In soil exploration, the Standard Penetration Test (SPT) is widely used. If the measured 'N' value is corrected for overburden pressure and dilatancy, the 'N' value is generally recorded for a penetration of
- AThe first 150 mm
- BThe last 300 mm of a total 450 mm penetrationCorrect
- CThe full 450 mm penetration
- DOnly the first 300 mm
Solution
Concept
Standard Penetration Test (SPT) N-value Determination
Formula
N/A
Step-by-step solution
1. Recall SPT procedure: The sampler is driven a total depth of 450 mm in three 150 mm steps. 2. Ignore the first 150 mm penetration as a seating drive due to top soil disturbance. 3. Record the N-value based on the hammer blows required for the last 300 mm penetration.
Explanation
In the Standard Penetration Test (SPT), a sampler is driven a total depth of 450 mm into the ground in three 150 mm steps. The first 150 mm drive is ignored because the soil at the top is disturbed by drilling. The N-value is calculated by counting the number of hammer blows needed for the last 300 mm of penetration.
Shortcut
To remember SPT penetration rules, use the rule "Ignore First 150, Count Last 300": - Total Depth: 450 mm divided into three 150 mm intervals. - First 150 mm: Seating drive (ignored due to surface disturbance). - Last 300 mm: Actual test interval used to calculate the N-value.
Common mistake
A common mistake is selecting option C by incorrectly assuming the N-value accounts for the total 450 mm drive depth, forgetting that the first 150 mm is treated as a seating drive and ignored due to soil disturbance. Another frequent mistake is selecting option A by confusing the initial 150 mm seating drive with the actual test interval used for counting blows.
Correct answer: B) The last 300 mm of a total 450 mm penetration