Except as otherwise provided by this Code, it shall be unlawful for any person to injure or obstruct any public street or alley by placing upon or across the same anything whatever that will injure the same or retard the public travel thereon, or by encroaching upon the same, by any fence, or by plowing or digging any ditch or opening thereon, or by taking any earth therefrom; provided, that this section shall not apply to the City in the making of such improvements and repairs as it deems necessary on its streets and alleys, or any work that may be done thereon by the authority of the City.