Updating Bearing Capacity – SPT Graphs - Maryland State Highway ...

CHAPTER IINTRODUCTION1.1 General OverviewThe graphs used by the Maryland State Highway Administration (MD SHA) fordetermining the allowable bearing capacity of shallow foundations are presented in Policy andProcedure Memorandum Memo No. D-79-18-(4), March 29, 1979. The graphs were originallypublished in Basic Soils Engineering by B.K. Hough, published by the Roland Press Company,1957. The graphs, Fig. 1.1 and Fig 1.2, related the Standard Penetration Test (SPT) in blows/ftto the maximum advisable presumptive bearing value. The major drawbacks to the use of thesegraphs are that they do not consider the effects of the depth, shape and the size of the footing, thelocation of the water table, or the factor of safety considered. They are also very conservative,which suggests that they do not provide accurate, cost effective designs, and in the case of a highwater table they provide unsafe values, however, their main advantage is that they are simple touse.1.2 Objective of the StudyThe objective of the study is to develop new graphs that relate the SPT N-value to thebearing pressure. The new graphs are to use the American Association of State Highway andTransportation Officials (AASHTO) bearing capacity equations. The new graphs will thusconsider the effects of the depth, shape and size of the footing, soil properties, factor of safety,and location of the water table. The challenge here is to have the new graphs as simple to use asthe current ones.1-1