3.7 Interrelationship of Effective Porosity, Specific Yield and Specific Retention

As shown in Equation 13, effective porosity is equal to the sum of the specific yield and specific retention. These characteristics can be determined at the laboratory sample scale and at the broader field scale. Laboratory determination of two of the three properties allows for calculation of the third (neSy = Sr; Sy + Sr = ne; neSr = Sy). Tables 1, 2 and 3, reveal that Sy is typically highest for granular materials with large diameter pore spaces and lowest in silts and clay even though their porosities may be high. It would be desirable to develop a relationship between some easily measured earth material property (like grain size) and effective porosity, specific yield and specific retention.

The relationship of effective porosity, specific yield and specific retention, to grain size of unconsolidated material has been described by experiment and observation. Unfortunately, the relationships are not simple linear functions, in part, because it is rare that a natural sediment is of a uniform grain size. Johnson (1967) compiled data from laboratory and field tests where porosity, specific yield and specific retention were estimated. Test results were reported based on grain-size distributions of unconsolidated samples collected under field conditions. Grain-size distributions are determined by passing the sediment through a series of screens with different mesh size and weighing the material remaining on each screen. Click here to link to Box 2 for additional information about analyzing grain-size distribution.

Johnson plotted the values of ne, Sy, and Sr, against the grain size (referred to as Grade Size on his figure) of the coarsest 10% of the grains (by weight) of each sample (Figure 14). The field samples did not have uniform grain size and, so, 90% of the sample (by weight) consisted of smaller grains. Johnson’s graph shows that, for his data set, the highest porosity and specific yield values were associated with sands. This is likely because, even though the boulder to gravel grain sizes would have large pores, the presence of finer grained particles in the natural sample would fill a portion of the large pore spaces reducing the porosity. Coarse sand samples showed the largest porosity and specific yield. It may be that the coarse sands had more uniform grain sizes. For samples with smaller values of maximum 10% grain size, more water is retained after gravity drainage. This is consistent with more retention being associated with materials that have large solid surface areas. A similar reduction in specific yield and increase in specific retention would be expected for samples of silt and clay.