(A) Where required by the Engineer, pipe lines shall be constructed in tunnel. This work will be conducted in accordance with requirements of any permits obtained by the owner from railroads or state or county highway departments for tunnel work or in accordance with the following requirements:
(1) Materials. Pipe materials shall be shown on the plans or as described in the special provisions.
(2) Excavation and Laying. Requirements for excavation, laying, and joints shall be those applicable for the type of pipe line involved, unless otherwise specified.
(3) Methods of Construction.
(a) The tunnel shall be only of sufficient width and height to provide free working space. The sides and roof of the tunnel shall be braced sufficiently to support the external loads and to prevent caving, bulging, and settling of the earth.
(b) The Contractor shall backfill all tunnels with well-compacted sand, fine gravel or stone screenings as rapidly as the conditions permit.
(c) The backfill material shall be deposited in the tunnel in such a manner as not to injure or disturb the pipe. The filling of the tunnel shall be carried on simultaneously on both sides of the pipe in such a manner that injurious side pressures do not occur. Special care shall be taken to compact the backfill under the haunches of the pipe. The remainder of the tunnel, or such portion of the remainder as may be possible, shall then be backfilled by one of the following methods, at the option of the Contractor, if in the opinion of the Engineer the method is practicable.
1. The material shall be deposited in uniform layers not to exceed 12 inches thick (loose measure) and such layer either inundated or deposited in water; or
2. The tunnel shall be backfilled with loose material or only partly backfilled at a time, if necessary, and settlement secured in either case by introducing water through holes jetted into the material to a point approximately two feet above the top of the pipe.
(d) If neither of the above methods is practicable or can be used for only a portion of the backfill, the remainder of the tunnel shall be completely backfilled with material carefully deposited in layers and each layer compacted by ramming or tamping with tools approved by the Engineer.
(e) When sheeting and bracing have been used, sufficient bracing shall be left across the trench as the backfilling progresses to hold the sides and top firmly in place without caving or settlement before the backfilling has been placed. This bracing may be removed as soon as practicable.
(f) Any depressions which may develop within the area involved in the construction operations due to settlement of the backfilling material shall be filled in a manner meeting the approval of the Engineer.
(4) Use of Casing Pipe. The Contractor may, subject to the approval of the Engineer, use metal casing pipe as a tunnel liner in place of timber shoring for tunnel sections. The diameter, gauge and type of such pipe, method of placing and method of installing pipe within it shall be subject to the approval of the Engineer. The entire void space between tunnel liners and pipe shall be filled with compacted sand or other approved material if such method of construction is used.
(5) Jacking or Boring of Pipe. The Contractor may, subject to the approval of the Engineer, use special cast iron or specially designed reinforced concrete pipe jacked and/or bored into position with or without tunnel liners, for tunneled sections of pipe. In such cases, all conditions of performance of the work shall be subject to the approval of the Engineer.