Selecting Water Main Materials for the Los Angeles Department of Water and Power

Most water mains in the Los Angeles water system are iron. Cast-iron was the dominant material until the 1970s. Ductile iron has been the dominant material since then. Welded steel has been used for special applications, like very high pressures, or in hilly areas where ground movement is a concern. A small amount of HDPE pipe has also been installed in recent years, but also for special applications like slip lining and subaqueous installations. This year, the staff and management of the Los Angeles Department of Water and Power were challenged to reconsider their selection of materials. To assure that this process would be without bias, the Department employed a "Blue Ribbon" committee, to investigate and compare the cost and performance of commonly used pipeline materials, preparing a report and a presentation to the Board of Commissioners. The four primary materials commonly used in the United States for small diameter water mains were examined: 1) Ductile iron. As a modern version of cast iron, a great deal of information about long-term performance is available. Based on this data and the performance of other iron pipes in its system, LADWP is confident that cement mortar lined, polyethylene-wrapped ductile iron will generally last 100 to 150 years, with occasional repairs. 2) Steel. Like ductile iron, the performance of steel water mains is also well understood. With cement mortar lining and coating, and with cathodic protection, LADWP believes this pipe will also last well over 100 years, with minor repairs. 3) Polyvinyl Chloride. This material is now used for the majority of small diameter mains constructed in the U.S. With only about 30 years of data, analysis is needed to forecast long-term performance. This analysis shows that low failure rates can be expected, and mains constructed with this material should also last 100 years or more, with occasional repairs. 4) Polyethylene. This material is widely used in gas distribution systems in the U.S., and in water systems overseas. With 40 years of good data, very low failure rates can be projected for a long service life. This paper looks at the cost, failure rates, and seismic performance of all 4 materials. Over the next few years, LADWP will be working with pipe manufacturers to test PVC and HDPE materials and possibly develop new standards for their installation. The objective is to achieve the lowest life-cycle cost, while providing dependable performance, particularly during earthquakes.