Building a Critical Piece of Infrastructure Adjacent to the San Andreas Fault
West Yost provided design and support during construction for a 2.0 MG prestressed concrete tank with a dome roof to replace a 60-year old concrete tank. The project site is located within 300 feet of the San Andreas Fault and sits between two faults just north and east of the tank. This tank is a critical component in the City’s day-to-day operation of its water supply system and provides emergency fire storage. During off-peak water usage this tank receives water from tank in a lower pressure zone and also pumps water to another tank in the next higher pressure zone; thereby, filling all the City’s water storage tanks during the off-peak periods. Then during periods of peak water usage water flows, by gravity, from tanks located in upper pressure zones to the lower pressure zones. Consequently, each tank is a key component in providing water to the City’s residents.
The original tank was a prestressed concrete structure built in 1950, and due to seismic and structural deficiencies the City had been operating the tank at 50 percent of its total capacity creating a water storage shortfall in the system. Consequently, the City’s primary goals were to enhance seismic survivability with low maintenance sustainable infrastructure. Performance of this tank is mission critical as it serves as the lifeline structure for fire flow, daily demand, and is vital to the local economy.
The Glenview Tank 3 is situated within a few hundred feet of the active San Andreas Fault, which posed unique geotechnical and structural challenges to the City and project designers. The tank sits between the surface rupture zone of the 1906 Great San Francisco Earthquake (280 feet west), and the Pampeyan fault trace (180 feet east). Observations made in 1908 indicated that a fence line near the current tank site was deformed by 1906 fault rupture. The post-earthquake fence alignment suggested about 7 feet of fault displacement abruptly occurred along the main San Andreas Fault trace.
Given the geologic evidence and observations following the 1906 event, it was concluded that similar distributed ground deformations are possible due to a future rupture of the adjacent active San Andreas Fault trace. This ground deformation may cause abrupt surface offset, differential settlement, tensile stress due to ground elongation, and tensile cracking of the ground. The following measures were implemented into the tank design in order to mitigate these potential effects.
- The tank is supported on engineered fill reinforced with several layers of geogrid. The reinforced fill is designed to enhance ductile behavior, which serves to partially decouple horizontal deformations from the foundation and mute vertical deformations.
- The tank is supported on a mat foundation designed to resist planer racking due to differential horizontal displacement and span/cantilever unsupported areas due to vertical ground deformation. The foundation is 30 inches thick with a total of 254,150 lbs of reinforcing steel and 900 cubic yards of concrete.
- Seismic shutoff valves, flexible pipe connections, and a secondary earthen containment berm around the perimeter of the tank were also implemented.
An AWWA D110 Type I tank was specified for the Tank 3 Project and designed with an anchored flexible base connection between the foundation and the wall. This “base isolation” style detailing reduces bending moments induced by hydrostatic, thermal, and seismic forces as the tank wall is separated or “isolated” from the wall footing by a neoprene bearing pad. Base isolation enhances operational and seismic performance of the tank by allowing the floor and wall to act independently of each other. Similar detailing is now used in critical buildings, such as hospitals, to help ensure survivability during earthquakes.
In areas of high seismicity the sloshing wave is also critical to the design of liquid storage structure. For the San Bruno project the calculated sloshing wave, per ASCE 7-10, is 12 feet. A cast-in-place free span concrete dome roof was used to accommodate the seismic forces and the sloshing wave. Through structural design modifications to the concrete dome roof and prestressed concrete tank wall, the freeboard was reduced to 6’-3”.
The Project also included mechanical design for tank and associated pipes and valves, grading plan, chloramine chemical feed system, tank mixer, and site security improvements.