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Determination of Relative Density of Sand



Standard Penetration Test (SPT). The standard penetration resistance value N_SPT , often referred to as the blowcount, is frequently used to estimate the relative density of cohesionless soil. N_SPT is the number of blows required to drive a standard splitspoon sampler (1.42" I.D., 2.00" O.D.) 1 ft. The split spoon sampler is driven by a 140-lb hammer falling 30 inches. The sampler is driven 18 inches and blows counted for the last 12 inches. N_SPT may be determined using standard method ASTM D 1586. In order to calculate the relative density, the N_SPT value is normalized to an effective energy delivered to the drill rod at 60 percent of theoretical free-fall energy

N₆₀ = C_ER * C_N * N_SPT Where:

N₆₀ = penetration resistance normalized to an effective energy delivered to the drill rod at 60 percent of theoretical free-fall energy, blows/ft

C_ER = rod energy correction factor, See Table Below

C_N = overburden correction factor, See Table Below

Rod Energy Correction Factor C_ER

Hammer Type	Hammer Release	CER
Donut	Rope and Pulley	1.12
Safety	Rope and Pulley	1.00
Donut	Free Fall	1.00

Correction Factor C_N

CN	effective overburden pressure (ksf)
1.6	0.6
1.3	1.0
1.0	2.0
0.7	4.0
0.55	6.0
0.50	8.0

Relative density of sands can now be calculated by:

D_r = 100(N₆₀/60)^0.5
D_r >= 35 percent. N₆₀ should be multiplied by 0.92 for coarse sands and 1.08 for fine sands.



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