The corrected N values given by

in which corrected value of observed N; = correction factor for overburden pressure.

Di1atancy correction is to be applied when obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when > 15) using the equation

=15+0.5 ( — 15)

where = final corrected value to be used in design charts.

If =

>15 is an indication of a dense sand. In such a soil, the fast rate of application of shear through the blows of a drop hammer, is likely to induce negative pore water pressure in a saturated fine sand under undrained condition of loading. Consequently, a transient increase in shear resistance will occur, leading to a SPT value higher than the actual one.