For the same foundation in Fig. P$7\mathrm{.}20,$ determine the average vertical stress increase $({}^{\text{'}})$ at $6m$ and $15m$ of depth applying the approximated method called $2$:$1$ Method $($Section $7\mathrm{.}11\mathrm{.}7)$ and sketch the problem. How do the estimated values compare with the values calculated in Problem $1$ for point $A$ at the same depth?

$7\mathrm{.}20$ A rectangular foundation $4m6m($Figure P$7\mathrm{.}20)$ transmits a stress of $100$kPa on the surface of a soil deposit. Plot the distribution of increases of vertical stresses with depth under points $A,B,$ and $C$ up to a depth of $20m.$ At what depth is the increase in vertical stress below A less than $10\%$ of the surface stress?