A soil has an apparent cohesion $c^{\text{'}}=5$kPa and an angle of friction ${}^{\text{'}}=35.$ A sample of

this soil is consolidated in a triaxial cell by applying an axial stress ${}_1=100$kPa, a cell

pressure ${}_3=70$kPa and a pore pressure $u=20$kPa. The sample is then failed by

increasing the axial stress under undrained conditions $({}_3$ remains constant$).$

Calculate the axial stress at failure if the pore pressure at failure $u=-10$kPa.