In light of these influence factors listed, the N60 may be revised as (Skempton 1986):
where N60 = SPT N value corrected for field procedures; Em = hammer efficiency; CB = borehole diameter correction; Cs = sample correction; CR = rod length correction; and N = measured SPT N value. In addition, the SPT data should be adjusted to accommodate the effect of effective stress and field procedures. This leads to a corrected SPT N’60 value of
where CN is a modification factor for overburden stress (see Figure 1.1) with CN = 2/(1+ ) v a p / (normally consolidated fine sands), CN = 3/(2 + ) /pa (normally consolidated coarse sands), and CN = 1.7/(0.7 + ) v a p / (overconsolidated fine sands) (Skempton 1986); pa = 100 kPa, atmospheric pressure; and v = vertical effective stress at the test location (see Figure 1.1).
