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

CHAPTER VAPPLICATIONS OF CURRENT AND NEW GRAPHSTo show the difference between bearing capacity values determined using the Stategraphs and the new graphs that are based on AASHTO equations, the following examples arepresented.The examples will assume that a footing for a retaining wall rests on level ground and issubjected to vertical loading. For the retaining wall, the footing can be considered continuouswith its length L larger than its width B (at least L > 5B).Example 1For the footing of a retaining wall, calculate the allowable bearing capacity if the footingwidth is B ft and its base rests 3 ft below the ground surface. Assume the soil below the footingis a uniform fine sand with an average value of standard penetration resistance (blows/ft) of 6.Solution:a) Using the MD, SHA charts (D-79-18(4)), (Fig. 1.1) the allowable bearing capacity for theuniform fine sand with 6 blows/ft is 2000 lb/ft 2 .b) Using the new graphs, the allowable bearing capacity is a function of the width B, hence,it will be calculated in this example for B = 2, 4, 6, 8 ft. Assume also that γ = 110 lb/ft 3 .For a Wall Footing:[ ]q ult= γ Dx 1+ Bx 2From Table 4.1, for N = 6, x 1 = 16.09 and x 2 =9.41.Thus: = 110 [ D × 16.09 + B × 9.41]q ultFor B = 2 ft and D = 3 ft[ 3×16.09 + 2×9.41] 7380q = 110 = psfult5-1