Calculation of true thickness of the bed

CSIR_NET_JUNE_2019

82) From a drill length of 200 cm individual cores of 30cm, 20cm, 40cm, and 12 cm lengths were obtained from a rock mass. What is the Rock Quality Designation (RQD) of a rock mass.

A) 55% 

B) 65% 

C) 25% 

D) 51%

(Thanks to Chandrasekhar, ANU)

Solution:

Rock Quality Designation:

It is a rough measure of the degree of joining (or) fracture in a rock mass, measured as a percentage of the drill core in lengths of 10 cm (or) more.

High quality Rock mass has RQD >75%

Low quality Rock mass has RQD <50%

Therefore RQD = $(\frac{l_{sum of the length of the core peices }}{l_{total sum of the core}})\times100 %$

Given that $l_{total}= 200cm$

$l_{sum of total of the core pieces}= 30+20+40+12=102 cm$

$RQD = \frac{102}{200}\times100$

RQD= 51%

Reference: https://en.wikipedia.org/wiki/Core_recovery_parameters