Figure 16-3.-Plasticity chart.plasticity index are used in specifying the laboratorycriteria for these groups. Reference also is made to theplasticity chart shown in figure 16-3 that is based uponestablished relationships between the liquid limit andplasticity index for many different fine-grained soils.The symbol M is used to indicate that the materialpassing the No. 40 sieve is silty in character. An Musually designates a fine-grained soil of little or noplasticity. The symbol C is used to indicate that thebinder soil is predominately clayey in character.GM AND SM GROUPS.— Typical of the soilsincluded in the GM group are silty gravels andgravel-sand-silt mixtures. Similarly, the SM groupcontains silty sands and sand-silt mixtures. For both ofthese groups, the Atterberg limits must plot below theA-line of the plasticity chart (fig. 16-3). The plasticityindex must be less than 4.GC AND SC GROUPS.— The GC group includesclayey gravels and gravel-sand-clay mixtures.Similarly, SC includes clayey sands and sand-claymixtures. For both of these groups, the Atterberg limitsmust plot above the A-line with a plasticity index formore than 7.Borderline SoilsCoarse-grained soils that contain between 5 and 12percent of material passing the No. 200 sieve are classedas border line and are given a dual symbol, such asGW-GM. Similarly, coarse-grained soils that containmore than 12 percent of material passing the No. 200sieve, and for which the limits plot in the shaded portionof the plasticity chart (fig. 16-3), are classed as borderline and require dual symbols, such as SM-SC. It ispossible in rare instances for a soil to fall into more thanone borderline zone. In this case, if appropriate symbolswere used for each possible classification, the resultwould be a multiple designation consisting of three ormore symbols. This approach is unnecessarilycomplicated. It is considered best to use only a doublesymbol in these cases. You should select the two thatyou believe to be most representative of the probablebehavior of the soil. In cases of doubt, the symbolsrepresenting the poorer of the possible groupings shouldbe used. For example, a well-graded sandy soil with 8percent passing the No. 200 sieve, an LL of 28 and a PIof 9 would be designated as SW-SC. If the Atterberglimits of this soil are such as to plot in the shaded portion16-12
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