Design of Laterally Loaded Deep Piers to Resist River Scour

Abstract

Expansion of the 91 Avenue Waste Water Treatment Plant in Phoenix, Arizona necessitated the relocation of a major Salt River Project (SRP) 500kV electric transmission power line. The project required installation of six reinforced concrete drilled shaft foundations at two structure sites within and along the Salt River banks to support the new single pole structures. WEST Consultants (WEST) performed hydraulic and sediment transport analyses to determine the local and longterm scour depths in the area near the supporting foundations. The hydraulic analysis consisted of determining average and maximum flow velocities as well as water surface elevations for various peak flow events. SRP utilized these values to determine the local scour that might be expected on the new foundations. WEST developed a sediment transport model for the study reach that was run through an observed 105-year flow hydrograph. Long-term scour near the new foundations was estimated from the sediment transport modeling results. WEST also conducted geomorphic analyses to evaluate the relative lateral stability of the channel in the vicinity of the proposed structures. SRP performed a geotechnical investigation to the full depth of proposed foundations to characterize subsurface soils and develop design parameters. Final design incorporating all study work resulted in the construction of 3-m diameter foundations ranging from 21.0-m to 23.9-m in length, with maximum reveals from scour and associated bank ground loss of up to 16.1-m. This paper discusses a probabilistic approach for design of deep pier foundations with large reveals where structures and their structural elements must resist the transient environmental conditions of wind, temperature and stream flow. INTRODUCTION The 91 Avenue Wastewater Treatment Plant, located just north of the Salt River and east of 91 Avenue in Phoenix, Arizona, treats wastewater from the cities of Phoenix, Glendale, Mesa, Scottsdale, and Tempe. Plant reclaimed water is used for agriculture, running the Palo Verde Nuclear Power Plant, and helping provide a wildlife refuge in the adjacent Tres Rios Demonstration Wetlands Project. As part of ongoing capital improvements, the plant is undergoing a multi-year expansion to boost capacity from 678 MLD to 772 MLD by 2014. A segment of the SRP Jojoba to Kyrene 500KV electric transmission power line extends through the planned waste water treatment plant expansion area and must be relocated to facilitate the work (see Figure 1). Work involves constructing five new structure sites using eleven single shaft poles that will wrap around the north, east, and south perimeters of the plant site and the demolition of two lattice tower