Scour in Cohesive Soils

APPENDIX A. RELATION BETWEEN PI AND CLAY CONTENT

In addition to the six soil mixtures used for erosion testing, another seven soils were prepared and tested for PI. The composition of Red art clay, commercial silt, and sand in each of these 13 soils varied from 10 to 40 percent, 10 to 40 percent, and 20 to 70 percent by mass, respectively. With these variations, it was found that the relationship between the clay percentage and PI is linear, as summarized in figure 67. The red squares represent the six original soils. The blue circles represent the expanded data, including the six original soils. The fittings between the clay percentage and PI for the 6 soils and 13 soils are both linear and are almost superimposed on each other.

Figure 67. Graph. Relationship between PI and clay percentage

The line fitting the data is described in figure 68:

Figure 68. Equation. Linear relationship between PI and clay percentage

The black triangles represent an additional four soils prepared with very low (2 to 4 percent) silt concentrations. They clearly scattered away from the other 13 points, which may indicate that when the silt content is very low, the particle size distribution of soils will dramatically change. This change will dramatically change the PI. Therefore soils with very low silt do not have a linear relationship between PI and clay percentage.