of the plasticity chart (for example, LL 20 and PI 5), the soil is designated either SW-SC or SW-SM; it depends on the judgment of the engineer from the standpoint of the climatic region in which the soil is located. FINE-GRAINED SOILS The fine-grained soils are not classified on the basis of grain size distribution, but according to plasticity and compressibility.  Laboratory  classification  criteria  are based on the relationship between the liquid limit and plasticity index as designated in the plasticity chart in figure 16-3. This chart was established by the deter- mination  of  limits  for  many  soils,  together  with  an analysis   of   the   effect   of   limits   upon   physical characteristics. Examination of the chart shows that there are two major groupings of fine-grained soils. Thase are the L groups, which have liquid limits less than 50, and the H groups,  which  have  liquid  limits  equal  to  and  greater than 50. The symbols L and H have general meanings of  low  and  high  compressibility,  respectively. Fine-grained  soils  are  further  divided  with  relation  to their position above or below the A-line of the plasticity chart. ML and MH Groups Typical  soils  of  the  ML  and  MH  groups  are inorganic silts. Those of low compressibility are in the ML group. Others are in the MH group. All of these soils plot below the A-line of the plasticity chart. The ML group includes very fine sands, rock flours (rock dust), and silty or clayey fine sand or clayey silts with low plasticity. Loess type soils usually fall into this group. Diatomaceous and micaceous soils usually fall into the MH group but may fall into the ML group when the liquid limit is less than 50. Plastic silts fall into the MH group. and CL and CH Groups In these groups, the symbol C stands for clay, with L and H denoting low or high liquid limits. These soils plot 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 inorganic clays of high plasticity. OL and OH Groups The soils in these two groups are characterized by the presence of organic matter; hence the symbol O. All of  these  soils  generally  plot  below  the  A-line.  Organic silts and organic silt-clays of high plasticity fall into the OL group, while organic clays of high plasticity plot in the OH zone of the plasticity chart. Many of the organic silts, silt-clays, and clays deposited by the rivers along the lower reaches of the Atlantic seaboard have liquid limits  above  40  and  plot  below  the  A-line.  Peaty  soils may have liquid limits of several hundred percent and plot  well  below  the  A-line  because  of  their  high percentage  of  decomposed  vegetational  matter.  A  liquid limit test, however, is not a true indicator in cases in which a considerable portion consists of other than soil matter. Borderline Soils Fine-grained soils that have limits that plot in the shaded portion of the plasticity chart are borderline cases and are given dual symbols, such as CL-ML. Several soil types that exhibit low plasticity plot in this general region on the chart where no definite boundary between silty and clayey soils exists. HIGHLY ORGANIC SOILS A  special  classification  (Pt)  is  reserved  for  the highly  organic  soils,  such  as  peat,  which  have characteristics that are undesirable for construction materials  and  foundations.  No  laboratory  criteria  are established for these soils, as they generally can be readily  identified  in  the  field  by  their  distinctive  color and odor, spongy feel, and fiequently, fibrous textures. Particles of leaves, grass, branches, or other fibrous vegetable matter are common components of these soils. COEFFICIENT  OF  UNIFORMITY In  table  AV-1  of  appendix  V,  you  can  see  that well-graded gravels (GW) and well-graded sands (SW) must meet certain requirements with regard to means the coefficient of uniformity  with  regard to the plotted grain size curve for the material. To see how the coefficient of uniformity is determined, let’s consider  an  example. Suppose that the sieve analysis of a soil sample identified as FT-P1-1 is as follows: 16-13