Case$1.$ A ring knife with an inner diameter of $79\mathrm{.}8mm$

and a height of $20mm$ was used to cut the in$-$situ sample.

Its specific gravity is $2\mathrm{.}7,$ water content is $40\mathrm{.}3\%,$ and the

mass of wet soil is $154g.$ Now, a confined compression

test is conducted. Under the pressure of $100$kPa and

$200$kPa, the total settlement of the sample is $S1=1\mathrm{.}4mm$

and $S2=2\mathrm{.}0mm$ respectively. Please calculate its

compression coefficient $a_{1-2}$ and judge the

compressibility of the soil.