A falling$-$head permeability test is to be performed on a soil whose permeability $($hydraulic conductivity$)$ is estimated to be $2\mathrm{.}810-6\frac{m}{s}.$ What diameter standpipe should you use if you want the head drop from $31\mathrm{.}2cm$ to $19\mathrm{.}4cm$ to occur in $5$ minutes? The sample's cross$-$sectional area is $12cm2$ and its length is $7\mathrm{.}4cm.$ Are there any benefits$/$drawbacks to using a falling head permeability test rather than a constant head permeability test?