Expert: Robert Cummings, P.E. - 12/2/2008

Question
Robert: I read an answer you gave in 2005 about rock foundations “As an aside I don't know why anybody would place a masonry foundation directly on a soil subgrade, even with normal soil compaction practices.  Even if a foundation is on rock or hardpan I would think a grade beam or leveling course of concrete would be required.”  The answer was very helpful.  I am planning to build a completely rock cabin, from the foundation up.  I had planned to embed t the first course of rock in a layer of concrete at the bottom of the foundation trench to ensure a uniform surfaced on the bottom.  Can you recommend any good reference books on rock walls and foundations?  A local community college has a construction trades program with a masonry class I will be taking in the fall.  But I would like to do some research on design and techniques ahead of time.  Thanks.  Also would you have any tips?  The cabin will be 30 x 30 on the inside with a center column to support the floor beams for a 3/4 loft.  Thanks for any help you can provide.  

Answer
There are dozens of references on foundations and soil mechanics, one of the best being by Bowles, but they tend to be quite technical and broad in their application.  In your case you are interested mainly in the design of a residential foundation to support a masonry wall.  The type of wall is not really critical except that masonry is relatively heavy, meaning the loads and potential for differential settlement are greater, and rock masonry is also somewhat less settlement-tolerant than other types of construction.  So you are well-advised to lay concrete first.

You will need to reinforce the concrete and do a thorough job of subgrade compaction to remedy soft/yielding zones and thereby prevent differential settlements.  Once again, Bowles will give some guidance on this, but there are other texts as well. Try http://searchwarp.com/swa320855.htm  and http://www.concretenetwork.com/concrete/footing_fundamentals/footing_dimensions.... as a starting point.  You may also download NAVFAC DM-7.2 off the Internet (I think vulcanhammer has a link) which gives a lot of practical, straightforward footing design equations.  But you are going to have to figure out the structural loads for yourself.

I don't know offhand of any texts on masonry wall construction (not really my central area of expertise); an Internet or library search is likely to help you zero in on practical advice.  There are many architectural trade groups that have publications available that would be practical in nature.  Check out PRG, Inc., an organization that deals with the rehabilitation and reconstruction of stone masonry and has publications available.  

Good luck.