The streets, cross-walks, and sidewalks shall be kept clean, clear, and free for the passage of carts, wagons, carriages and street or steam railway cars, or pedestrians, unless otherwise authorized by special permission in writing from the Engineer. In all cases a straight and continuous passageway on the sidewalks and over the cross walks of not less than three feet in width shall be preserved free from all obstruction. Where any cross walk is cut by the trench it shall be temporarily replaced by a timber bridge at least three feet wide, with side railings, at the Contractor’s expense. The placing of planks across the trench without proper means of connection or fastenings, or pipe or other material, or the using of any other makeshift in place of properly constructed bridges, will not be permitted. This is equally applicable to certain wagon bridges to be fixed upon by the Engineer, on the basis of traffic requirements. In streets that are important thoroughfares or in narrow streets the material excavated from the first one hundred feet of any opening or from such additional length as may be required, shall upon the order of the Engineer, be removed by the Contractor, as soon as excavated. The material subsequently excavated shall be used to refill the trench where the sewer has been built. The preceding specifications are applicable to open-trench excavation. Rigid restrictions are placed about tunneling because of the greater difficulty of doing good work, the greater danger to life and property and the possibility of later surface subsidence if the backfilling is done improperly. A common clause in specifications is: All excavations for sewers and their appurtenances shall be made in open trenches unless written permission to excavate in tunnel shall be given by the Engineer.