def _calc_mean_confining_stress(sigma_v0, K0):
    """
    Calculate mean (of three directions) confining stress.

    Parameters
    ----------
    sigma_v0 : numpy.ndarray
        (Effective) vertical stress of each layer. Unit: Pa.
    K0 : numpy.ndarray
        Lateral stress coefficient of each layer. Unit: 1.

    Returns
    -------
    sigma_m0 : numpy.ndarray
        Mean effective confining stress (of three directions). Unit: Pa.
    """
    sigma_m0 = (2 * K0 + 1) / 3.0 * sigma_v0
    return sigma_m0
