The magnificent Hoover Dam, which was constructed 80 years ago, still stands strong and serves the US in the fields of irrigation, flood control, and power production. In this article we will explore all engineering secrets of hoover dam. Hoover dam was designed by engineer John savage which is constructed in Arizona’s Colorado river.
Mr. John savage’s surveying team zeroed in on the black canyon mountains (refer fig 1a) beside the colorado river. Can you guess the reason? Yeah, It is because the mountains have a decent height and narrow gaps between them, allowing for huge savings on construction materials (refer fig 1b).
Let’s first assume the design with a straight concrete wall of uniform width (refer fig 2a). The strong water pressure obviously causes the wall to deform and bend (refer fig 2b). Due to this bending, the outer fibres become elongated, and the inner fibres get compressed. This results in tension on the wall’s downstream side and compression on its up stream side. When tensile stress is applied to concrete, it easily develops cracks. Generally, modern buildings use steel bars to overcome this issue, as steel rods can readily carry a large tensile load. However, Mr. John savage came up with a much simpler solution, one that does not require steel rods - the arch dam technology.
Giving curvature to a dam, it becomes an arch dam (refer fig 3a). This arch dam deforms under the water loading. Now, if you compare this dam’s deformed shape with its original shape, you’ll notice that both the upstream and downstream fibres are undergoing a length reduction, which means the whole dam body will be under compressive loading (refer fig 3b). Concrete can withstand strong compression forces. This is the beauty of arch dam technology.
When the dam goes under service, it still has a good chance of toppling due to the water pressure. This issue was solved by increasing the dam’s width gradually towards the base. This will lower the dam body’s centre of gravity. The lower the centre of gravity, the higher an object’s stability. The newly achieved design is called a gravity arch dam (refer fig 4), and this design can overcome the issues of tensile stress and stability. This increasing width design can also resist shear forces. The water pressure on the dam body is not uniform, but is triangular in shape and increases towards the base. However, since the area of the dam increases toward the base, the shear stress value at every cross section is nearly identical.
In my opinion, the next challenge Mr. Savage faced was the height of the dam. The higher the dam, the greater will be its water storage capacity. This is obviously an advantage for electricity generation and flood control, but is it possible to construct a dam that is the same height as the mountain walls? Now, I will explain you, how this was done. Building a taller dam requires significantly more materials, increasing its construction cost. Therefore, Mr. Savage selected a height that was cost-effective, met the water demand of nearby cities and also helped in controlling floods. The height chosen by Mr. Savage was 726 feet.
The important thing, I would like to mention that before the engineers began the construction work, they had to first divert the river flow in another direction. To do so, they constructed temporary cofferdams and diversion tunnels as shown in Fig:5 below.
Being an arch gravity dam, it needs strong mountain walls to transfer the load. As the cross section of mountains was taken, they found that the rocks on the surface are weathered and quite weak. Therefore, the first task during hoover's construction was to remove all those weathered rocks until only the virgin ones remained. To reach the virgin rocks, the workmen drilled holes using jack hammers and blasted them using dynamite. After blasting, acrobatic workmen were sent with ropes to clear loose rock from walls and the excavated material was taken away via trucks. To have this dam a strong joint with the sidewalls, they excavated the mountain wall in the shape of an arch again using dynamite explosion (refer fig 6a). The dam body takes from these deep holes, making the mountain wall-dam connection really strong (refer fig 6b).
Now, I will explain how concreting was carried out. For this, they first arranged the form work, which was made up of wood, for the concreting. Once the form work or mold was done, they started pouring the concrete. However, the main issue here they faced was, cement reacts with water, it produces heat. Considering the scale of the project, pouring all the concrete at once will create an enormous store of heat which will result in material expansion and thermal cracks in the concrete, making the project a failure.
Engineers came with a construction innovation to solve this issue. They cleverly divided the entire dam area into a number of blocks (refer fig 8a), each approximately 50 by 50 feet, and poured concrete in each block mold work one by one. These small quantities of concrete took much less time to cool. In addition, they embedded 2-inch diameter steel pipes into these blocks (refer fig 8b). The pipes carried cool water, which controlled the temperature in the concrete and set it quickly and easily. Once the concrete hardened, workers filled these steel pipes with a grout-cement slurry. This technique proved so effective that the Hoover dam hasn’t shown any cracks to date.
Do you know that you can actually walk inside the hoover dam body? Several tunnels are hidden inside the dam body. They constructed these tunnels because of a simple phenomenon with which you might be familiar - water seepage. A liquid under pressure always wants to escape through porous material. Here the water molecules flow through the soil below the dam body due to the seepage effect. The issue is that this flow generates a high uplift pressure on the base of the dam, reducing its stability. This is why Mr. Savage designed a gallery, a tunnel that collects all seepage water from the dam body and base. This action reduces the uplift pressure considerably. The collected water is then discharged safely(refer fig 9). These galleries also provide passageways for leak or crack inspections.
That’s all in this article, you can read more about the hoover dam in next article.
I hope you enjoy reading. Thank you!