A. Modified Unified Classification System for Field Classification of Soil

Transcription

1 Field Classification of Soil Using the USCS CVEEN 6340 Page 1 of 12 I. General A. Modified Unified Classification System for Field Classification of Soil Soil descriptions given in this lecture are based on a modified version of ASTM standards D 2487 and D B. Field Soil Description Consistency (fine-grained soils) or apparent density (coarse-grained soils) Water content Color description Minor soil type name with "y" added if 30 percent Descriptive adjective for main soil type Particle-size distribution adjective for gravel and sand Plasticity adjective and soil texture (silty or clayey) for inorganic and organic silts or clays Main soil type s name (all capital letters) Descriptive adjective, some or trace, for minor soil type if 30 percent Minor soil type(s) Inclusions Unified Soil Classification System (USCS) Group Name and Symbol appropriate for the soil type in accordance with ASTM D 2487, with few exceptions, and (symbol in parenthesis) Geological name, if known, (in parenthesis) The various elements of the soil description should generally be stated in the order given above. For example: Fine-grained soils Soft, wet, gray, high plasticity CLAY, trace f. sand - Fat CLAY (CH); (Alluvium) Coarse-grained soils Dense, moist, brown, silty m-f SAND, trace f. gravel to c. sand - Silty SAND (SM); (Alluvium) 1. Consistency and Apparent Density Empirical values for the consistency of fine-grained soils and the apparent density of silts and coarse-grained soils have been developed for the blow count (N-value) resistance using a 2-inch OD, 24-inch long split-barrel sampler and standard penetration test energy

2 Field Classification of Soil Using the USCS CVEEN 6340 Page 2 of 12 (a 140-pound hammer dropped 30 inches) ASTM D SPT N-Value (corrected) Apparent Density of Coarse-Grained Soils 0-4 Very loose 5-10 Loose Medium Dense Dense > 50 Very Dense Apparent Density SPT N-Value (uncorrected) Consistency Consistency of Fine-Grained Soils Compressive Strength (ksf) Results of Manual Manipulation < 2 Very Soft < 0.5 Specimen (height = twice the diameter) sags under its own weight; extrudes between fingers when squeezed 3-4 Soft > Speciment can be pinched in to between the thumb and forefinger; remolded by light finger pressure 5-8 Medium stiff > Can be imprinted easily with fingers; remolded by strong finger pressure 9-15 Stiff > Can be imprinted with considerable pressure from fingers or indented by thumbnail Very stiff > Can be barely imprinted by pressure from the fingers or indented by thumbnail > 30 Hard > 8.0 Cannot be imprinted by fingers or difficult to indent by thumbnail The apparent density and consistency of the soil formation can vary from these empirical correlations for a variety of reasons. Judgment remains an important part of the visual identification process. Mechanical tools such as the pocket (hand) penetrometer and the Torvane are suggested to guide estimates of consistency. In some cases the sampler may

3 CVEEN 3310 Lecture Notes #3b Page 3 of 12 pass from one layer into another of markedly different properties; for example, from a dense sand into a soft clay. In attempting to identify apparent density, an assessment should be made as to what part of the blow count corresponds to each layer; realizing that the sampler begins to reflect the presence of the lower layer before it reaches it. Empirical criteria requires that the blow count from the modified California sampler be converted to an "equivalent" standard penetration test blow count by multiplying the modified California blow count by Water Content (Moisture) The amount of water present in the soil sample or its water content adjective should be described as dry, moist, or wet as follows: Description Dry Moist Wet Conditions No sign of water and soil dry to touch Signs of water and soil is relatively dry to touch Signs of water and soil definitely wet to touch; granular soil exhibits some free water when densified The descriptor "damp" should not be used (use "moist"). The descriptor "saturated" should not be used ( use "wet"). 3. Color The color should be described when the sample is first retrieved at the soil's as-sampled water content (the color will change with water content). Primary colors should be used (brown, gray, black, green, white, yellow, red). The simplified color chart presented below can be used as a reference for color determination. Soils with different shades or tints of basic colors are described by using two basic colors; e.g., gray-green. Note that some clients may require Munsell color and carry no inferences of texture designations. When the soil is marked with spots of color, the term mottled can be applied. Soils with a homogeneous texture but having color patterns which change and are not considered mottled can be described as streaked.

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5 CVEEN 3310 Lecture Notes #3b Page 5 of Type of Soil The constituent parts of a given soil type are defined on the basis of texture in accordance with particle-size designators separating the soil into coarse-grained, fine-grained, and highly organic designations. Soil with more than 50 percent of the particles larger than the (U.S. Standard) No.200 sieve (0.074 mm) is designated coarse-grained. Soil (inorganic and organic) with 50 percent or more of the particles finer than the No. 200 sieve is designated fine-grained. Soil primarily consisting of organic matter, dark in color, and with an organic odor is designated as highly-organic soil (peat). The soil type designations follow ASTM D 2487; i.e., gravel, sand, clay, silt, organic clay, organic silt, and peat. a. Coarse-Grained Soils (Gravel and Sand) Coarse-grained soils consist of gravel, sand, and fine-grained soil, whether separately or in combination, and in which more than 50 percent of the soil is retained on the No.200 sieve. The gravel and components are defined on the basis of particle size as follows: Soil Component Grain Size Determination Boulders * 12 inch + Measurable Cobbles * 12 inch to 3 inch Measurable Gravel Coarse 3 inch to 3/4 inch Measurable Fine 3/4 inch to #4 Measurable Sand Coarse #4 to #10 sieve Measurable and visible to eye Medium #10 to #40 sieve Measurable and visible to eye Fine #40 to #200 sieve Measurable and barely discernible to the eye * Boulders and cobbles are not considered soil or part of the soil's classification or description, except under miscellaneous descriptions; i.e. --, with cobbles at about 5 percent (volume). The particle size distribution is identified as well graded or poorly graded. Well graded coarse-grained soil contains a good representation of all particle sizes from largest to smallest, with 12 percent fines. Poorly graded coarse-grained soil is uniformly graded with most particles about the same size or lacking one or more intermediate sizes, with 12

6 CVEEN 3310 Lecture Notes #3b Page 6 of 12 percent fines. Use the flow chart given earlier in this lecture to determine the group symbol and group name for coarse-grained soils. 1) Capitalize soil type; i.e., GRAVEL, SAND. 2) Add particle-size distribution adjective in front of the soil type following the criteria given below: Particle-Size Adjective Coarse Coarse to medium Medium to fine Fine Coarse to fine Abbreviation Size Requirement c. < 30% m-f sand or < 12% f. gravel c-m < 12% f. sand m-f < 12% c. sand and > 30% m. sand f. < 30% m. sand or < 12% c. gravel c-f > 12% of each size The following simple field identification tests can be used as an aid in identifying granular soils. Feel and Smear Tests: A pinch of soil is handled lightly between the thumb and fingers to obtain an impression of the grittiness or the softness of the constituent particles. Thereafter, a pinch of soil is smeared with considerable pressure between the thumb and forefinger to determine the degree of roughness and grittiness, or the softness and smoothness of the soil. Coarse to medium-grained sands typically exhibit a very harsh and gritty fell and smear. Coarse to fine-grained sands have a less harsh feel, but exhibits a very gritty smear. Medium to fine-grained sands exhibit a less gritty feel and smear which becomes softer and less gritty with an increase in the fine sand fraction. Fine-grained sands exhibit a relatively soft feel and a much less gritty smear than the coarser sand components. Silt components less than about 10 percent of the total weight can be identified by a slight discoloration of the fingers after smear of a moist sample. Increasing silt increases discoloration and softens the smear. Sedimentation Test: A small sample of the soil is shaken in a test tube filled with water and allowed to settle. The time required for the particles to fall to a distance of 4 inches is about ½ minute for particle sizes coarser than silt. About 50 minutes would be required for particles of 0.005

7 CVEEN 3310 Lecture Notes #3b Page 7 of 12 or smaller (clay size) to settle out. Visual Characteristics: Sand and gravel particles can be readily identified visually but silt particles are generally indistinguishable to the eye. With an increasing silt component, individual sand grains become obscured, and when silt exceeds about 12 percent, it masks almost entirely the sand component from visual separation. Note that gray fine-grained sand visually appears siltier than the actual silt content. b. Fine-Grained Soils Fine-grained soils are those which 50 percent or more of the particles pass the No. 200 sieve. Use the flow chart given earlier in this lecture to determine the group symbol and group name. 1) Capitalize soil type; i.e., CLAY, SILT, ORGANIC CLAY, ORGANIC SILT, PEAT, etc. Also, the group name of SILTY CLAY would be capitalized for dual symbol fine-grained soils (CL-ML). 2) Use plasticity adjectives and soil types as adjectives to further define the soil type s texture, plasticity, etc. using the table below. Plasticity Index Range SOIL PLASTICITY DESCRIPTIONS Plasticity Adjective ML & MH (SILT) CL & CH (CLAY) OL & OH (ORGANIC SILT OR CLAY) 0 nonplastic ORGANIC SILT 1-10 low plasticity > medium plastic > highly plastic ---- silty ORGANIC SILT clayey silty to no adj. ORGANIC clayey SILT clayey ---- ORGANIC silty CLAY > 40 very clayey ---- ORGANIC

8 CVEEN 3310 Lecture Notes #3b Page 8 of 12 plastic CLAY The following simple field identification tests can be used to estimate the degree of plasticity of fine-grained soils. Shaking (Dilatency) Test: This test involves lightly squeezing the soil pat between the thumb and forefinger and releasing it alternatively to observe it reaction and speed of response. Soils which are predominantly silt (nonplastic to low plastic) will show a dull dry surface upon squeezing and a glassy wet surface immediately upon releasing of the pressure. With increasing fineness (plasticity) and the related decreasing dilatency, this phenomenon becomes less pronounced. Dry Strength Test: A portion of the sample is allowed to dry out and a fragment of the dried soil is pressed beteen the fingers. Fragments which cannot be crumbled or broken are characteristic of clays with high plasticity. Fragments which can be disintegrated with gentle finger pressure are characteristic of silty materials of low plasticity. Thread Test: Moisture is added or worked out of a small ball (about 1 1/2-inch diameter) and the ball is kneaded until it consistency approaches medium stiff to stiff and it breaks, or crumbles. A thread is then rolled out to the smallest diameter possible before disintegration. The smaller the thread achieved, the higher the plasticity of the soil. Fine-grained soils of high plasticity will have threads smaller than 1/32 inch in diameter. Soils with low plasticity will have threads larger the 1/8 inch in diameter. Smear Test: A fragment of soil smeared between the thumb and forefinger or drawn across the thumbnail will, by the smoothness and sheen of the smear surface indicate the plasticity of the soil. A soil of low plasticity will exhibit a rough texture and dull smear, while a soil of high plasticity will exhibit a slick, waxy smear surface.

9 CVEEN 3310 Lecture Notes #3b Page 9 of 12 Plastic Range FIELD METHODS TO DESCRIBE PLASTICITY Plasticity Adjective Dry Strength Smear Test Thread Smallest Diameter, in. (mm) 0 nonplastic none-crumbles into powder with mere pressure gritty or rough ball crakcs 1-10 low plasticity low-crumbles into powder with some finger pressure rough to smooth 1/4 to 1/8 (3 to 6) > medium plastic medium - breaks into pieces or crumbles with considerable finger pressure smooth and dull 1/16 (0.5 to 1) > high plasticity high- cannot be broken with finger pressure; will break into pieces between thumb and a hard surface shiny 1/32 (0.75) > 40 very plastic very high - can t be broken between thumb and a hard surface very shiny and waxy 1/64 (0.5) c. Organic Silts and Clays Organic silts and clays (with colloidal and amorphous organic materials) are identified and classified in accordance with their decrease in plasticity upon oven drying (ASTM D 2487). Further identifiers are: (1) dark gray and black and sometimes dark brown colors, although not all dark colored soils are organic; (2) most organic soils will oxidize when exposed to air and change from dark gray/black to a lighter brown, i.e., the exposed surface is brownish, but when the sample is pulled apart the freshly exposed surface is dark gray /black; (3) fresh organic soils usually have a characteristic odor which can be recognized, particularly when the soil is heated; ( 4) compared to nonorganic soils, less effort is typically required to pull the material apart and a friable break is usually formed with a fine granular or silty texture and appearance; (5) their workability at the plastic limit is weaker and spongier than an equivalent nonorganic soil; and (6) the smear, although generally smooth, is usually duller and appears more silty.

10 CVEEN 3310 Lecture Notes #3b Page 10 of 12 A fiber remnant root mat structure (mostly decomposed) is also sometimes detected within organic soils, as are fossiliferous twigs and leaves. Such fine-grained soils would be described as - - -; with pockets/lenses/layers of (give size/thickness) dark brown decomposed PEAT (PT). Finally, as organic soil deposits get older, it appears that their plasticity characteristics on the plasticity chart tend to go from below the "A"-line to above it. d. Highly Organic Materials These materials containing a predominance of undecomposed plant or woody fiber are described as follows: (1) Root Mat: Pronounced structure of living root fibers characteristic of marsh or swampy deposits. (2) Peat: Fossiliferous root mat with a varying degree of decomposition, often containing a matrix of amorphous, colloidal organic clays and silts. (3) Humus: Decomposed root and leaf litter, characteristic of organic forest cover in well-drained areas. (4) Lignite: Immature coals having a woody appearance, brown in color, which slake and crumble on exposure. 5. Minor Soil Type(s) In many soils two or more soil types are present in the soil. If the percentage of the minor soil type is equal to or greater than 30 percent and less than 50 percent of the total sample, the minor soil type is indicated by adding a "y" to its name; i.e., f. gravelly, c-f. sandy, silty, clayey, silty clayey, organic silty, etc. (Note: the gradation adjectives are given for the coarse-grained soils, while the plasticity adjective is omitted for the fine- grained soils.) When the minor soil type percentage is between 12 and 29 percent of the total sample, the minor soil type's name is given along with the adjective "some." When the minor soil type percentage is between 1 and 12 percent of the total sample, the minor soil type s name is given along with the adjective trace. 6. Inclusions

11 CVEEN 3310 Lecture Notes #3b Page 11 of 12 Additional inclusions or characteristics of the sample can be described by using "with" and the descriptions described above. Examples are given below: with petroleum odor with organic matter with foreign matter (roots, brick, etc.) with shell fragments with mica with parting(s), seam(s), etc. of (give soils complete description) 7. Layered Soils Soils of different types can be found in repeating layers of various thickness. It is important that all such formations and their thicknesses are noted. Each layer is described as if it is a nonlayered soil using the sequence for soil descriptions discussed above. The thickness and shape of layers and the geological type of layering are noted using the following descriptive terms: Type of Layer Thickness Occurrence Parting Seam Layer Stratum Pocket Lens Varved (also layered) Occasional Frequent < 1/16 in. 0.5 in. to 1/16 in. 12 in. to 0.5 in. > 12 in. Small erratic deposit Lenticular deposit Alternating seams or layers of silt and/or clay and sometimes f. sand One or less per foot of thickness or laboratory sample inspected More than one per foot of thickness or laboratory sample inspected Place the thickness designation before the type of layer, or at the end of each description and in parentheses, whichever is more appropriate. Examples of descriptions for layered soils are: Medium stiff, moist to wet 1/4"-3/4" interbedded seams and layers of: gray, medium plastic, silty CLAY (CL); and lt. gray, low plasticity SILT (ML); (Alluvium).

12 CVEEN 3310 Lecture Notes #3b Page 12 of 12 Soft moist to wet varved layers of: gray-brown, high plasticity CLAY ( CH) (1/4"-3/8"); firm, gray, medium plastic, silty CLAY (CL) (3/8"-1/2"); and nonplastic SILT, trace f. sand (ML) {1/2"-3/4"); (Alluvium). Medium stiff, moist, red-brown, medium plastic silty CLAY, trace f. sand - lean CLAY (CL); with occasional 1/8" to 1/4" lenses of red-brown f. SAND, trace silt -SAND with silt (SP-SM); (Alluvium). 8. Geological Name The soil description should include the Field Representative's assessment of the origin of the soil unit and the geologic name, if known, placed in parentheses at the end of the soil description or in the field notes column of the boring log.