All pipes shall be laid fair and true to line, and upon foundations of assured stability, either natural or artificial, and the ends of each length of pipe shall abut squarely and truly against the adjoining pieces, so as to present an absolute continuity and uniformity in the interior of drain, particularly at the bottom line. The joint rings shall be carefully placed in position, concentrically around the pipes, the space between the latter and the rings thoroughly flushed with mortar, and finally as the work progresses, and each length of pipe is permanently fixed in place, the interior shall be thoroughly cleansed and wiped out, and all projecting mortar or other substances carefully removed, so that the internal area of the drain shall be left absolutely unobstructed, smooth and clean throughout its entire length. Any change in the alignment of the pipes in any direction to be made on true curves, with pipe, only.