Krishnendu,
My answer mainly assumes sandy conditions. I would tend to not want to use a canned plot of settlement vs. N value for settlement analysis. There are many site specific items that are used to complete a settlement analysis which include N value among other items such as structure type and usage, other underlying soils types for which the Standard Penetration Test (SPT) N Value may not be applicable, etc. Structure size, type, soil/structure interaction, and usage is a very important consideration in the final settlement estimate that deserves more attention than it is usually given.
I've found the best way to deal with settlement issues in Sandy soils is to take the N values and make a good estimate of relative density based on the existing overburden pressure, sand type, and SPT methods using an appropriate relationship such as the one provided by Gibbs and Holtz. When you have the best estimate of relative density, you can make a good estimate of the modulus or compressibility of the soils. This infomation can be used directly in most well-known settlement analyses. It should be noted that soil type affects the N value very much. Coarse sand or gravel can raise the N value artificially high. Pore pressures developed during the SPT in silty sands can raise or lower the N value remarkably. The idea is to look at all aspects of the project to get "the best" estimate(s) for settlement. So to answer your question directly, your charts were developed with many assumptions so I wouldn't be able to give you the mathematical relationship between the two. Please let me know if any of this helped.
