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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
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B
30
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C
25
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D
28
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Solution

The correct option is A 22
Number of blows for last 300 mm penetration
N = 12 + 13 = 25
Correction for overburden, N1=1.2 N = 30
Since the water table is 2 m below G.L hence correction for dilatancy will be applied
N2=15+(N115)×0.5
=15+15×0.5
=22.522

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