Question

In a standard penetration test, what is the weight of the hammer and the dropping height used as per IS 1231-1981 ?

63.5 kg, 450 mm

4.89 kg, 450 mm

2.6 kg, 310 mm

63.5 kg, 750 mm

Answer ( Option D)

63.5 kg, 750 mm

Solution

• The values of 'N' is determined at Number of Selected bore holes and average value of corrected (N) is calculated for the depth from [D_f + (1.5 – 2)B_f].

• While calculating the average value of (N), any value greater than 50% of average value is discarded.

• Suitable for Granular soil.

• Split spoon samples is used in the bore hole.

• 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 impact load of 63.5 kg having a free fall of 75 cm.

• The sampler is allowed to penetrate for 150 mm depth, but reading is not noted i.e. (No. of blows required for 150 mm penetration is not Noted).

• Then the sampler is allowed to penetrate, further for 300 mm and No. of blows required to penetrate the sampler to this 300 mm is the SPT N-value

Corrections required for N-values :

1. Over burden correction

• It is necessary because the N-value will have effect on it due to confinement of soil at various depth.

• Two granular soils possessing the same relative density but having different confining pressures are tested, the one with a higher confining pressure will give higher N value.

• If N₀ = observed S.P.T. value then,

Where,

= Effective stress at level of test (kN/m²)

N₁ = Corrected N-value of overburden.

• Overburden correction will not be applied if

2. Dilatancy Correction

• It is applied to the already corrected N-values for overburden pressure. Dilatancy correction is required only if [N₁ > 15] in saturated fine sand and silt (i.e., when water table is above test level). Corrected N value after Dilatancy Correction i.e. N₂ is

• This correction becomes more significant for fine dense sand.