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

For a factor of safety of 3, the allowable q isqall7380 = =22460 lb/ft3The computations for the other widths use the same equation, with the following results:B(ft)q ult(psf)q all(psf)2 7380 24604 9450 31506 11520 38408 13590 4530Figure 5.1 shows the allowable bearing pressure as a function of the width B plotted using thestate graphs and using the new graphs. As can be seen, the bearing pressure from the Stategraphs is very conservative. The result of this is that their use will not provide a cost effectivedesign.Example 2In Example 1, a low blow count was used for the sand. In this example, all parametersare the same as in Example 1, except the blow count is 30.Solution:a) Using the MD, SHA chart, (D-79-18(4)), the allowable bearing capacity for the uniformfine sand with 30 blows/ft is 6000 lb/ft 2 .b) Using the new charts, for a blow count of 30, x 1 = 37.75 and x 2 = 28.15.Thenq ult[ D × 37.75 + 28.15]= 110 B ×For B = 2 ft5-2