Constructing Hillside Ozone Facilities in a High Seismic Zone

The Metropolitan Water District of Southern California (Metropolitan) has undertaken the addition of ozone disinfection facilities at all five of its water treatment plants as part of its Oxidation Retrofit Program (ORP). The ORP Site Preparation Project for the Robert B. Diemer Water Treatment Plant (Diemer Plant) is an integral part of this work. This project will prepare the plant site for construction of the ozonation facilities, i.e., ozone contactor, ozone generation building, and liquefied oxygen tank (LOX) farm. The Diemer plant, with a rated treatment capacity of 520 million gallons per day (mgd), is one of Metropolitan's five treatment plants to be upgraded with ozonation facilities. The plant is located about 37 miles east of Los Angeles and provides treated water by gravity flow to Metropolitan's central pool area, which serves approximately 3.5 million people in the greater Orange and Los Angeles County areas. The plant was constructed during 1961 to 1963 in the Chino Hills 820 feet above sea level to allow for gravity flow to Metropolitan's service area without resorting to pumping. As such, the site was created by removing the top of a broad ridge and placing the excavated materials in adjacent ravines to provide a large level pad for construction of various treatment facilities. Since the completion of the initial construction work, the plant has experienced a major expansion, which includes the construction of many smaller facilities, and currently the addition of ozonation facilities. Design and construction of the new facilities are challenging due to stringent seismic criteria caused by the nearby Whittier fault located 0.5 kilometer north of the plant area and capable of generating an earthquake event with a moment magnitude of about 7. Based on site-specific deterministic seismic hazard analysis, the 84^th percentile peak-horizontal ground acceleration is approximately 1.15g. Specifically, this paper describes the challenging issues and major design considerations that were associated with the preparation of construction documents for the Diemer Site Preparation Project. Major project elements that will be discussed in this paper include: the limited area of the Diemer site; the geology of the Diemer site and its nearby seismogenic sources; selection of site specific acceleration design criteria; alternative slope remediation methods investigated; selected slope remediation method and the design considerations; shoring and construction concerns during the design process; large excavation and backfill with roller compacted concrete; and current status of the construction project.