SPT-N Convertion to N60 and N1,60

SPT correction Several correlations, make use of the energy corrected SPT blow count, denoted as N60 where 60 is the percentage of  the theoretical free-fall hammer energy. Measured blow counts may additionally be adjusted to an effective overburden pressure of 100 kPa which produces the N1,60 value. In some cases various other factors that influence the SPT results can be taken into consideration when calculating N1,60 such as bore-hole diameter, diameter, rod length e.t.c. SPTCorr provides calculations for both N 60 and N1,60.

For most geotechnical applications N60 and N1,60 values are calculated based on the following equations: N60 = Nfield x Ce N1,60 = Nfield x Ce x Cn [1] Equation [1] is used only when the Simplified calculation for N1,60 is checked. When unchecked, N1,60  value is calculated according to: N1,60 = Nfield x Ce x Cn x Cr x Cb x Cs where:

Ce is the energy correction factor which depends mainly on the way that hammer is lifted and released. Some typical values are presented on the table below (after Skempton, 1986) Factor

Energy ratio

Equipment variable

Correction

Donut Hammer

0.50 to 1.00

Safety Hammer

0.70 to 1.20

Automatic-Trip Automatic-Trip Donut-type 0.80 to 1.30 Hammer

Cn is the overburden correction factor. Measured blow count is adjusted to a 100 kPa overburden pressure according to an equation that you can select from the Calc. method   pop-up menu. Clicking on the down arrow opens a menu where you can select one of the suggested equations. Clicking on the button opens a new dialog where these functions are visualized. SPTCorr applies a constraint on C n value as suggested in various publications (e.g. SP117). Cr is the rod length correction factor which depends on the total length of the drill rod. Cb is the bore-hole diameter correction factor  and its is set according to the selected diameter from the drop down list. Cs is the liner correction factor and its value depends on the sampler used to perform the test. The split spoon sampler may contain liner or not. The Cs value for the sampler with liners is 1.00 while for samplers without liners value ranges from 1.10 to 1.30. Copyright © 2006, GeoLogismiki