GEOTECHNICAL INSIGHTS FROM MAPPING A 3.2-KM-LONG TUNNEL THROUGH SAN ONOFRE BRECCIA, LAGUNA BEACH, CALIFORNIA

The 1954 Beach Interceptor Sewer Tunnel in Laguna Beach, California, which services several thousand residents, had been in a state of collapse and hazard since 1957. In 2016, the South Coast Water District began the expansion of the dilapidated 1.5-meter-diameter tunnel to 2.4 meters, with Parsons Corporation as Construction Manager and Drill Tech Drilling and Shoring, Inc. as contractor. Excavation was completed in 2022, and the project is expected to be completed by the end of 2024. The excavation provided an unprecedented opportunity to map characteristics of the San Onofre breccia along freshly-exposed, continuous rock surfaces.

The San Onofre breccia is unique among middle Miocene sedimentary units of coastal California as it forms impressive cliffs 100 m tall at Dana Point unbroken by wave action, with pebble-cobble-boulder clasts that occasionally exceed the size of cars. Many of the clasts are derived from the offshore Catalina schist, thought to be a core complex exhumed during extension. At a larger scale, Miocene microplate capture initiated transrotation of the western Transverse Ranges and uplift of Mesozoic-Paleogene subduction strata, including Catalina schist. This process created deposits of breccia with distinctive blueschist clasts that now form widely scattered outcrops at Anacapa Island, Malibu coast, south Orange County, and Tijuana. The Beach Interceptor Tunnel passes exclusively through the San Onofre breccia along a transect parallel to the shoreline.

From its inception at the prestigious gated community of Three Arch Bay to its terminus at Aliso Beach, the tunnel passes through several facies of rock, including sandy-matrix breccia, silt-matrix breccia, and fat clay layers. Many microfaults were intercepted, some of which introduced up to 0.02 m³/min of water into the tunnel. Special observed features include complex folding and channel scouring, and areas of past and potential landslides. A significant challenge was realignment of the tunnel along a 143-m path that bypasses a damaged section of tunnel that underlies the 1990 Arch Bay landslide. The project will provide an essential service for many Laguna Beach and Dana Point residents, and the incidental geologic mapping of the San Onofre breccia reveals stratigraphic, structural, and hydrogeologic data previously unknown to the geologic community.