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
Where
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 dissipated 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”.
Also Read: Standard Penetration Test Theory
Also Read: How to Calculate Bearing Capacity of Soil From Standard Penetration Test?
Also Read: 20 Factors Affecting Standard Penetration Test Result
Also Read: 9 Precautions to Take While Performing Standard Penetration Test
Also Read: How to Calculate Pile Load Capacity Using SPT or N Values?
Also Read: Corrections Applied to SPT Values
several correction factors are applied to the feild N value before using it to interprete the soil condition in a borehole,name the correction factor
Any other correction which related to depth of water table ?
Why N” value is greater than 15 when what we have to do is to correct it coz its already higher than 15
Why N (penetration number) value is not corrected for overburden and dilatancy correction for clay soil?
why the overburden pressure correction is applied before the dilatancy correction.
is there any variation in the design when using corrected N Value instead of the actual spt N value from field work (pile design)
Yes. Because what value we gain from field. That is not accurate. Now you can ask me. Why they are not accurate?
Because-
1.Sometimes human error can happened.
2.In our field worker are not properly skillful.
3. When we hammer the sampler for 6 inch, it can pass 6 inch or not. Because this is not mechanical work result.
There are so many things for not accurate the SPT-N value. Thats why we don’t use field value
How do i correct N-spt in relation to overburden pressure and atmospheric pressure.Does it matter the type of soil?
There are so many articles relating this aspect. Just type your question on google and see the result. Or check any geotechnical textbook and you will fond it also.
This website is very nice for discussion.