The new outlet facilities at Lake Mathews Reservoir in Riverside County, California, will include a new outlet tower constructed within a 27 to 34 m deep excavation. Fully grouted rock bolts will provide support for removable wedges exposed in the excavation, which will be subjecto a peak horizontal earthquake acceleration of 0.62 times gravity under the design earthquake loading conditions. Since large wedge failures could damage the new tower, the performance of the rock support under seismic loading was considered explicitly in the design process. A displacement-based inelastic design methodology was developed to determine support requirements for the tower excavation under seismic loading. The maximum design load on the rock bolts, when permitted to yield inelastically, is considerably less than the peak dynamic loads associated with the design earthquake. This design method justified the use of a pseudostatic acceleration that is approximately 35 percent less than the peak ground acceleration.


Lake Mathews is the terminus reservoir for the Colorado River Aqueduct (CRA) and is located in Riverside County, California. The facility is owned by the Metropolitan Water District of Southern California (Metropolitan), and serves as the distribution source of CRA water for Los Angeles, Orange and Western Riverside Counties, a service area of 13,500 km 2 with a population of 16 million. The main components of the existing outlet facility include a 54 m tall, 6 m diameter reinforced concrete outlet tower and a 732 m long, 4.3 m diameter steel lined outlet tunnel.

Studies and inspections conducted by Metropolitan and specialist consultants determined the existing tower is not capable of resisting the maximum credible earthquake (MCE) on either the Elsinore or San Jacinto faults (see Figure 1). The decision was therefore made to design new outlet facilities to assure serviceability following a major earthquake. A general plan of the existing and proposed new outlet facilities is shown on Figure 2. The new tower will be a reinforced concrete structure, 35 m high with an inside diameter of 9.75 m and six tiers of butterfly valves (with 5 valves per tier). The excavation required to constructhe new tower is 52 m long by 33.5 m wide and approximately 27 m deep within the footprint of the tower foundation. The excavation is a maximum of 34 m deep where the new outlet tunnel ties into the excavation. The inclination of the sidewalls ranges from 60 degrees (from the horizontal) to vertical. The geometry of the excavation was developed to allow subsequent underwater blasting to construct the approach channel, which will tie into the tower excavation, without damaging the new tower. To protect the tower during underwater blasting, temporary excavation walls that will be removed during the construction of the approach channel are a minimum of 12 m from the new tower.

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