def strain_static(p, pu, yc):
    """
    Static strain using p-y model developed by Matlock
    
    Parameters
    ----------------
    p -- stress
    pu -- ultimate stress
    yc -- ultimate displacement
    
    Output
    ----------------
    y -- soil strain
    """
    if p <= pu:
        y = (p / (0.5 * pu)) ** 3 * yc
    else:
        raise ValueError('The applied stress is LARGER than ultimate strength')
    return y
