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The most recent empirical correlations of NSPT, relative density and angle of shearing resistance of granular soils are calculated using the penetration resistance in terms of N60 (NSPT=N60 according to Cestari, 1996) or of N1,60, normalized value at a geostatic vertical pressure of 1 bar by applying the coefficient CN (Liao and Withman 1986).
N60=NSPT*CE
N60: Value of corrected NSPT for an efficiency of 60% (according to Cestari N60=NSPT)
CE=ER/60
The energies of the different systems vary from 45% to 98%
N1,60= NSPT*CE*CN*CR*CB*CS
CN=(Pa/σ’v)^1/2 Liao and Withman ( 1986)
pa=atmospheric pressure = 98.1
CN=2/(1+σ’v/100) Skempton (1986) Fine sands
CN=3/(2+σ’v/100) Skempton (1986) Coarse sands
Normalization at an effective stress of 100 Kpa
CN: Overburden correction factor
CE: Energy correction factor
CR: Rod length correction factor
CB: Bore-hole diameter correction factor
CS: Liner correction factor
ER: Hammer energy transfer ratio according to ASTM D-4633-86
CN in practical application cannot exceed 2 and preferably not exceed 1.5. The correction of CN is applied only for the computation of relative density and angle of shearing resistance, and is not applied to the computation of the undrained parameters and deformation parameters.
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