The Contractor may, at his option, where working conditions and right-of-way permit (as determined by the Engineer), excavate pipeline trenches with sloping sides, but with the following limitations:
(A) In general, only braced and vertical trenches will be permitted in traveled streets, alleys or narrow easements.
(B) Where trenches with sloping sides are permitted, the slopes shall not extend below the top of the sewer, and trench excavations below this point shall be made with vertical sides with widths not exceeding those specified herein for the various sizes of pipe.