Development of a Prescriptive Approach for Seismic Design of Water Mains

Many water systems have performed poorly in major earthquakes. Water supplies needed to fight fires or for health and safety of the public during post-disaster recovery have been disrupted. Although many water utilities have seismically upgraded their tanks, pump stations, and other “vertical” structures, most U.S. water utilities still have vulnerable watermain networks. Over one thousand watermain failures occurred in each of the 1994 Northridge, 1995 Hyogo-ken Nanbu (Kobe), 2011 Christchurch, and 2011 Tohoku earthquakes. These watermain failures led to depressurization in large areas of the affected water systems and lengthy restoration times. Japan has pioneered the use earthquake-resistant design standards for watermains and the installation of seismic resistant pipe. Watermains built using these earthquake-resistant design standards and seismic resilient pipe systems have been shown to provide excellent seismic performance. Although many U.S. utilities located in areas of high seismicity are beginning to use earthquake-resistant watermains, there is not a uniform U.S. standard that stipulates seismic design requirements for new watermains. To decrease the seismic vulnerability of watermain networks in the United States, the American Society of Civil Engineers’ (ASCE) Utility Engineering and Surveying Institute (UESI), in collaboration with ASCE’s Infrastructure Resilience Division (IRD), is developing a manual of practice (MOP) for water and wastewater pipeline seismic-resistant design. The intention of this MOP is to serve as a pre-standard for seismic design of all new water and wastewater piping in the U.S. and Canada. Detailed, site-specific analysis is warranted for transmission mains and some other critical watermains. However, most pipelines do not require this type of analysis. To simplify the design methodology for the less critical pipelines that comprise the majority of most water system networks, a prescriptive procedure is being developed. The prescriptive procedure will utilize the pipelines level of service (criticality), level of redundancy, the seismicity, and geotechnical hazard exposure to assist the watermain designer in selecting pipeline performance criteria. The pipeline performance criteria will be established in terms of required system flexibility (e.g., joint deflection for segmented pipelines), ductility (e.g., strain capacity), and strength for continuous and segmented pipelines. It is expected that pipe manufacturers would test their pipe products and demonstrate different performance levels. The goal is to ensure that the pipe materials perform as expected after a seismic event. The prescriptive procedure is intended to be a simple, straight forward method that will allow civil engineers to design and specify new pipe without specialized geotechnical and earthquake engineering expertise for most commonly encountered design conditions. This paper describes the proposed prescriptive procedure and presents design examples.