Standard penetration test SPT results in a granular soil dep

Standard penetration test (SPT) results in a granular soil deposit are summarized in Table below. Assume that 7- 18.5 kN/m and correct the SPT numbers using the Skempton\'s relationship (1986): . (i) What will be the net allowable bearing capacity of a foundation for a foundation 2m x 2m? Assume Dr 1.5 m. The allowable settlement 20 mm. (ii) Why do we use SPT method when there is a rigorous method such as the Meyerhof\'s general the bearing capacity method for the design of foundations? Field standard penetration number, N60 10 12 13 4 16 19

Solution

Let energy correction factor be C_e=o.8

and P_a=100KPa

N_avg of the wholw strata=9.85 (we does not include value at 16m depth due to irregular variation)

From Fig. 13.5 Q = 10KPa for B = 2 m and N_avg=9.85=10

safe bearing pressure = q=16xQ = 160 KPa

net safe bearing pressure = q - 18.5*1.5=131.95KPa

Advantage of SPT method over other rigorus method are:-

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Depth(m)	Nfield	Cn	Overburden pressure(KPa)	N160=Cn*Nfield*Ce
1	10	1,6875	18.5	13.5
4	12	1.15	74	11.04
7	13	0.87	129.5	9.048
11	11	0.66	203.5	5.808
16	19	0.505	296	7,676