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This report is an archived publication and may contain dated technical, contact, and link information
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Publication Number:  FHWA-HRT-15-033    Date:  May 2015
Publication Number: FHWA-HRT-15-033
Date: May 2015

 

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.

In this graph, the abscissa represents the clay percentage ranging from 0 to 16 percent. The ordinate represents the plasticity index ranging from 0 to 10 percent. A regression line based on 13 soil samples is also shown and specified in a subsequent figure. The 13 soils, including the 6 original soil samples used for erosion testing, plot in the vicinity of the regression line, as expected. A set of low silt soil samples plot above the regression line and are apparently distinct in their behavior from the 13 soil samples.
Figure 67. Graph. Relationship between PI and clay percentage

The line fitting the data is described in figure 68:

The equation calculates PI as equal to 70 times the clay percentage minus 0.57.
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.

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