Not all the pores of a rock or soil are available for flow of water. Some water

Question

Not all the pores of a rock or soil are available for flow of water. Some water does not flow because of molecular forces, surface tension, and dead-end pores. Effective porosity (n) is the amount of pore space that is available for transmitting water. (In some coarse-grained materials effective porosity approximates specific yield and gravity drainage.) Effective porosity is difficult to measure and is often approximated from total porosity and lab test data. Effective porosity is a factor in velocity of groundwater flow and is expressed as a percentage (Table

Explanation / Answer

Effective porosity is one of the best indicators of the hydraulic properties of a soil medium. It is a more reliable parameter than the porosity itself. Table 12.1 demonstrates this more clearly. The porosity of clay, for instance, is more than that of the sand, however, the hydraulic conductivity is less. The effective porosity, on the other hand, is indicating a better understanding of the hydraulic properties of the medium as it is proportionately decreasing as the hydraulic conductivity decreases. Even the range is proportional to the hydraulic conductivity range and thus effective porosity serves as the better indicator of the hydraulic properties of a medium.

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