The galleries within the Hoover Dam are an impressive feat of engineering. They play a crucial role in draining water from beneath the dam’s body, and without them, the consequences could be catastrophic. To truly appreciate the clever engineering behind the Hoover Dam, it’s essential to understand the unique approach taken in its construction. Let’s begin with how the concrete was placed.
One of the most unusual choices made by the dam’s chief engineer, Mr. John Savage, was to leave a 6-foot-wide gap in the center of the dam during its construction. You might wonder why such a gap was left. If the engineers had not included this gap and instead built the dam as solid, continuous blocks, the concrete would have cracked within one to two weeks. This happens because concrete naturally contracts as it cures. However, when the concrete blocks are trapped between the solid mountain rock at each end, they can’t shrink, which creates immense stress and cracks the dam. By leaving a gap, the concrete had room to contract freely during curing, preventing any stress from building up. Once the concrete had fully cured, the gap was filled with specially graded concrete, showcasing a brilliant solution.
But wait—there’s more to the dam’s construction story. The site where the dam was built was originally occupied by the violent Colorado River. With the river flowing through, it would have been impossible to safely begin construction. The engineers had only one option: divert the river. Diverting the river allowed construction to begin in the dry area while the river continued its path through a separate route. However, diverting such a massive and unpredictable river presented significant challenges.
To tackle this, engineers constructed four enormous diversion tunnels around the site. After the tunnels were built, temporary dams, called coffer dams, were erected to ensure water only flowed through the tunnels. These tunnels were large enough to not interfere with the river’s original flow, providing a safe dry area for construction.
A unique machine known as the “jumbo drill” was developed specifically for drilling the diversion tunnels. At the time, tunnel boring machines were not yet popular, and the only method available was controlled explosions using dynamite. The jumbo drill could hold 30 drills simultaneously, creating holes for dynamite in the rock. These drills were powered by compressed air. Once the holes were ready, the dynamite was placed, and the drill and workers were moved to a safe distance before the dynamite was electrically detonated.
After the explosions, workers began clearing the debris. Instead of discarding the broken rocks, they were repurposed for later use. The workers had to carefully deal with dangerous hanging rocks before continuing with the construction. The tunnel’s bottom portion was relatively easy to concrete, but more effort was needed for the rest of the tunnel. Reinforcement bars were placed and bonded to the rock by spraying concrete. Then, an expandable form machine, which could grow in size inside the tunnel, was used to complete the side concreting. The top portion was finished using pneumatic concrete guns. Once the concrete work was done, the form machine was removed, and the jumbo drill continued its work.
This process was repeated many times, and after 18 months of hard labor, all four diversion tunnels were completed. To open the tunnels, workers removed the soil blocking the entrance, allowing a small stream of river water to flow through. But here’s a question: What would happen if trucks started dumping rocks and soil upstream of the tunnels? The water level would rise, as the trucks would be raising the height of the rock mountain, making it taller than the tunnel’s entrance. This would completely divert the river, successfully isolating the construction site and creating a dry environment.
This rock mountain is known as a coffer dam. The broken rocks from the tunnel construction were used to build it. The diverted river now flowed through the tunnels, occupying the original riverbed. About 900 meters downstream, another coffer dam was built to ensure no water could reach the construction site. Through this meticulous process, the engineers were able to safely complete the construction of the Hoover Dam.
