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Earthfill exchange Two huge earthfill dams under construction in a dry valley outside Los Angeles will nearly double the stored water capacity of Southern California. Report and photographs by Helena

The Eastside reservoir project just outside Los Angeles is one of the biggest civil engineering projects currently under way in the US.

Main components in the $1.9bn project are two earthfill dams, 7.2km apart. By blocking off a section of a dry valley these will create a 986M.m3 reservoir, nearly doubling the surface storage capacity for Southern California. A third and smaller earthfill dam, Saddle dam, fills a low point on the northern side of the valley.

Because the construction is in a dry valley, there are no river diversion works. However the scheme instead includes a $1.2bn pipeline which will carry pumped water to the reservoir from the Colorado river at Devil Canyon, some 70km from the reservoir site.

Two main requirements underlie construction of the reservoir. The first is to meet the region's endless thirst for more water on a day to day basis. Additionally, the Metropolitan Water District of Southern California needs the new reservoir to fulfil its obligations for earthquake preparedness, whereby MWD is required to hold six months water supply.

Work on the two main dam contracts started in 1996 and construction is due for completion in 1999. For these winning and moving materials makes up the majority of the workload.

Both dams are significantly large - the east dam is 3.2km long to the west dam's 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 dam design, recalls MWD lead design engineer Joe Ehasz, is that the two have a different seismic design. The nearest fault is 8km from the east dam, but because the distance between the two, the west dam is almost twice as far from the fault.

A considerable amount of time was spent on ground investigation at the design stage, says Ehasz, to establish what materials could be used for building the dams. The team achieved its 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.

The best rock is at the west end of the reservoir area, so logistics dictate that quarrying of this is being carried out by west dam contractor Atkinson Washington Zachry. Under its contract, though, AWZ has to supply 10Mt of the hard quartzite to east dam joint venture contractor Kiewit Granite, which then crushes the rock into four different products.

'We were lucky to find deposits of hard, strong, durable rock, particularly in this area of California,' says MWD con-struction manager Chuck Nichols. The reservoir is sited in the middle of what is known geologically as the 'Perris block' - a massive granitic intrusion, which is actually one of the more stable areas of southern California. Despite being bounded by faults, it behaves as a single and relatively stable block.

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

The muckshifting muscle power evident on the two dam sites is considerable, with Caterpillar equipment making up the large majority of the fleet.

On the Kiewit Granite-run east dam, for example, there are 19 Caterpillar rock trucks with up to 200t capacity, and seven excavators of varying sizes.

At the west dam, a fleet of Caterpillar trucks, dozers, and compactors grinds up and down the line of the dam all day long. The massive trucks, which weigh 350t when loaded, are undoubtedly contributing to site compaction - perhaps more than the com pactors themselves, suggests Ehasz.

Both dam structures are 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 mater- ial,' 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.

At the east dam, there were no ravines; the dam is founded entirely on bedrock, but this rock is moderately weathered so a cut-off concrete wall was needed along the length of the dam. The rock below is being grouted before construction of the core starts.

Grouting has been going on for three months. For operational purposes, KG has split the dam into two sections referred to as the north and south dams, explains KG construction manager Rod Genny, and the grouting team, subcontractor Sudakara, 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 sub- contractor Nicholson-Rodio is drilling and grouting down at least 46m, and in some cases as deep as 60m below ground level. Here the rigs are working on a steep slope fastened to winches attached to the top of the slope, which allow them to be wound down as they move on to the next grout hole.

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