Foundation Engineering Please With Deatailed Information

As shown in the figure, consider a soil element in a slope. The slope consists of a dry sand, and the ground water table is far below.

Let ${}_x=90k\frac{N}{m^2},{}_y=150k\frac{N}{m^2},{}_{xy}={}_{yx}=30k\frac{N}{m^2}.$ Using Mohr's stress circle, show the principal stresses and the angle of the major principal stress plane $$ measured from the horizontal direction.

A consolidated undrained triaxial compression test was conducted on the soil specimen sampled from the site. Under the lateral pressure of ${}_3=280k\frac{N}{m^2},$ the axial stress of ${}_1=680k\frac{N}{m^2}$ and the pore water pressure of $u=80k\frac{N}{m^2}$ were measured at failure. Assuming $c^{\text{'}}=0,$ obtain the value of ${}^{\text{'}}.$

Due to the rainfall, the ground water table rose and the soil element was fully saturated. Answer how much the pore water pressure $u$ will cause the failure of the soil element at that point. Here, the total stress remains unchanged due to the rise of ground water.