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Las Vegas draws from ‘Third Straw’

0 169 Technology

by KORKY KOROLUK

When work began on a third intake to feed water from LaktEe Mead to nearby Las Vegas, Nev., headline writers were quick to dub it “the Third Straw.”
Las Vegas draws from ‘Third Straw’

Others dubbed it "the Last Straw."

Whichever way you look at it, it's an impressive project costing US$817 million over six years — for the first part. The second part will cost another US$650 million, and take...well, big tunnelling jobs often run late. That's not because it's anyone's fault. But if you want to dig a big, long tunnel, you can expect surprises.

One of the biggest surprises faced on the Third Straw job was a change in alignment because of clay-rich deposits that were found.

As a result, the custom-designed tunnel boring machine sat unassembled at the top of the access shaft for more than a year while planning was completed for the new alignment.

The tunnel boring machine was immense — 6.7 metres wide by 180 metres long.

Getting it down to where it had to work was a project in itself.

First an access shaft 180 metres deep by nine metres wide was excavated to where the horseshoe-shaped launch chamber was created. That chamber is 65 metres long, 15 metres wide and 11.5 metres tall.

From the launch chamber a starter tunnel was dug, 110 metres long, with a 7.8-metre opening.

Then the boring machine was lowered, piece by piece to the launch chamber, where it was assembled.

When it finally got to work, progress was uneven, depending upon geologic conditions encountered, and by how quickly crews could get materials down the access shaft. Some days the boring machine moved only a few centimetres; other days it achieved as much as 20 metres.

Behind it, the tunnel was tiled with more than 2,400 sections of curved concrete 35 centimetres thick. The result is a clean tube just over six metres in diameter, big enough to run a subway train through it.

While geologic conditions were one factor in the machine's progress, logistics were another. All the materials, the grout, the rail, the piping extensions, had to be lowered 180 metres down the access shaft, then hauled by locomotive to the boring machine.

The tunnel is about 4.8 kilometres long, reaching from a pumping station on shore to the intake out in the lake.

Constructing the intake itself was no walk in the park. It involved a lot of barge and underwater activity.

Contractors used "shaped" charges, which direct the force of their explosions, to blast much of the area where the intake's base was to be constructed. After the blasting and excavation, a tugboat and barge placed the intake structure within the pit created on the lake bottom by the blasting. The pit is 29 metres by 49 metres long and 25 metres deep. Tremie concrete backfilled the area. Tremie concrete placement method uses a pipe, though which concrete is placed below water level. The lower end of the pipe is kept immersed in fresh concrete so that the rising concrete from the bottom displaces the water without washing out the concrete's cement content.

In two years, crews excavated almost 11,000 cubic metres of overburden and about just under 24,000 cubic metres of rock.

In all, it was a lot of time, effort and money expended to provide water to Las Vegas. The city has grown from about 126,000 residents when it began drawing water from Lake Mead in 1971, to about two million residents today. As well, it draws 40 million tourists a year, drawn by the gambling casinos, the entertainment, and only about 10 centimetres of rain a year.

And if the drought continues, and the level of Lake Mead continues to fall, perhaps a fourth intake might be necessary. And that, for many, might really be the last straw.

Korky Koroluk is an Ottawa-based freelance writer. Send comments to editor@dailycommercialnews.com.

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