CHAPTER 5,14 SANITARY SEWAGE SYSTEMS The development of the design of the sanitary sewage system serving the constructed facilities for each island installation was naturally predicated upon the correlation of the building arrangement with the topography of the area in which these buildings or facilities were located, In general, the topography of all improved sites requiring sanitary sewers was similar in that the ground contour of all islands consisted of a comparatively level plane, the elevation of which was but a few feet above extreme high tide. If pumping of sewage was to be eliminated, upper ends of sewer lines would have to be shallow, the lower ends would be as deep as these factors would permit, and the length of any one line would be limited by the allowable gradient consuming the difference in elevation between the surface of the lagoon and the em of the line. Gradients, therefore, were generally established as being the minimum which would produce sufficient velocities of flow for proper functioning of the pipe sewer. As a result of these conditions, lengths of runs were also minimized where possible; thus excessive depths of trenching for the pipe lines were avoided. The Reconnaissance Report conceived the alternate possibility of utilizing cast iron or transit pipe as a substitute for vitrified clay pipe which is the standard material for this type of service and installing substandard sizes as a compensating econamic balance in the total cost, This alternative was predicated upon the probability of excessive breakage in the transshipment and rehandling required by the location of the Project. During the early design stages, a trial shipment of 3400 lineal feet of 8 inch vitrified clay pipe for initial construction was made to the Jobsite, and although considerable breakage and damage was experienced (approximately 15 percent), an economic analysis indicated that the use of vitrified clay pipe for sewer construction would be the least expensive, and subsequent design specified the use of this kind of pipe. The next shipment involved 40 percent breakage, due primarily to the method of packaging. Vendors were advised and subsequent shipments experienced considerably less breakage, overall job figures reflecting less than 10 percent loss. The design of all sewer systems followed the pattern established by standard practice, incorporating access manholes at about 400 foot intervals along the line, at breaks in gradient, and at junctions of two or more lines, Wye branches providing for connection with service lines to buildings were inserted in the line at strategic locations. Service con- nections to individual buildings were constructed of either 4 inch or 6 inch vitrified clay pipe, in accordance with requirements of individual buildings, 5-197