Question

Describe how the Standard Penetration Test (SPT) is conducted. How is the raw number of blow count (N) computed? What corrections are performed to obtained (N₁)₆₀?

Answer 1

Procedure of SPT

SPT is suitable for granular soil.Bore hole is advanced to a depth at which N value is to be calculated.The split spoon sampler is allowed to penetrate into the soil by applying load of 65kg having a free fall of 75 cm.The sampler is allowed to penetrate for 150 mm depth but reading is not noted.Then sampler is further allowed to penetrate 300 mm and number of blows required to penetrate 300 mm is noted.The next test is carried out at level of 750mm below the previous test reference level.thats how N value is collected.

The correction applied to observed N value are

1) over burden correction,

Necessary because the N value will be effected due to confinement of soil at various depth

2)dilatency correction

It is applied to already corrected N value for overburden pressure.It is required only when N>15 in saturated fine soil and silt