Question

A borehole log of \(1.2\ \text{m}\) in length has recovered rock cores of lengths \(20,\ 8,\ 15,\ 8,\ 8,\ 4,\ 3,\ 9,\ 10,\ 1,\ 5,\) and \(10\ \text{cm}\). The RQD (in percentage) is

• A. \(29.2\)
• B. \(31.8\)
• C. \(45.8\)
• D. \(50.0\)

Hint:

Only core pieces of length \[ \boxed{\ge 10\ \text{cm}} \] are considered while calculating RQD.

Answer 1

The Correct Option is C

Step 1: Recall the definition of Rock Quality Designation (RQD). RQD is calculated as \[ \boxed{ \text{RQD} = \frac{\text{Total length of core pieces } \ge 10\text{ cm}} {\text{Total core run}} \times100. } \]

Step 2: Find the total length of qualifying core pieces. Core pieces having length at least \(10\) cm are \[ 20,\ 15,\ 10,\ 10\ \text{cm}. \] Hence, \[ \text{Total qualifying length} = 20+15+10+10 = 55\ \text{cm}. \] The borehole length is \[ 1.2\ \text{m} = 120\ \text{cm}. \] Therefore, \[ \text{RQD} = \frac{55}{120}\times100 = 45.83\%. \] Thus, \[ \boxed{45.8\%} \] is the correct answer. Therefore, \[ \boxed{(C)} \] is the correct answer.