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Winning rock

By its very definition, building an earth dam involves a lot of earthmoving. At Eastside, where two such massive structures are being built, winning and moving materials makes up the majority of the workload.

Both dams are significant. The east dam is 3.2km long, to the west dams 2.7km. Each is 12.2m wide at the crest, but the west dam is 87m high compared with just 56m at the east dam. Almost 69M.m3 of clay, sand and rock is required.

One of the quirks of the design, recalls MWD lead design engineer Joe Ehasz, is that the two dams have a different seismic design. The nearest fault is 8km from the east dam, but almost twice that distance from the west dam because the two are 7.5km apart.

Considerable time was spent on ground investigation at the design stage, says Ehasz, to establish what materials could be used. The team achieved their aim of sourcing everything needed from the area between the two dams. This cuts down considerably on the financial and environmental cost of importing rock and fill material. Quarrying from this area also enhances the capacity of the reservoir.

We were lucky to find deposits of hard, strong, durable rock, particularly in this area of California, says MWD construction manager Chuck Nichols. The reservoir is sited in the middle of what is known by geologists as the Perris block a massive intrusion of granitic material, which is one of the more stable areas of southern California.

Blasting takes place every day to supply rock to the processing plant, which can handle a tonne of rock every second. When approximately 153,000m3 of rock is being moved every day, this plant is a vital link in the chain of supply.

At the west dam, a fleet of Caterpillar trucks, dozers, and compacters grinds up and down the line of the dam all day long. Over the next few months the shape of the embankment will become more pronounced as it begins to rise above grade. Erhasz reckons the massive trucks, which weigh some 350t when loaded, are probably doing more of the compaction work than the compactors themselves.

Watching the rollers bounce around over the well-compacted material is reassuring for Erhasz the dam is designed to be dense for earthquake resistance. I dont think we will get any settlement at all, he smiles. These dams will be better compacted than any other dam in California.

Both dam structures are of a fairly standard design, but special care has had to be taken at the footings. On the west dam there were three deeply incised ravines channels which had filled up with alluvial material.

We chose to excavate the ravines down to stronger material, explains MWD construction administration manager Chuck Thomas, but the rock was still permeable so we had to build a cut-off wall. The cut-off walls, one at each channel, are up to 37m deep and formed in plastic concrete.

The east dam is founded entirely on bedrock, but this rock is moderately weathered so a cut-off concrete wall was needed along the whole length. The rock below is now being grouted before construction of the core starts.

Grouting has been going on for some three months. For operational purposes, Kiewit & Granite has split the dam into two sections referred to as the north and south dams, explains KG construction manager Rod Genny. Grouting subcontractor Sudhakar started at the centre and is moving out in both directions. The grouting programme is large, but the take is minimal, Genny says. Grout holes go as deep as 60m, and are typically 46m deep.

Grouting is also continuing at the south abutment of the west dam where Italian-owned subcontractor Nicholson-Rodio is drilling and grouting down to at least 46m, and in some cases as deep as 60m below ground level when there is a high grout take.

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