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.

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. 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.

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

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.

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.

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 C_M the C_c, C_M plotted grain size curve for the material. To see how the coefficient of uniformity is determined, lets consider an example.

Suppose that the sieve analysis of a soil sample identified as FT-P1-1 is as follows:

Figure 16-4.Grain size distribution chart.

You should plot these values on a form like the one shown in figure 16-4. The graph on this form is a logarithm type of layout; coordinates horizontally are sieve sizes (at the top) and grain sizes in millimeters (at the bottom). Vertical coordinates are percents passing. 

The formula for determining C_M is as follows:

D₆₀ means the grain size, in millimeters, indicated by the gradation curve at the 60-percent passing level. In figure 16-4, follow the 60-percent passing line to the point where it intersects the gradation curve for FT-P1-1; then drop down and read the grain size in millimeters indicated below. You read about 1.25mm.  

D₁₀ means, similarly, the grain size indicated by the gradation curve at the 10-percent passing level. In figure 16-4, this is about 0.11mm.  

C_M for this sample, then, is 1.25/0.11, or about 11.4. is as follows: