Cheesman's stone and mortar surface was unique and controversial. Many contemporary engineers doubted the long-term durability of this type of dam construction. Binding it together was newfangled Portland cement. Would it hold?
Ultimately, Cheesman was designed as both a gravity dam and as an arch dam. It would be a huge pyramid of stone with the curved upstream wall of an arch dam. Gravity dams are built in a wedge, like a doorstop, so that their weight stabilizes them, with thickness that increases dramatically at the bottom of the dam.
Arched dams, on the other hand, use the keystone principle that supports the arches of a bridge. The weight of the water pushes the stones together and against the cliff sides, making it even stronger. A pure arched dam is roughly the same width from top to bottom, because its strength comes from its shape, not its weight. Cheesman, the hybrid, is 10 times as thick at the bottom as it is at the top.
The first try
The hybrid was not the original design. It was the response to a harsh lesson delivered by the South Platte River. The original design specified a pure gravity dam nearly twice as thick as Cheesman. The rock-fill dam would be a pile of rubble protected by steel plates from the destructiveness of water. Between 1894 and 1900, the work proceeded. Tunnels were bored for bypasses and outlets. The stream bed was excavated down to solid bedrock. Fill for the dam was dumped and compacted, and the upstream face was sheathed in steel. But in the spring of 1900, when the dam was only 50 feet high, the flooding South Platte demonstrated the power of nature to upset the best-laid plans.
On May 3, 1900 a flood caused record-making problems all up and down the South Platte. It wrecked the rails of the South Park Railroad and destroyed bridges as far north as Colfax Avenue in Denver. And far upstream from downtown Denver, it had washed over the partially completed dam, eroding the rubble and collapsing the plates.
New hybrid design
The destruction of the dam called the whole concept into question. Days later, a new engineer, Charles Harrison, was designing a new hybrid he believed could withstand the power of the river. His replacement dam would use the existing outlets, which crews had spent years drilling through the canyon walls, and the existing foundation. In appearance, it would be a breed apart, a state-of-the-art landmark.
The wall arched against the canyon, so that the weight of upstream water pressed the stones more firmly together. The new design employed Portland cement to solidify the fill, and a surface of finely set and mortared granite blocks, beveled to enhance its strength, replaced the steel. Using Portland cement required a complex system for transporting the cement up the canyon and monitoring its placement. Although the curved surface was known to be stronger than a flat one, there were no mathematical models to calculate its strength, but the result was both strong and solid.
Harrison did not supervise the entire construction project, but in 1904, just a few months before water filled Cheesman Reservoir, he wrote a paper for The Transactions of the American Society of Civil Engineers on the history, design and construction of the dam. In 100 pages of complex math, engineers analyzed and argued over the design.
The dam was sound, and it remains sound more than a century later. The beauty of the mortared granite face, built by Italian master masons, was only the surface of a technological masterpiece that would stand for decades, a landmark of civil engineering, Cheesman Dam.