Material Marvels: Tame the river
In the 1930s, during a time of global depression, the Hoover Dam project brings work to the western American states and valuable lessons for future dam engineering. Ines Nastali reports.
About 30 miles southeast of the dry desert city that is Las Vegas, Nevada, sits a giant water reserve in the Black Canyon, spanning the Colorado River between Arizona and Nevada – the Hoover Dam. It was named after US President Herbert Hoover, who was in office from 1929 to 1933, so during constrcution of the project.
The concrete arch-gravity type was built to give farmers a secure water supply, generate electricity with hydropower, tame the river as well as to improve flood control.
Prior to construction, the Colorado River had breached over its borders several times and flooded surrounding valleys.
The dam, authorised by the US Congress as the Boulder Canyon Project, was built between 1931 and 1936 by the US Bureau of Reclamation, during a time of global depression that ultimately ended in the beginning of the Second World War. When news of the massive undertaking spread around the country, thousands of men looking for employment came to the borders of Arizona and Nevada. Records show that eight men from far away Rhode Island on the east coast of the US came to find work.
In total, more than 21,000 men built the dam and around 100 died during construction. It is reported that all US states contributed to the project at some point by delivering material.
Divert the river
Before construction could begin, a chief engineer had to be appointed. The choice was Frank Crowe who had previously made a name of himself by successfully building other dams in the USA.
A Fortune Magazine article, published during construction of the Hoover Dam in 1933, describes him in the following way, ‘Frank Crowe's last vacation was his honeymoon 20 years ago. He avoids cities except for required directors' meetings and an occasional football game. He plays the stock market a bit, buys Buicks exclusively for work on the job and can be seen matching quarters with US$4-a-day muckers while waiting for a big dynamite explosion.’
Crowe decided to build an arch-gravity type dam, which is convex towards the upstream river and directs most of the water against the canyon rock walls, creating a force that compresses the dam. In order to start construction, the river had to be diverted and a cofferdam built. Killing two birds with one stone, Crowe got to enjoy many explosions when workers blasted the canyon rocks away, creating four tunnels, each three miles long, on both sides of the river, and reusing the excavated material to build the cofferdam up- and downstream (something that is still being done today on the Crossrail train line).
This proved a difficult undertaking, so roads and walkways were built, as prior to this, workers and materials came by boat. Wanting to finish the dam in record time – to cash in bonuses – workers came up with an ingenious way to speed up the drilling for the holes for the dynamite. On top of a lorry, they created a scaffold that enabled them to work simultaneously on different heights on one side of the tunnel. The scaffold was mobile and they achieved their goal in 1932.
For the first time, the Colorado River was under human control – the four tunnels were fitted with concrete plugs and valves, which could release water on command and formed Lake Mead upstream of the dam.
Prepare the concrete
Part of the reason why the Black Canyon, with its rock of volcanic origin – andesite breccia – was chosen as the dam site has to do with the supply of concrete. Surveyors searched for a good supply of sand and rock aggregate – two of the four components to make concrete alongside water and Portland cement – near prospective sites. More than 2 million cubic metres of concrete and 800,000 cubic metres of cement were used – although this includes the amount needed to secure infrastructure like roads, railways and contractors’ housing.
Surveyors found a source around six miles away from the dam site. ‘Here, floodwaters had been depositing stones for millions of years,’ The Bureau of Reclamation states.
‘Some of the rounded stones were as much as 30cm in diameter and had been washed down from as far away as the Grand Canyon. The deposit covered more than 100 acres up to 10 metres deep,’ The Bureau states. A dragline was used to excavate the aggregate and load it into rail cars. The cars hauled the aggregate to a screening and washing plant on the Nevada side of the river at Hemenway Wash.
At this screening plant, the aggregate was grouped into different sizes – fine, intermediate and coarse gravels, and cobbles 3-9 inch in diameter. Anything over 9in was run through a crusher and screened again. The separated gravel and cobbles were loaded on a train and moved to the mixing plant closer to the dam site.
The first of these mixing plants was located at river level. From there, concrete was transported to the diversion tunnels, to fit out their lining, and the basis of the dam. Using buckets, the concrete was moved first by truck and then by electric train.
Pour the concrete
Work on the actual dam began in 1933. From the trains, the concrete was transported by buckets and cable ways to the construction site, reducing workers’ physical labour. With the dam growing, a new mixing plant was set up at the edge of the canyon. This automated plant improved efficiency because it was capable of measuring ingredients, mixing them together and distributing the concrete into the buckets. ‘To produce the concrete strength required, a very dry mix had to be used. There was therefore very little time to move the concrete from the mixing plant to the dam. If too much time was taken, the concrete would take its initial set still in the dump buckets and would have to be chipped out by hand,’ The Bureau of Reclamation states. ‘For this reason, the men who operated the cranes, which moved the buckets into place, were some of the highest paid on the project, earning US$1.25 per hour. As each bucket was dumped, seven puddlers used shovels and rubber-booted feet to distribute the concrete throughout the form and pneumatic vibrators to ensure there were no voids.’
The placement of concrete took two years to complete, a lot of chemical heat was generated during the process, which slowed it down as the workers needed to let the material cool. Embedding 582 miles of steel pipes within the concrete, through which ice water circulated, accelerated this. In 24 hours, 900 tonnes of ice was produced by a specially engineered refrigeration plan. The time it took to place the concrete is also why dam engineers decided to construct the dam from concrete-filled blocks, rather than pouring it in one go. In total, 215 blocks, ‘varied in size from about 6m2 at the upstream face of the dam to about 2m2 at the downstream face’, were used, The Bureau of Reclamation states. ‘Adjacent columns were locked together by a system of vertical keys on the radial joints and horizontal keys on the circumferential joints – think giant Lego set.’
Concrete placement in each block was limited to a height of one metre in 72 hours. After it had cooled, a grout mixture of cement and water was used to fill spaces between columns to form a monolithic structure and to prevent hairline fissures.
Bureau of Reclamation engineers have calculated that if the dam had been built in a single pour, the concrete would have gotten so hot that it would have taken 125 years to cool it down to ambient temperatures. The resulting stresses would have caused the dam to crack and crumble, hence why they decided to fill the dam in blocks.
Don’t spill it
A key purpose of the dam is to generate electricity, and so part of the construction was to build a power house for the hydro energy plant. It is located at the foot of the dam and contains 17 turbines, which generate 4bln KWh of hydroelectric power each year, serving 1.3 million people and the dam.
As water and electricity don’t play well together, water never flows over the top of the dam. There is also a road bridge spanning the river, which would be flooded if the dam overflows. Spillways have been installed to prevent this from happening. ‘The spillways are located eight metres 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 15m in diameter and 180m long, which are inclined at a steep angle and connected to two of the original diversion tunnels,’ The Bureau states. ‘Each spillway can handle 5,600m3/s of water – the flow at Niagara Falls is about the same.’
During its years of operation, the dam has been repaired and amended. The grout curtain was retrofitted as the dam foundation moved due to the lack of geological exploration prior to construction. ‘There was insufficient exploration and characterisation of the foundation materials beneath and adjacent to the dam, specially the faults, shears, and breccia zones. The grouting programme was not sufficiently deep to provide an adequate seepage cut-off. The cost of the supplemental was US$1.84 million, about 2.37% of the cost of the dam,’ writes Richard Wiltshire, Reclamation engineer, on the dam’s 75th anniversary in 2010. Secondly, the spillways’ walls were upgraded with a smooth concrete finish in 1941 and 1983, and their elbow tunnels fitted with aeration slots. These let air into the water, thereby easing the power of the flow to avoid creation of craters and holes and the clogging of tunnels caused by debris.
While taller dams have been built, reaching around 300m in height, the Hoover Dam, at 220m high, is the oldest among the five tallest concrete arch-gravity dams in the world, more than 80 years after its completion.
A heavy load
The principal materials purchased by the federal government:
- Reinforcement steel: 20,410 tonnes
- Gates and valves: 11,190 tonnes
- Plate steel and outlet pipes: 400,000 pounds
- Pipe and fittings: 840 miles
- Structural steel: 3.7 tonnes
- Miscellaneous metal work: 2,400 tonnes