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

qult[ 3×37.75 + 2×28.15] 18650= 110 =For a factor of safety of 3qallThe computations for the other widths are thus:18,6502= = 6217 lb/ft3B(ft)q ult(psf)q all(pfs)2 18,650 6,2174 24,844 8,2816 31,036 10,3468 37,230 12,410Figure 5.2 shows the allowable bearing pressure as a function of the width B plottedusing the State graphs and using the new graphs. Again the State charts produce veryconservative results.Example 3In this example, the footing of the retaining wall of Example 1, is now resting on clayeysoils with an average blow count of 10 blows/ft.Solution:a) Using the MD, SHA charts (D-79-18(4)), (Fig. 1.2) the allowable bearing capacitywill range from 1000 psf to 2800 psf.b) Using the new graphs, the allowable bearing capacity is a function of the type of clay.If the clay is of medium plasticity, q all for a factor of safety of 3 will vary from 1680to 3913 psf. If the clay is of high plasticity, q all will vary from 3913 to 5967 psf.Again the State charts produce very conservative results.5-4