Section 70. A gas company may, with the written consent of the aldermen or the selectmen, dig up and open the ground in any of the streets, lanes and highways of a town, so far as necessary to accomplish the objects of said corporation; but such consent shall not affect the right or remedy to recover damages for an injury caused to persons or property by the acts of such corporation. It shall put all such streets, lanes and highways in as good repair as they were in when opened; and upon failure so to do within a reasonable time, shall be guilty of a nuisance.