Standard Penetration Test is the most used test in India and many other countries like USA. The test employs a split spoon sampler, consists of driving shoe, split barrel and coupling.
- The bore-hole is advanced up to the required depth and bottom is cleaned.
- Sampler ( split spoon ) is attached to the standard drill rod, lowered into the bore hole and rested at the bottom.
- Then split spoon sampler is driven into the soil for a distance of 450 mm by blows of drop hammer of 65 k.g falling freely from the height of 750 mm.
- The number of blows required to penetrate soil up to 150 mm is recorded during the driving of sampler and still continuous up to a depth of 450 mm.
- The number of blows required for the last 300 mm of penetration is added together and recorded as the N value at that particular depth of the bore hole. The number of blows required for first 150 mm penetration, called seating drive is rejected.
- The sampler is then take out from the bore hole and is detached from the drill rods. Sampler is then separated from the cutting shoe and the coupling. The soil sample collected from the barrel is carefully preserved and transported to the lab for tests.
The standard penetration test is carried out at every 0.75 m vertical interval in a bore hole. Interval distance can be increased up to 1.5 m if the depth of bore hole is large.Due to the presence of boulders, it may not be possible to drive the sampler to a distance of 450 mm. Then in this case, value of N can be recorded for the first 300 mm penetration.
- sampler must be in good condition and cutting shoe must be free from wear and tear.
- Bottom of bore hole must be cleaned before the test carried out.
- Falling of drop hammer must be right weight and falling should be free and vertical.
- The drill rod attached with sampler should not bent.
- The height of falling of drop hammer must be 750 mm exactly.
Correction for overburden pressure:-
If two soil possessing the same relative density but having different confining pressure during the test, the one with a higher confining pressure gives a higher N value. This confining pressure is directly proportional to the overburden pressure might be due to the presence of boulders during the test and it increases the value of N. Then the N value at shallow depths are underestimated and the N value at larger depths are overestimated. So, if no correction is applied to recorded N value, the relative density of soil at shallow depths is underestimated and at higher depths, it will be overestimated.
Then to correct N value from this overburden pressure, given below equation is used,
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 dilatancy:-
It is applied when N’ obtained after overburden correction exceed in saturated fine sand. Dilatancy correction is carried out when N’ > 15
N” = 15 + 0.5 ( N’ -15)
Where N” = final corrected value to be used .