One Tough Reservoir

California utility upgrades to a prestressed concrete tank

The Padre Dam Municipal Water District, located in Southern California, serves water to more than 103,000 residents in the San Diego suburbs of Alpine, Blossom Valley, Crest, Dehesa, El Cajon, Flinn Springs, Harbison Canyon, Lakeside and Santee. As part of its commitment to serving clean water to the community, the district supplemented its existing 27 water storage tanks by replacing a 1.5-million-gal tank with a 2.5-million-gal prestressed concrete tank and pump station in the suburb of Alpine. The tragic Cedar Fire of 2003, which destroyed many homes in the area, heightened the need for additional fire flow capacity in the area.

Originally, the “East Victoria” tank was one of five partially buried hopper-bottom concrete reservoirs in the district’s Five Reservoirs Retrofit Program. All five reservoirs were constructed more than 50 years ago and required continual maintenance to keep them in safe operating condition and in compliance with California Department of Public Health regulations. The district’s 2001 Master Plan recommended that all five be replaced.

Even though structural evaluations determined that all five reservoirs could be retrofitted, hydraulic modeling indicated that the East Victoria site required a 1-million-gal capacity increase to provide emergency storage, increase fire flow availability and improve operational reliability. The project also included the replacement of an existing antiquated pump station with a new modern one offering increased and efficient pumping capacity.

The district hired a project team consisting of AECOM, civil and structural engineer; Ninyo & Moore, geotechnical consultant; Spiess Construction Co., general contractor; Ward Henshaw, tank subcontractor; and DN Tanks, tank prestressor.

Tank Design & Seismic Considerations

After careful consideration of the options, environmental impacts and long-term benefits, the district and AECOM determined that a circular, prestressed concrete tank offered the optimal solution for water storage on site. Concrete was a natural choice because it offered the highest quality, most economical and lowest maintenance storage structure with a long service life for this project. With a capacity of 2.5 million gal, the circular tank’s shape improved water quality and circulation.

The East Victoria tank floor, footings, columns, walls and roof all were designed and constructed with concrete. With an inside diameter of 125 ft and a wall height of 28 ft, the tank incorporated a 10-in.-thick, poured-in-place core wall and a flat slab roof. The walls were constructed of cast-in-place concrete, and each wall section was poured full height, which eliminated horizontal jointing.

Because the tank is located in California and might be subjected to high seismic forces over its service life, seismic design was of particular importance. Specialized seismic connections were incorporated at the wall base and wall top, similar to a base isolation detail in order to maximize the ductility of the structure under a seismic event. Post-tensioning the concrete core wall both ver- tically and circumferentially yielded an efficient structure to contain the large loads produced by the stored water and the subsequent backfill and design live loads. The latest prestressing technology was utilized by DN Tanks for the project. Due to hydrostatic, hydrodynamic, differential temperature and differential moisture wall loads, the reservoir wall was prestressed in two directions—both vertically and circumferentially.

Vertical prestressing utilized 96 1.25-in.-diameter threadbars embedded in the wall and spaced approximately 48 in. on center. The vertical post-tensioning process elongated each of the high-strength steel threadbars with a hydraulic ram. The hydraulic ram was threaded onto the vertical threadbar at the top of wall and was stressed to 137.3 kips of force.

A specialized strandwrapping machine, which stressed galvanized 7-wire strand with a high degree of precision to 14,950 lb, completed the process of circumferential prestressing. The galvanized strand application was continuously recorded and electronically monitored, ensuring that a tight tolerance of the applied force was maintained. A total of 145 wraps of stranded cable were applied to counteract the hydrostatic and hydrodynamic loads. Prestressing not only ensured an economical project but also gave the structure the strength and durability to undergo horizontal and vertical ground accelerations. The state-of-the-art prestressing machine allowed the tank to be constructed faster and stronger with a high degree of precision.

Site Constraints & Challenges

Careful planning and coordination were crucial for successful completion of the project. One of the key challenges was to establish an operational strategy to maintain service while the existing reservoir was offline. It was determined that the reservoir could be offline only for seven months and that the construction of the new Type I prestressed concrete tank would be able to accommodate that time schedule. The existing pump station, however, was needed to maintain service to district customers until the new pump station was constructed. Through collaboration with the district’s engineers and operations staff and AECOM’s design team, a well-planned construction phase plan was initiated to facilitate this goal and avoid operational and customer service issues.

As identified in Ninyo & Moore’s geotechnical investigation, excavation for the East Victoria tank site was difficult due to numerous and sizeable large rocks and boulders, which required removal. Blasting the rocks in this neighborhood location would be difficult, dangerous and hard to control. Spiess Construction used hydrofracturing to break up the rocks and boulders into smaller, more manageable pieces. This was a safer method and it avoided excessive fragmentation, scattering and noise.

The project posed challenges for the management team, given the tank’s proximity to rural residential housing. Both during and after construction, several measures were taken by the project team to ensure that the surrounding community was not adversely impacted. The district partnered closely with the community through a community outreach effort and workshops that facilitated two-way dialogues between the neighboring residents and the district. Excessive noise and aesthetics were the primary concerns for them.

AECOM and the district developed several measures to mitigate the construction and traffic noise generated by grading, excavation and the pump house. Acoustical insulation was installed inside the building that housed the pump station, and the standby diesel generator was placed in a sound-attenuated enclosure.

Mitigating the project’s aesthetic impact on the area also was a major concern because many residents had a direct view of the site. The selection of a prestressed concrete tank gave the district the ability to partially bury it, leaving only the top 2 ft exposed, thereby reducing the aesthetic impact to the surrounding neighborhoods. Upon completion, the visible portion of the tank, the pump house building and the retaining walls all were top-coated in the same paint color. Low-intensity lighting was installed and native drought-resistant landscaping was planted to enhance the facility and help prevent soil erosion. The finished appearance of the completed facility far exceeded the community’s expectations.

Award-Winning Project

Water storage tanks are an integral part of the Padre Dam Municipal Water District’s long-term program to create a safe and reliable water distribution system, which will meet current and future water quality standards, capacity requirements and operational needs. It also enabled the district to provide emergency water storage in the event that water supplies are cut off due to fire, earthquake or other catastrophic events.

The project won several awards, including the American Society of Civil Engineers San Diego section’s Award of Excellence in the Water Supply category, the American Public Works Assn. San Diego and Imperial Counties Chapter’s Public Works Project of the Year award in the $2 million to $5 million Utilities category and the Structural Engineers Assn. San Diego section’s Award for Special Use Structure in the excellence and merit categories.