code: def get_v_total_stress_at_depth(self, z):
          
  
          if not hasattr(z, "__len__"):
              return self.one_vertical_total_stress(z)
          else:
              sigma_v_effs = []
              for value in z:
                  sigma_v_effs.append(self.one_vertical_total_stress(value))
              return np.array(sigma_v_effs)
docs: Determine the vertical total stress at depth z, where z can be a number or an array of numbers.