Friday, May 26, 2017

Blankets and Curtains for Calaveras Dam?

Here at the Calaveras Dam Replacement Project we are building what is called an earth and rock-fill dam. The replacement Calaveras Dam, like all so-called embankment dams, is made of mostly compacted earth (see a cross section of the future dam below). An important aspect of this type of dam is the management of water. All earthen dams leak, the important thing is to control where the water flows to protect the integrity of your dam.  In the case of the Calaveras Dam Replacement project, we accomplish this with blankets and curtains. 

No fooling! Here’s how it works.  For years, the crews onsite have been constructing an entire dam that extends more than 100 feet below the base of the future dam.  We call this a grout curtain. Grout for those of you not in the construction industry is a special mix of cement that can be injected or literally blown onto a surface to seal it.  

Grout Curtains

As you can see in the photo from 2015, crews drilled countless small diameter holes into the bedrock a hundred feet down below the future dam and injected the holes with pressurized grout. The grout effectively seals up cracks and fractures within the rock underneath the new dam.  We did this for years. Why is this grout curtain so important? It prevents water from seeping underneath the new dam and undermining it.  In essence we drilled an entire new dam underneath the future dam. 

Drainage Blanket

If you thought grout curtains alone were enough to protect the future Calaveras Dam, guess again! We need blankets, too. More specifically a drainage blanket. The drainage blanket prevents water from exiting to, and damaging, the down-stream face of the dam (the face of the dam away from the reservoir).  At the end of the drainage blanket we are constructing a ‘mini dam’ made out of the same clay material as the core to capture any water in the drainage blanket. You can see the drainage blanket being constructed here.  This ‘mini dam’ is also known as the seepage barrier.  This seepage water is then collected in an inspection well to monitor the condition of the dam.

The Calaveras Dam Replacement Project, along with its blankets and curtains, is more than 80% complete.

See you around the Valley!

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