Eastside Reservoir Project, California, USA

Although construction of the three dams in the Eastside Reservoir project in California finished in 1999, the process of filling the 4,500-acre reservoir site will not be complete until 2003.

The Eastside Reservoir is located in Riverside County about 90 miles (145km) southeast of Los Angeles, USA.

METROPOLITAN WATER DISTRICT PROJECT

At a cost of US$2 billion, the Eastside Reservoir will almost double the storage capacity for the Metropolitan Water District of Southern California (MWD).

The main aim of the project was to help the MWD meet its seasonal, drought, and emergency needs. When full, the reservoir will be capable of supplying most of southern California with six months worth of water in an emergency.

The project's three dams, the longest of which extends more than 2 miles, required an embankment volume of more than 84 million cubic metres of earth and rock. The process of filling the reservoir, which will inundate 4,500 acres of land, began in November 1999 and will take four years.

DAM CONSTRUCTION

The three dams built in Riverside County, California, for the Eastside Reservoir project, will hold back 987 million cubic metres of water. The dams, which were completed in December 1999, required the excavation of 31 million cubic metres of foundation material.

Extensive foundation excavation was needed for the dams, which also required careful placement of embankment material to enclose portions of two valleys between a pair of bedrock ridges.

The two larger embankments - the West and East dams - block the west and east ends of the valleys. A relatively narrow ridge 500ft (152m) above the valley floor at its highest point forms the north rim of the reservoir. The lowest point of the ridge, where the Saddle Dam was built, is 190ft (58m) above the valley floor. The reservoir's south rim is formed by a broad bedrock highland with peaks as high as 1,100ft (335m) above the valley within a mile of the reservoir.

The reservoir will be 8,000ft (2,438m) wide at its west end, 10,000ft (3,048m) in the east and about 2,500ft (762m) wide near the middle. When full the reservoir will be between 160ft and 260ft (49m and 79m) deep.

EARTH-CORE ROCK-FILL DAMS

The three earth-core rock-fill dams are made from soil and rock materials obtained from borrow areas within the confines of the project. Core materials were obtained from the silty and clayey sandy alluvium in the floor of the reservoir and the rock fill came from the bedrock highlands of the reservoir's south rim.

The embankment designs of the three Eastside Reservoir dams are similar. The upstream and downstream slopes of the rock-fill shells are two horizontal to one vertical. The crest widths of the West and East dams are 40ft (12m) and the crest width of the Saddle Dam is 30ft (9m).

Earthquake movement was a major consideration when designing the dams. The San Jacinto Fault, located about 6 miles from the reservoir, is capable of 7.5 on the Richter Scale. The San Andreas Fault, located about 19 miles from the reservoir, is capable of 8.

The planning and site selection studies for the Eastside Reservoir began in 1987 with design studies starting in early 1993. The first major construction contract for the excavation of the West Dam foundation was awarded in May 1995, and construction of the East Dam began early in 1996. All three dams were completed in December 1999 and the reservoir is now being filled.

WEST DAM

The West Dam rises to 285ft (87m) above the valley floor. Its alignment spans a nearly flat alluvial valley floor 1.5 miles wide between bedrock abutment ridges.

About 65% of the dam's foundation area is founded on quartzite and phyllite bedrock; the remainder is founded on dense alluvium that fills the three buried channels, which are up to 120ft (37m) deep. The foundation excavations were extended 90ft (27m) below the original ground surface to remove the liquefiable soils.

To minimise foundation seepage through the alluvium, 3ft (0.9m) thick cut-off walls of plastic concrete were constructed across the three alluvial channels and excavated at least 2ft (0.6m) into bedrock.

To reduce seepage through the rock foundation the entire core width was consolidation grouted and a double-line grout curtain was installed to a depth of approximately 125ft (38m).

The West Dam grouting took about two years to complete and required 1 million feet (304,800m) of grout holes.

EAST DAM

The 2.1 mile East Dam, constructed by a Kiewit-led joint venture, is the longest of the three dams. It measures 185ft (56m) long, 1,200ft wide at the base and 40ft wide at the top. Before embankment of the dam could begin, more than 18 million cubic yards of alluvium had to be excavated to reach a solid bedrock foundation.

The embankment required 43 million cubic yards of crushed rock. An on-site crushing plant crushed and processed over 14 million tons of rock in 20 months to supply the dam's materials.

The cut-off wall, which consists of a 3ft (0.9m) thick plastic concrete mix constructed through the weathered portion of bedrock, varies in depth from 10ft to 110ft (3m to 34m) and totals 250,000ft² (23,225m²).

A two-line grout curtain reaches depths of 150ft (46m) below the foundation in the north segment of the East Dam and 100ft (30m) in the south segment.

SADDLE DAM

The Saddle Dam rises 130ft (40m) above the lowest point in the ridgeline of the north rim and is about 0.5 miles (0.8km) long.

The Saddle Dam is founded completely on phyllite and schist bedrock. To minimise foundation seepage, a two-line grout curtain extends up to 100ft (30m) below the foundation.

METROPOLITAN WATER DISTRICT OF SOUTHERN CALIFORNIA

The Metropolitan Water District of Southern California is a consortium of 26 cities and water districts that provides drinking water to nearly 17 million people in parts of Los Angeles, Orange, San Diego, Riverside, San Bernardino and Ventura counties.

Metropolitan currently delivers an average of 1.7 billion gallons of water per day to a 5,200 square mile service area.