Question

A clean sand deposit has a total unit weight gabove the groundwater table of

18.9 kN/m3and a submerged unit weight of 9.84 kN/m3. The groundwater table

is located 1.5 m below ground surface. Standard penetration tests were

performed at 3 m below the ground surface with a blow count of 3 blows for

the first 150mm, 4 blows for the second 150mm and 5 blows for the third

150mm. Estimate the shear wave velocity of the sand at 3 m below the ground

surface. If a cone penetration test was performed at 3m below the ground

surface and the cone tip resistance was 3.9 MPa, estimate the shear wave velocity.

Compare the values obtained from both approaches.

Answer 1

Ans) We know, the relation between SPT-N values and shear wave velocity as:

V_s = 80.6 N_c^0.331

where, V_s = shear wave velocity

N_c = corrected SPT N value due to overburden pressure

Number of blows for first 150mm penetration is neglected therefore,

N = 4 + 5

Hence , N =9

This value of N is to corrected for overburden pressure

N_c = N x 350 /(+ 70)

where, N_c is corrected N value

= effective vertical stress

Also, according to question,

= (18.9 x 1.5) + (9.84 x 1.5)

= 43.11 kN/m²

Putting values,

N_c = 9 x 350 / (43.11 +70)

= 28

Putting value of N in above formula ,

V_s = 80.6 x(28)^0.331

On solving,

V_s = 242.85 m/s

Now, shear wave velocity in terms of cone tip resistance can be given as ,

V_s = 1.75 q^0.627

where, q is cone tip resistance in kPa

Putting values,

V_s = 1.75 (3900)^0.627

On solving,

V_s = 312.35 m/s

As we can see value of shear wave velocity obtained from cone penetration test is about 22% more then what obtained from SPT- N value