The recently installed underground stormwater storage system at Gates Canyon Park is part of an increasing number of innovative stormwater capture and treatment projects within Southern California. The project, located adjacent to the City of Calabasas in the western reaches of unincorporated Los Angeles County, aims to capture, treat, infiltrate and re-use local stormwater runoff. The cistern component of the project required up to 3.5 acre-feet of storage in an underground structure with up to 14 feet of fill above. Precast concrete has quickly become the preferred alternative for the construction of underground stormwater storage cisterns due to their quick construction time and high load capacity. The modular precast concrete system provided for the project by Precon Products utilizes a patent pending cantilevered column system to resist the high earth, seismic and hydrodynamic loading conditions.
Competing precast manufacturers have been developing various modular precast concrete systems, most utilizing wall systems for the structural support. These wall systems are often slender and have been observed on previous projects to have issues during transport and installation, including. large cracks at the base of the walls where the vibrations and lateral forces from truck transport have been over stressed. Also, fit up issues at the horizontal and vertical joints when stacking and laying multiple adjacent units creates issues that shimming and grouting cannot always overcome.
The patent pending Storm Prism System, provided by Precon Products, utilizes a column system to support the roof structure, is similar approach used for traditional cast-in-place structures that have historically performed well. Utilizing the cantilevered column system allows for a more efficient vertical element section while also providing area for the required confinement steel in the vertical reinforced core of the column element. The column system utilizes a flared cross-section at the top and bottom of each respective modular unit to provide increased rigidity to handle site transportation issues, and also exceed the required moments from the seismic loading conditions. As the profile of the column elements are much less than equivalent wall based modular units, the hydrodynamic loading is reduced on the interior elements of the structure resulting in more efficient structural sections. The modular units are comprised of a top and bottom section that fit together at four column point locations. The fit-up at these four bearing points provides a much tighter overall fit and any required adjustments or shimming can easily be accounted for with thin bearing pads.
The speed of construction met the expectations associated with the use of precast concrete while also providing for quicker installation and fit-up times due to the efficient nature of the system. Utilizing the cantilevered column system allowed for the rigid frames to easily survive transport to the site and resulted in not a single unit being rejected on-site. The Storm Prism System also has potential usage beyond the use in stormwater capture projects as increased open space allows it to be used for many other applications, including underground storage, shelters and agriculture.
Editor's Note: Scranton Gillette Communications and the SGC Water Group are not liable for the accuracy, efficacy and validity of the claims made in this piece. The views expressed in this content do not reflect the position of the editorial teams of Water & Wastes Digest, Water Quality Products and Storm Water Solutions.
