Exploring Our Industrial Heritage – The Hoover Dam (Part II)
I'm sure by now, you can appreciate the Hoover Dam for it societal contribution. As such, we owe some respect to the country's leadership at the time, those who had the vision and negotiation skills to see the project through to completion. Now in part two of this article series about our exploration of the Hoover Dam, I will discuss the dam's design and the engineering marvels that supported the building of the dam.
THE HOOVER DAM: DESIGN & ENGINEERING
Let's Divert a River
If you have ever painted a room in your house, then you can appreciate how sometimes doing good prep work is just as important and often just as time consuming as the painting itself. The Hoover Dam build experienced a similar hurdle. Before dam building could even begin, the Colorado River had to be completely diverted away from the build site. This step alone was a massive undertaking and it is one of the most impressive aspects of the dam's engineering.
To divert the river, four tunnels, which averaged 4000 feet long were drilled and blasted through the canyon walls. Constructing these tunnels took almost as much time and money as constructing the dam itself. To speed up the work, the "Drill Jumbo", a mobile, double-decker drill rig was invented. The jumbo allowed as many as 30 miners to simultaneously drill the canyon walls. The miners drilled holes into the canyon walls for dynamite to be inserted into canyon walls.
Each of the tunnels measured 56 feet in diameter before being lined with three feet of concrete. Over 600,000 tons of concrete where used just to line the tunnels. Completing the tunnels took over 13 months of work. In November of 1932 the Colorado River was successfully diverted away from the dam build site. Afterwards, dirt and stone levies were installed and the dams foundational area was pumped dry so that site excavation could take place.
Let's Build a Dam
Once the river was diverted, concrete could begin to be poured into the block forms. Hoover Dam’s base alone required 230 blocks of concrete and each block was five feet in height. Widths of the blocks varied from 25 square feet to 60 square feet depending on the blocks’ location. Columns of blocks were linked in alternating and interconnecting arrangements.
Under normal curing methods and time, the Hoover Dam concrete would cure in 125 years (yikes!). Engineers however, used some innovative methods to hasten the curing process. Nearly 600 miles of steel pipes were woven through the concrete blocks in order to significantly reduce the chemical heat produced by the curing concrete blocks. Crews then relied on 1,000-pound blocks of ice produced daily at the site’s ammonia-refrigeration plant to cool the air in the pipes and thus hasten the cure time of the concrete.
The dam's final dimensions were 726 feet in height, 1244 feet in length, 660 feet in thickness at it's base and 45 feet in thickness at the top. With its 80-year-old technologies and construction methods, the Hoover Dam maintains water-pressure levels of up to 45,000 pounds per square inch. The project used engineering and physics concepts known as arch-gravity and gravity dam, meaning some of the force of the Colorado River water is redirected upstream or against canyon walls. Simply put, the arch-gravity Hoover Dam curves upstream to direct water pressure outward and against the canyon walls.
Let's Make a Power Plant
How about a shoutout to Arthur Powell Davis who had the vision to add a hydro-power generation plant to the design and build of the Hoover Dam! Even though building the power plant raised the cost of the project by 80%, the power plant provided a basis for financial payback. Once this phase of the project was approved, the engineers went to work on its design.
Essentially, water enters intake towers from Lake Mead, where four, 30-foot diameter pipes (penstocks) carry water from the intake towers to the generators. Water is then forced via natural water pressure through ever narrowing penstocks, increasing the water velocity enough to turn the turbines. These turbines power the 17 powerplant generators to create electricity.
Converting gravity fed water into enough electrical power for the needs of over 500,000 homes required some excellent design work! Kudo's to the Hoover Dam Hydro-Power Engineers of the 1930's!
Let's Design-in some Safety Precautions
Did you know that water has never gone over the top of the Hoover Dam? They never want it to either and that is for a couple of reasons. First, the powerhouse is located at the foot of the dam and all of that river water would be bad for the electrical generators. Second, at its peak there were about 20,000 vehicles a day going across the top of the dam, and obviously they didn't want those vehicles to get swept away. (The road across the top of the dam was replace with a separate bridge in 2010 so this second reason no longer exists, but it did at the time of the spillway design).
To prevent water from going over the top of the dam, engineers designed-in spillways. The spillways work just like the overflow hole in your sinks which prevent overflows. At the Hoover Dam, the spillways are located 27 feet below the top of the dam, one on each side of the dam. Any water getting up that high will go into the spillways then into tunnels 50 feet in diameter, and 600 feet long which are inclined at a steep angle and connect to two of the original diversion tunnels. Each spillway can handle 200,000 cubic feet per second (cfs) of water. The flow at Niagara Falls is about 200,000 cfs, so there is the potential for two Niagara Falls here.
It is a rare occurrence, as these spillways have only ever been used twice. The first time was in 1941 to test the spillway system and the second time, in 1983, was due to an actual flood.
Design and Engineering Summary
There are a lot of things to marvel at when it comes to the Hoover Dam. The leaders who had the vision to have it built, the men who actually built it and the logistics of making it all happen however the sole reason that the Hoover Dam is number one on the list of the seven wonders of the industrial world is clearly its design aspects and all of the wonderful engineering innovations that went into the dam. These things are pretty impressive especially considering the fact that engineers of the 1930's were not aided by computers, CAD software or even MS Project!
(continue reading on 7/31 at T4T)
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