of the plasticity chart (for example, LL 20 and PI 5), thesoil is designated either SW-SC or SW-SM; it dependson the judgment of the engineer from the standpoint ofthe climatic region in which the soil is located.FINE-GRAINED SOILSThe fine-grained soils are not classified on the basisof grain size distribution, but according to plasticity andcompressibility. Laboratory classification criteria arebased on the relationship between the liquid limit andplasticity index as designated in the plasticity chart infigure 16-3. This chart was established by the deter-mination of limits for many soils, together with ananalysis of the effect of limits upon physicalcharacteristics.Examination of the chart shows that there are twomajor groupings of fine-grained soils. Thase are the Lgroups, which have liquid limits less than 50, and the Hgroups, which have liquid limits equal to and greaterthan 50. The symbols L and H have general meaningsof low and high compressibility, respectively.Fine-grained soils are further divided with relation totheir position above or below the A-line of the plasticitychart.ML and MH GroupsTypical soils of the ML and MH groups areinorganic silts. Those of low compressibility are in theML group. Others are in the MH group. All of these soilsplot below the A-line of the plasticity chart. The MLgroup includes very fine sands, rock flours (rock dust),and silty or clayey fine sand or clayey silts with lowplasticity. Loess type soils usually fall into this group.Diatomaceous and micaceous soils usually fall into theMH group but may fall into the ML group when theliquid limit is less than 50. Plastic silts fall into the MHgroup.andCL and CH GroupsIn these groups, the symbol C stands for clay, withL and H denoting low or high liquid limits. These soilsplot above the A-line and are principally inorganic clays.In the CL group are included gravelly clays, sandy clays,silty clays, and lean clays. In the CH group are inorganicclays of high plasticity.OL and OH GroupsThe soils in these two groups are characterized bythe presence of organic matter; hence the symbol O. Allof these soils generally plot below the A-line. Organicsilts and organic silt-clays of high plasticity fall into theOL group, while organic clays of high plasticity plot inthe OH zone of the plasticity chart. Many of the organicsilts, silt-clays, and clays deposited by the rivers alongthe lower reaches of the Atlantic seaboard have liquidlimits above 40 and plot below the A-line. Peaty soilsmay have liquid limits of several hundred percent andplot well below the A-line because of their highpercentage of decomposed vegetational matter. A liquidlimit test, however, is not a true indicator in cases inwhich a considerable portion consists of other than soilmatter.Borderline SoilsFine-grained soils that have limits that plot in theshaded portion of the plasticity chart are borderlinecases and are given dual symbols, such as CL-ML.Several soil types that exhibit low plasticity plot in thisgeneral region on the chart where no definite boundarybetween silty and clayey soils exists.HIGHLY ORGANIC SOILSA special classification (Pt) is reserved for thehighly organic soils, such as peat, which havecharacteristics that are undesirable for constructionmaterials and foundations. No laboratory criteria areestablished for these soils, as they generally can bereadily identified in the field by their distinctive colorand odor, spongy feel, and fiequently, fibrous textures.Particles of leaves, grass, branches, or other fibrousvegetable matter are common components of thesesoils.COEFFICIENT OF UNIFORMITYIn table AV-1 of appendix V, you can see thatwell-graded gravels (GW) and well-graded sands (SW)must meet certain requirements with regard to means the coefficient of uniformity with regardto the plotted grain size curve for the material. To seehow the coefficient of uniformity is determined, let’sconsider an example.Suppose that the sieve analysis of a soil sampleidentified as FT-P1-1 is as follows:16-13
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