Design Guidelines for the Steel Pipelines of a Major Project in San Antonio, Texas

The Central Water Integration Pipeline (CWIP) Project by the San Antonio Water System (SAWS) in San Antonio, Texas, included: three lap-welded steel pipelines totaling 3 miles in length, classified as Segments 5-1, 5-2, and 5-3, with 54-, 36-, and 48-in. diameters respectively; the agua vista treatment plant with steel pipelines and fittings up to 60-in. in diameter; and major improvements to an existing pump station with steel pipelines and fittings up to 54-in. in diameter. The 54-in. Segment 5-1 Pipeline included approximately 8,200 ft of a two-pass tunnel system with five shafts. Minimum wall thicknesses were established by the owner’s engineer for the different pipe diameters based on the internal hydrostatic pressures including full vacuum, joint analysis, and an external grout pressure of 15 psi in case of the tunnel pipe; back-up analysis will be presented. The lap-welded joint details were also established and the adequacy of the single lap-welded joint in non-thrust areas and tunnel liners was checked based on ASME, AWS D1.1 codes and joint stress analysis; analysis results will be presented. However, in thrust areas of bends, the first joint at either end of the bend was mandated to be double lap-welded due to the additional bending stress at the first joint due to the bend movement against the soil. A summary of the preliminary designs conducted and lap-welded joint details will be presented. Regarding the steel fitting reinforcements, the project specifications mandated the use of crotch plate reinforcement at equal diameter tees at critical locations which will be discussed. The project specifications required the successful bidder to perform the steel pipe and fitting designs in conformance with AWWA M11 manual and per the project specification requirements. In addition, analysis was required for the horizontal and riser pipe in the tunnel shafts which will have flowable fill backfill up to 86 ft deep. A summary of design checks and procedures used will be presented.