Picture a cruise ship swimming pool during a storm with 30-foot waves. The ship is pitching up and down causing the pool water to slosh back and forth, with water escaping on to the deck. Now picture the inside of an underground wastewater tank or stormwater retention system. What happens to the water during a seismic event?

To get some perspective…water weighs 8.345 pounds per gallon. The Gates Canyon stormwater system, (our Featured Project this month) is 14′ tall and covers 3.5 acres, and has the potential of storing enough stormwater to fill a small pond or lake, equal to:

• 153,000 cubic feet
• 1,144,440 gallons
• 4,255 tons
• 8.5 million pounds

This 15-second video illustrates fluid motion in a tank, and, measures its kinetic energy and is similar to what can take place inside an underground infrastructure or stormwater retention system within an earthquake zone.

Within these earthquake zones live more than 143 million Americans or approximately half of the US population according to the Environmental Protection Agency (EPA). After the powerful 1994 Northridge, California quake caused major damage, above-ground precast structures including parking garages, hotels, stadiums, prisons, and commercial office buildings became a safety concern.

These concerns led to advanced precast concrete design safety measures for above and below grade structures, as well as an effort to further promote the benefits of utilizing precast in earthquake-prone areas. A consortium of industry experts started the Diaphragm Seismic Design Methodology project or (DSDM) for the purposes of scientific research and formed a task force including experts from academia, and the precast industry including the Precast/Prestressed Concrete Institute, (PCI).  Many of the DSDM experiments have been performed at the University of San Diego, Englekirk Structural Engineering Center (ESEC), which features the largest outdoor shake table, enabling earthquake simulation on scaled-down precast structures.

What is interesting about the Gates Canyon structure is that it is one of the largest stormwater detention systems created by Precon Products in California, and the only system to date designed to current seismic codes, (including the American Association of State Highway and Transportation Officials, (AASHTO), LA County DPW Structural Design Manual for HS20, ACI 318-14, ACI 350.3-06-Seismic Design of Liquid-Containing Concrete Structures).

You can find out more about standards and building codes (page 29) for Utility Earthquake Resilience and/or determine if your project is within a high-risk area for earthquakes by visiting the USGS site. 

CONCLUSION
Unless advanced engineering design techniques are implemented, including some type of baffling to impede the flow or ‘sloshing’, to lessen the impact within a precast stormwater retention system, the hydrodynamic energy of thousands or, even millions of gallons of water slamming laterally into walls during a seismic event will be the major cause of damage and/or destruction to your client’s underground precast structure. (Note: the Gates Canyon project is designed with interior baffle units for this reason).

At Delta Precast, we have the engineering expertise to assist you with all your projects, including those projects located in areas with seismic activity that require an understanding of codes and seismic design for precast concrete structures. Contact us today!