Correct Answer - Option 1 :
\(15 + \left( {\frac{{N - 15}}{2}} \right)\)
Explanation:
Dilatancy correction:
It is to be applied when N obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when N > 15) using the equation
N’ = 15 + 0.5(N – 15)
N 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.
Correction for overburden pressure:
(i) Several investigators have found that the penetration resistance or the N value in a granular soil is influenced by the overburden pressure.
(ii) Of two granular soils possessing the same relative density but having different confining pressures, the one with a higher confining pressure gives a higher N value.
(iii) Since the confining pressure (which is directly proportional to the overburden pressure) increases with depth, the N values at shallow depths are underestimated and the N values at larger depths are overestimated. To allow for this, N values recorded from field tests at different effective overburden pressures are corrected to a standard effective overburden pressure.
Let No = observed SPT number
N1 = Corrected SPT Number for overburden pressure
N1 = No.CN
\({N_1} = {N_o}\left( {\frac{{350}}{{σ + 70}}} \right)\)
Where, σ = Effective overburden pressure at test level
σ should be less than 280 kN/m2
(where CN is the correction factor for overburden)