Question

During the subsurface investigation for design of foundation a SPT test was conducted at 3 m below ground surface. The record number of blows are as given:

Penetration depth	No. of blows
0 -75 mm	6
75 -150 mm	7
150 -300 mm	12
300 -450 mm	13

The water table is 2 m below ground surface. The correction factor for overburden is 1.2, the corrected N value for the soil will be:

• A. 22
• B. 30
• C. 25
• D. 28

Answer 1

The correct option is A 22

Number of blows for last 300 mm penetration
N = 12 + 13 = 25
Correction for overburden, $N_1=1.2$ N = 30
Since the water table is 2 m below G.L hence correction for dilatancy will be applied
$N_2=15+(N_1-15)\times 0.5$
$=15+15\times 0.5$
$=22.5\simeq 22$