Hoover Dam (1)

The technical visit to the Hoover Dam (originally called Boulder Dam), was in my opinion an excellent activity. Being part of an engineering institution meant that we were able to get the full tour of the dam rather than just the 'public areas'. One disappointment was that we didn't get to wear the bright yellow Hoover Dam fake hard hats :( as we needed to wear proper safety headwear, but we all got a book about the dam at the end of the tour, which was a bonus! Our guide was very knowledgeable about the dam & commented that he often took film crews around the dam so his voice is on numerous documentaries but he hasn't actually been seen on TV. Another interesting fact was that when a film was being shot shortly after the dam was constructed the concrete face of the dam was too bright so thousands of nails were hammered into the dam face so that the rust staining 'dulled down' the reflectivity. Fast forward quite a few years & a James Bond film was being shot at the dam which included abseiling down the face of the structure. After the first repel the stuntmen were stuck to the dam & couldn't move - the nail heads had rusted away & the stuntmen's shoes were impaled on the nails, so cue removing thousand of nails from the dam face! Not to sure if this is just folklore, but our tour guide seemed pretty sincere & it sounded pretty cool anyway!

The dam is located in the Black Canyon on the Colorado River (approximately 30 miles southeast from Las vegas) & spans the border (& time zone boundary) between Nevada & Arizona. The technical term for the dam is a 'concrete arch gravity dam' & when it was completed (1936) it was largest electric-power generating station (the dam was actually built for irrigation purposes but the power generation capability was a bonus by-product that actually paid for the construction of the dam & continues to pay for dam maintenance; however, even today irrigation needs have priority over power generation) & the largest concrete structure in the world. It is named after Herbert Hoover who was instrumental in its construction, firstly as the US Secretary of Commerces & then as the President of the United States. The dam & hydroelectric power generation plant are operated by the Bureau of Reclamation & it is a designated US National Historic Landmark.

Originally, the dam was going to be constructed in Boulder Canyon (which is how it got its original name) but the site proved unsuitable so the dam site was moved downstream eight miles along the Colorado River to Black Canyon. To protect the construction site from flooding, two cofferdams (temporary walls) were constructed. To enable the river water to pass around the construction zone four diversion tunnels were driven through the canyon walls (two in Nevada & two in Arizona). The tunnels were 56 ft in diameter (with a combined length of more than three miles) & then lined with three feet thick concrete (making the finished internal diameter, 50 ft). When the dam was completed the entrances to the outer diversion tunnels were sealed at the upstream opening & half way along with concrete plugs. The downstream tunnel sections now act as spillway tunnels. The inner diversion tunnels also have two concrete plugs in them; one is half way & the other about three quarters of the way along. The section sandwiched between two plugs is used as part of the tunnels which water travels along from the outer intake towers to the generators (the inner intake towers have separate tunnels). Part of our tour included one of the water supply tunnels where we got to stand on the Arizona side generator water feed header pipe. Since our visit was on a Sunday, power requirements were low & the tunnel wasn't supplying any generators. When water is flowing you can apparently feel the flow through the pipe!

An interesting aspect of constructing such a huge concrete structure that needs to be water retaining is the need to prevent cracking of the concrete (get ready for some interesting civil engineering). Concrete doesn't harden by drying (otherwise how could you use concrete underwater?) the cement actually chemically reacts (hydrates) with the water in an exothermic reaction. Exothermic reactions create heat & due to the massive size of the dam, if it had been continuously poured (ignoring the logistics of providing that much concrete continuously) the temperature differential would be so great that stresses created would lead to severe cracking & probably dam failure. To resolve this issue, the dam was constructed as a series of interlocking trapezoidal columns approximately six inches deep. Sacrificial cooling coils were also used to circulate river water through the curing concrete to regulate the temperature. The cooling pipes were disconnected & pressure grouted when each pour had cured sufficiently.