In the falling head permeability test, in the duration of 3 hours the initial head of 1000 mm is dropped to 350 mm. The diameter of the stand pipe is 0.5 cm. The length and diameter of the soil specimen are 20 cm and 10 cm respectively. What is the coefficient of permeability (mm/s) of the soil?

A

$$ 4.86 \times 10^{-8} $$

B

$$ 4.86 \times 10^{-5} $$

C

$$ 4.86 \times 10^{-3} $$

D

$$ 5.23 \times 10^{-5} $$