A CU triaxial test is conducted on two clay samples and the results are:

Sample no$\mathrm{.}1$: $\backslash$sigma $1=350$ kPa, $\backslash$sigma $3=200$ kPa

Sample no$\mathrm{.}1$: $\backslash$sigma $1=700$ kPa, $\backslash$sigma $3=400$ kPa

The shear stress on the failure surface is $\_\_\_\_\_\_\_\_$ kPa