Design Considerations for Large Diameter Buried Pipelines at a Water Treatment Facility with Limited Space, Large Differential Settlements, and Corrosive Soils

Abstract

Orange County Water District (OCWD) and Orange County Sanitation District (OCSD) have embarked on a joint Ground Water Replenishment (GWR) System. One of the projects included in the GWR System is the construction of a new Advanced Water Purification Facility (AWPF) at Water Factory 21, adjacent to OCSD’s Treatment Plant No. 1. The AWPF is a 70 mgd (upgradeable to 130 mgd) reverse osmosis (RO) facility, with microfiltration (MF) pre-treatment and ultraviolet (UV) disinfection. Due to the large size of the required facilities and limited space available at the existing Water Factory 21 site, special considerations were needed to route the site piping, which included steel pipelines up to 96-inches in diameter. Additional constraints included corrosive RO brine concentrate, highly aggressive RO product water, soft and highly corrosive alluvial soils with pockets of organic peat, high groundwater, highly variable potential settlements due to seismic-induced liquefaction, and the requirement to keep Water Factory 21 and Treatment Plant No. 1 operational during construction of the new AWPF. These challenges required the use of several special analysis, design, and construction methods, including: installation of buried pipe on piles, special coating systems, large diameter stainless steel pipe, soil-cement slurry backfill, a large, buried, multi-layered pipe gallery backfilled with run-able granular material, and detailed, continuous, pipe-as-beam bending analyses to deal with the widely varying potential liquefaction settlements. This paper provides an overview of the project, followed by a review of the demanding site conditions and constraints. Specific analysis techniques and design approaches will be reviewed, along with some unusual techniques employed during construction of the project.