code: def stress_vert(self, depth_within=0, effective=False):
          
          assert depth_within <= self.thickness
          stress_vert = self._stress_vert + depth_within * self.unit_wt
          if effective:
              pore_pressure = self._profile.pore_pressure(self.depth +
                                                          depth_within)
              stress_vert -= pore_pressure
          return stress_vert
docs: Vertical stress from the top of the layer [kN//m²].