Vertical stress from the top of the layer [kN//m²].

def stress_vert(self, depth_within=0, effective=False):
        """Vertical stress from the top of the layer [kN//m²]."""
        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