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<br />Unltled Classiflcatlon - The Unltled Soll Classif)catlon system Is based on
<br />identlflcatlon of soils according to their texture and plasticity and their.
<br />performance as engineering construction material. (Corps of Engineers, U: 5.
<br />Army, Technical Memorandum No. 3-357, Vol. 1, March 1953). In this system,,so.ll
<br />materials divided into 15 classes: 8 classes are for coarse-grained material .:.:.
<br />(GW, GP, GM, GC, SW, SM, SC), for find-grained (ML, CL, OL, MH, CH, OH); and 1 .
<br />for organic material (Pt). ",
<br />AASHO Clesslffcatlon - Most highway engineers classify Boll materiels accord- '
<br />Ing to the system approved by the American Association of State Highway Offi-
<br />cials. (Highway Research Board Proceedings of the 25th Annual Meeting, 1445).
<br />This classiflcatlon is based on the gradation, liquid Iimlt, and plasticity
<br />Index of the soil. Highway performance has bean rolatod to this system of .
<br />classiflcatlon. All soil matorlats are placed In seven principle groups.' The
<br />groups range from A-1 (gravelly soils of high bearing capacity, the Dest rolls
<br />for subgrades) to A-7 (clay soils having low strength when wet, the poorest
<br />rolls for subgrades). ~.
<br />.Percent of Material Passing Slevo - The measured or estimated percentages of
<br />material passing the numbers 4, 10, 40 b 200 sieves is given for each maJor '.
<br />horizon. When Chore is very little gravel-sizo material (No. 4 and f0 sieve)
<br />present, the percent passing the 200 sieve approximates the amount of silt and
<br />clay. Yalues are rounded oft to the nearest 5 percent. A range Is listed be-.
<br />cause of the varlabltity for a givon soil.
<br />Permeability - Yalues Ilstod are estimates of the range in rate and timo.,(t
<br />takes for downward movement of water In Cho maJor Boll layers when saturated, "
<br />but allowed to dra(n freely. Tho estimates are based on soil texture, soli
<br />.structure, available data on permeability and Intiltratlon tests, and draln-
<br />age observations of Cho water movement through soils. In most cases, partl-
<br />cularly with Boll horizons that are high In clay or organic matter, permeability.
<br />rates under unsaturated conditions are considerably higher than Cho values
<br />given hero. On a given soil type, percolation through Cho surface layer varies
<br />according to land use and management as well as with Initial moisture content,
<br />1lvallable Water Capacity - The available water capacity is givon In inches per,
<br />"Inch of soil for the maJor horizons. These ostlm3tos are for cultivatod,solls',
<br />with moderato structure and organic matter content, and average bulk derisltios.,
<br />Available water capacity of Cho soli In Inches Is the dtfforonco between"tiolA
<br />capacity (i/3 atmosphere) and the wilting percentage (15 atmosphere) times bulk
<br />density times the thickness in Inches of the soil. Tho water rotontton by roll
<br />is rolatod to the particle size and to the arrangement and sizo of the soil
<br />pores. Fine-textured soils tend to have higher water retention duo to small
<br />pores than do sandy soils with largo pores. Estimator of Cho available water
<br />capacity for soils w(th normally high water taml~• r•.ay appear moaningle55 until
<br />one considers the posslblttty of artificial drainage or the natural lowering
<br />of the water table during dry ~oasons, or late summer or fall. Sells of the
<br />same series vary from place to place; therefore, values can deviate consider-.
<br />ably from those Ilstod.
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