Key Developments in the History
of Gravity Dams

Ancient Times

Jawa
The world's most ancient, well researched and documented dams to this day are the Jawa gravity dams, located in Jordan. They were part of an elaborate water supply system for the town of Jawa, which experienced a brief heyday around 3000 BC. Located in an inhospitable basalt desert, the city, laid out for some 2000 inhabitants, must have been built very rapidly. It is likely that its construction had to start together with that of the water system at the beginning of the rainy season, if the builders and their herds should survive the following dry period.

The Jawaites satisfied their water needs during construction of the city by daming runoff to form artificial reservoirs. In order to fill these, canals were built from the Rajil river. Half of the storage capacity of the reservoirs was concentrated in Reservoir #4, which was impounded by a 4.5 m high and 80 m long dam.

The internal structure of this dam was quite complex. It had two dry masonry walls which enclosed an earth core of thickness 2 m. Additionally an impervious blanket was provided in front of the upstream heel. The stability of the structure was assured by a downstream embankment. The heightening of the dam by 1 m followed essentially the same principles, although the width of the earth core was now increased to some 7 m. A previous stone fill was provided behind its upstream wall for easy drainage during emptying of the reservoir. Thus the wall was protected against the dangers of water-backpressures, a precautionary measure that was reinvented only in modern times! For unexplained reasons, this miraculous desert city collapsed as swiftly as it rose. It might even have become a victim of its success--too many people may have overstressed the water supply systems.

The Roman Empire

Emperor Nero (AD 54-68) had a 40 m high, 13.5 m wide, and 80 m lonldam built for a pleasure lake near his villa at Subiaco, Italy. The dam was one of the earliest Roman dams and remained the highest the Romans ever built. Moreover, the Subiaco dam and two smaller dams nearby are the only Roman dams in Italy. Although the dam was too thin, it remained intact until it failed in 1305. Records place the blame on two monks who took it upon themselves to remove stones from the dam, apparently in an attempt to lower the level of the lake which was flooding their fields.

Inside a monastary near the dam hangs a painting from 1428 showing St. Benedict fishing from the crest of the Subiaco Dam. Incedentally, this painting is the oldest surviving illustration of a dam.

In extreme contrast to the number of Roman dams in Italy, Roman gravity dams were abound in the Iberian peninsula, North Africa, and the Middle East. The largest reservoir impounded by the Romans was created by a dam located near Homs, Syria in 284 AD. The dam had the extraordinary length of 2000 m and impounded approximately 90 million m³ of water. The main body of the dam consisted of concrete lined by masonry on both slightly inclined faces and on the crest. It was also grossly overdesigned: in its central part, the dam was 7 m high and 14 m wide . The crest width was smaller, but still measured 6.6 m for the upper 1.3 m of height.

Later Dams
The record height of the Roman Subiaco dam wasn't broken until 1594 with the construction of the 46 m high Tibi dam in Spain. However, the Roman concept of a rectangular wall was mostly maintained, with only a few, hesitant attempts to use trapezoidal, let alone the correct triangular, cross sections. The use of concrete by the Romans for the dams' interior or as a building material in general, fell into oblivion, while the construction techniques essentially remained the same: pick and shovel.

Evolution of the Modern Gravity Dam

The Correct Shape
In 1765 and 1800, the first triangular gravity dams were built in Mexico. Unfortunately, it remains unknown who their ingenious builder was. Amazingly, he/she had adopted the modern shape almost one century before it was developed in France.

In 1850, French engineer J. Augustin Tortene de Sazilly (1812-1852) showed in a lecture that the most advantagous profile for a gravity dam is triagle with a verticle upsteam face. Sazilly also analyzed three recent French navigation dams in a paper published posthumously in 1853. He used the cross sections of these three dams to illustrate the confusion and uncertainty in the design of gravity dams. In fact, two of the dams were wrongly inclined on the upstream side!

Materials: The Return of Concrete
Concrete--now on the basis of Portland cement--was used for the first time since the Romans for the Boyds Corner gravity dam completed in 1872 in New York. The Lower Crystal Springs Dam in California, completed in 1890, was the first dam in which the water content of the concrete was specifically controlled. Because the cement industry was nonexistent in California, the cement had to be imported from England. In 1892 in France and in 1918 in the United States it was scientifically determined that by decreasing the amount of water in concrete and increasing the amount of cement, the stronger it is. However, a minimum amount of water must be there to ensure the workability of concrete while the amount of cement may need to be limited on account of the heat it develops during the hardening process.

BIG Gravity Dams: New Ideas in Construction!

Hoover Dam
The next big development was the construction of the Hoover Dam, built by the US Bureau of Reclamation. It was a quantum jump in dams. Not only was it 60% higher and 2.5 time larger than any any dam in existence, but it was also to impound the unheard of quantity of 38,550 million m³ of water. The dam was built between 1931 and 1936 and was designed as a curved gravity dam. Concreting the Hoover Dam barely took two years with some 5000 men placing up to 8000 m³ of concrete per day!

Grand Dixence Dam
The tallest of the concrete gravity dams is the Grande Dixence dam in the Swiss Alps constructed from 1951 to 1962. It's 285 m high!

Latest Ideas in Gravity Dams

Steel
A relatively new development in the construction of gravity dams is incorporation of post-tensioned steel into the structure. This helped reduce the cross section of Allt Na Lairige Dam in Scotland to only 60 percent of that of a conventional gravity dam of the same height. A series of vertical steel rods near the upstream water face, stressed by jacks and securely anchored into the rock foundation, resists the overturning tendency of this more slender section. This system has also been used to raise existing gravity dams to a higher crest level, economically increasing the storage capacity of a reservoir. However, since reaching their prime in the 1960s, gravity dams have become something like a dying species, the dinosaurs of an era which especially the idustrialized countries with their high labor cost no longer can afford.