MUHAMMAD FIRAS AL-WA'IE BIN WADI

The shearbox cannot measure pore pressure. In an undrained shearbox, you shear quickly so water can't escape — but you have no idea how much pore pressure u has built up inside. This creates two separate envelopes on the same graph: one in total stress (what you measure), one in effective stress (what actually governs failure). They are parallel — offset by the pore pressure u.