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Observed N-value of an SPT test is 21. The N-value after correcting for dilatancy is
- a)18
- b)21
- c)19
- d)15
Correct answer is option 'A'. Can you explain this answer?
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Observed N-value of an SPT test is 21. The N-value after correcting fo...
Concept;
Dilatancy correction:
It is to be applied when No obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131-1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when No > 15) using the equation
N0 - SPT value after overburden correction
N0 - SPT value after overburden correction
Calculation:
Given: N0 = 21
N = 15 + 1/2(21−15)
⇒ N = 18
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Observed N-value of an SPT test is 21. The N-value after correcting fo...
Explanation:
The N-value in a Standard Penetration Test (SPT) is a measure of the resistance of the soil during the test. It is the number of blows required for the penetration of the standard sampler to achieve a depth of 12 inches (or 300 mm). However, the observed N-value may not accurately represent the soil's true strength due to various factors such as dilatancy.
Dilatancy:
Dilatancy is a phenomenon exhibited by certain soils, particularly granular soils, where the soil particles tend to separate and rearrange when subjected to shearing forces. This results in an increase in volume or dilation of the soil mass. Dilatancy can affect the observed N-value and may lead to an overestimation of the soil strength.
Correcting for Dilatancy:
To correct for dilatancy and obtain a more accurate N-value, the correction factor is subtracted from the observed N-value. The correction factor for dilatancy typically ranges from 2 to 6 blows per foot (or 6.6 to 19.7 blows per meter) of penetration.
In this case, the observed N-value is 21. After correcting for dilatancy, the correction factor of 3 is subtracted from the observed N-value.
Therefore, the corrected N-value is:
21 - 3 = 18
Conclusion:
The corrected N-value after accounting for dilatancy is 18. Therefore, the correct answer is option 'A' (18).
The N-value in a Standard Penetration Test (SPT) is a measure of the resistance of the soil during the test. It is the number of blows required for the penetration of the standard sampler to achieve a depth of 12 inches (or 300 mm). However, the observed N-value may not accurately represent the soil's true strength due to various factors such as dilatancy.
Dilatancy:
Dilatancy is a phenomenon exhibited by certain soils, particularly granular soils, where the soil particles tend to separate and rearrange when subjected to shearing forces. This results in an increase in volume or dilation of the soil mass. Dilatancy can affect the observed N-value and may lead to an overestimation of the soil strength.
Correcting for Dilatancy:
To correct for dilatancy and obtain a more accurate N-value, the correction factor is subtracted from the observed N-value. The correction factor for dilatancy typically ranges from 2 to 6 blows per foot (or 6.6 to 19.7 blows per meter) of penetration.
In this case, the observed N-value is 21. After correcting for dilatancy, the correction factor of 3 is subtracted from the observed N-value.
Therefore, the corrected N-value is:
21 - 3 = 18
Conclusion:
The corrected N-value after accounting for dilatancy is 18. Therefore, the correct answer is option 'A' (18).
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Observed N-value of an SPT test is 21. The N-value after correcting for dilatancy isa)18b)21c)19d)15Correct answer is option 'A'. Can you explain this answer?
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