The N-value observed during testing is not utilized directly in assessing soil properties. These values are corrected to account for
- The overburden pressure
- Dilatancy in saturated fine sands and silts
Correction for overburden pressure
The penetration resistance of soil depends on the over burden pressure. At deeper depth in-situ soil will have higher overburden pressure hence its response to SPT test will be better when compared to the behavior of the same soil at shallow depth.
Bazaraa (1967 Bowels, p99) proposed the following corrections to the actual count N, based on the over burden pressure
For p0 <= 75 kPa
For p0 > 75 kPa
N’ = corrected N value
N = observed N-value
P0 = over burden pressure, (kPa) = γ x D
D = depth of testing (m)
γ = unit weight of soil at the time of testing
- N’ is increased from the actual blow count when p0 <=75 kPa
- N’ is decreased from the actual blow count when p0 >75 kPa
Correction for the dilatancy in saturated fine sands and silts
When dynamic loads are applied on silty and fine sandy soils in saturated state the pore pressure in such soil will not be in a position to get dissipitated due to low permeability. Hence, during dynamic loading (i.e. application of blows) the pore water will offer a temporary resistance to dynamic loads. This leads to higher value of N-value which is unsafe. Therefore when SPT is performed in saturated silts and fine sands and if the observed N-value is more than 15, a correction has to be applied to reduce the observed values. This correction is applied on the N-value corrected for over burden pressure (N’).
If the stratum (during testing) consists of fine sand & silt below water table, the corrected N-value (N’) has to be further corrected to get the final corrected value N”.