Consider the following statements related to ultimate bearing capacity of footing based on the SPT and CPT values: 1.The empirical correlations established in the USA between N and soil properties indicate the value of N conforms to certain standard energy ratios. 2.The value of N to be used for getting ϕ is the corrected value for standard energy. 3.The angle ϕ obtained by this method can't be used for obtaining the bearing capacity factors. Which of the above statements are correct?
- A1 and 2 onlyCorrect
- B2 and 3 only
- C1 and 3 only
- D1, 2 and 3
Solution
Concept
N values used in correlations must be corrected for standard energy, and the φ obtained from N can be used to get bearing capacity factors.
Formula
N60 = N × (Er / 60)
Step-by-step solution
1. Statement 1: US correlations between N and soil properties are based on a standard energy ratio (about 60%). This is correct. 2. Statement 2: Before getting φ from N, the field N must be corrected to the standard energy. This is correct. 3. Statement 3: The φ found from corrected N is used to find Nc, Nq and N_γ for bearing capacity. So 'can't be used' is wrong. 4. Only statements 1 and 2 are correct, so the answer is (A).
Explanation
SPT N values depend on the hammer energy, so the correlations use a standard energy ratio, and the field N must be corrected to it. The φ obtained from the corrected N is then used to find the bearing capacity factors. So statement 3 is false.
Why the other options are wrong
B: statement 3 is false. C: statement 3 is false and statement 2 is true. D: statement 3 is false.
Shortcut
The main purpose of getting φ from N is to find the bearing capacity factors, so statement 3 must be false.
Common mistake
Thinking that φ from SPT is only approximate and so cannot be used for bearing capacity factors.
Correct answer: A) 1 and 2 only