Protecting Existing PCCP Subject to External Transient Loads

As more land areas are urbanized, the need for Tarrant Regional Water District (TRWD) to protect or maintain their pipelines and for construction contractors and utility companies to cross existing pipelines will increase. The contractors performing the work will often request to cross or work over pipelines with heavy equipment; these crossings should be evaluated to determine if the pipe will withstand the additional loading. This paper presents a process for reviewing crossing requests and how to analyze transient loads through the soil and how to use the Unified Design Procedure (UDP) software to determine the pipe's allowable dead and transient load capacity. In addition, this paper presents a discussion on the crossing construction and the crossing reliability based upon the loading frequency. Freese and Nichols, Inc. (FNI) recently investigated construction loads over prestressed concrete cylinder pipe (PCCP) for a paralleled construction project. The preferred construction sequence required operating and staging large construction equipment and materials over the existing pipeline. Therefore, the loads generated by the construction equipment were analyzed and compared to the capacity of the pipeline, which was determined by the UDP software. The pipe was found to be adequate if the equipment is treated as a transient load, but inadequate if the equipment is treated as a dead load. Since parked equipment and staged materials were assumed to cause dead loads on the pipe, no equipment or materials were allowed to be operated or staged over the pipeline. The results of this study led to subsequent pipeline crossing studies for a concrete- aggregate mining operation and a new business development center. The crossing for the mining operation required a slab bridge to protect the PCCP by spanning across the pipe. The crossing for the business development required the installation of Geofoam to offset the additional weight of four feet of fill over the pipeline. A later project was undertaken to standardize a crossing design procedure and standardize crossing details to speed up the application and approval process for third parties' permit requests to cross TRWD's 90-inch and 72-inch PCCP pipelines.