That table has been in most building codes in the Northeast for a long time. The values are averages based on soil type and allowing for a maximum of 1” settlement.
Anyone interested in this as to where it came from, get out the 1948 edition of "Soil Mechanics in Engineering Practice" by Terzaghi and Peck. Art. 54, page 413, 1948 edition John Wiley & Sons. They have a rather compete history and explanation of these tables, so many out there. In my review the 1" settlement is not mentioned. Hey you youngsters, that was the textbook for my first Soil Mechanics course, the most popular at the time.
Some geotechnical guys I work with have always said those values were based on a factor of safety of 3 and would ensure a maximum settlement of 1”. I don’t know if that is true.
The numbers appear to be conservative, but do not consider any property other than classification. If the granular soil is very loose, or the foundation is both narrow and shallow, those number could be unconservative, Same comment if the fine-grained soils are soft. Good judgement is still needed.
Thank you for all your response. I just thought that, maybe there is some statistical basis behind these values. Does the author perform monte carlo simulation or something.
Again, I’ve only seen these in the Residential Building Code. The Residential Code is limited to dwellings up to three stories, so you are talking very light loads for which these presumptive values are fine for use. For a much more expansive table see AASHTO LRFD Chapter 10. The list is much longer with better descriptions for the service limit state. The same tables are probably published in other codes too. The expansive tables will give you more soil classifications and values for dense to loose soils.
