4 and No. 200 sieves are used for separating gravel, sand, and fines.

. PIONEER TOOLS. Use a pick and shovel or a set of entrenching tools for collecting samples. A hand auger is useful if samples are desired from depths of more than a few feet below the surface.

. STIRRER. The spoon issued as part of the mess equipment serves in mixing materials with water to the desired consistency. It also can aid in collecting samples.

. KNIFE. Use a combat knife or pocketknife for collecting samples and trimming them to the desired size.

. MIXING BOWL. Use a small bowl with a rubber-faced pestle to pulverize the fine-grained portion of the soil. Both may be improvised. You could use a canteen cup and wood pestle.

. PAPER. Several sheets of heavy paper are needed for rolling samples.

. PAN AND HEATING ELEMENT. Use a pan and heating element to dry samples.

. SCALES. Use balances or scales to weigh samples. The Unified Soil Classification System, as shown in appendix V, considers three soil properties: (1) percentage of gravel, sand, or fines, (2) shape of the grain size distribution curve, and (3) plasticity. Other observed properties should also be included in the soil description, whether made in the field or in the laboratory.

The following descriptions represent some of the typical characteristics used in describing soil:

. Dark brown to white or any suitable color shade description

. Coarse-grained, maximum particle size 2 3/4 inches, estimated 60-percent gravel, 36-percent sand, and 4-percent fines (passing through No. 200 sieve)

. Poorly graded (gap-graded, insufficient fine gravel)

. Gravel particles subrounded to rounded, or predominately gravel

. Nonplastic

. Mostly sand with a small amount of nonplastic fines (silt)

. Slightly calcareous, no dry strength, dense in the undisturbed state


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