Borderline Soils

Figure 16-3.-Plasticity chart. plasticity index are used in specifying the laboratory criteria for these groups. Reference also is made to the plasticity chart shown in figure 16-3 that is based upon established relationships between the liquid limit and plasticity index for many different fine-grained soils. The symbol M is used to indicate that the material passing the No. 40 sieve is silty in character. An M usually designates a fine-grained soil of little or no plasticity. The symbol C is used to indicate that the binder soil is predominately clayey in character. GM AND SM GROUPS.— Typical of the soils included in the GM group are silty gravels and gravel-sand-silt mixtures. Similarly, the SM group contains silty sands and sand-silt mixtures. For both of these groups, the Atterberg limits must plot below the A-line of the plasticity chart (fig. 16-3). The plasticity index must be less than 4. GC AND SC GROUPS.— The GC group includes clayey gravels and gravel-sand-clay mixtures. Similarly, SC includes clayey sands and sand-clay mixtures. For both of these groups, the Atterberg limits must plot above the A-line with a plasticity index for more than 7. Borderline Soils Coarse-grained soils that contain between 5 and 12 percent of material passing the No. 200 sieve are classed as border line and are given a dual symbol, such as GW-GM. Similarly, coarse-grained soils that contain more than 12 percent of material passing the No. 200 sieve, and for which the limits plot in the shaded portion of the plasticity chart (fig. 16-3), are classed as border line and require dual symbols, such as SM-SC. It is possible in rare instances for a soil to fall into more than one borderline zone. In this case, if appropriate symbols were used for each possible classification, the result would be a multiple designation consisting of three or more symbols. This approach is unnecessarily complicated. It is considered best to use only a double symbol in these cases. You should select the two that you believe to be most representative of the probable behavior of the soil. In cases of doubt, the symbols representing the poorer of the possible groupings should be used. For example, a well-graded sandy soil with 8 percent passing the No. 200 sieve, an LL of 28 and a PI of 9 would be designated as SW-SC. If the Atterberg limits of this soil are such as to plot in the shaded portion 16-12