The Reservoir Lubricant

Vajont Dam and the Pore-Pressure Detachment of Monte Toc

A double-curved concrete arch dam designed to withstand hydrostatic water pressure at 262 meters height can survive a direct hit from an entire mountainside and still fail to protect the valley below. In 1959, Italian chief engineer Carlo Semenza completed the Vajont Dam across a narrow limestone gorge in the Dolomite Alps ninety kilometers north of Venice. Standing 262 meters tall with a crest width of just 3.4 meters, the thin concrete shell wedged into the vertical rock walls of the canyon was the tallest arch dam in the world.

What the builders did not initially recognize was the internal structure of Monte Toc, the mountain forming the southern wall of the reservoir immediately upstream of the dam. In the summer of 1960, the reservoir began filling for the first time. Carlo Semenza's son Edoardo Semenza—a newly graduated geologist—walked the slopes of Monte Toc and mapped a two-kilometer-long M-shaped fissure cutting across the mountain face. Deep inside the gorge, folded beds of Jurassic limestone dipped sharply toward the reservoir like the back and seat of a chair. Those limestone beds were separated by millimeter-thin seams of soft, slick montmorillonite clay. Edoardo realized that the entire northern flank of Monte Toc was a colossal prehistoric landslide that had slipped after the last Ice Age and sat perched on those clay layers.

Senior geotechnical consultants believed they could tame the creeping slope by raising and lowering the reservoir level in stages. They hoped the rock mass would settle gently against the lower seat of the gorge. Instead, every meter of water added to the reservoir infiltrated the fractured limestone and pressurized the thin montmorillonite clay seams at the base of the slide. By Archimedean buoyancy, submerging the lower toe of the mountain reduced the effective weight holding the flat seat of the rock mass down against the canyon floor. High pore-water pressure inside the inclined clay layers pushed the overlying limestone slabs apart.

In September 1963, engineers raised the water level to 710 meters above sea level. When the mountainside began creeping at four centimeters per day, they ordered an emergency drawdown to lower the reservoir. That rapid drawdown sealed the mountain's fate. Lowering the lake removed the stabilizing water pressure pushing against the outer face of the slope faster than trapped groundwater could drain out of the tight clay seams inside the mountain.

At 10:39 p.m. on October 9, 1963, deformation accelerated past twenty centimeters per day. Frictional heating along the sliding clay seam flash-vaporized the trapped pore water into high-pressure steam, dropping the friction coefficient to virtually zero. Within forty-five seconds, 260 million cubic meters of forest and limestone—a block two kilometers wide and 150 meters thick—slid into the reservoir at 110 kilometers per hour. The impact displaced fifty million cubic meters of water in a 250-meter wave that vaulted straight over the top of the Vajont Dam. It dropped into the Piave River valley below, erasing the town of Longarone and killing 1,917 people. Inspecting the canyon afterward, Austrian rock-mechanics pioneer Leopold Müller observed that the concrete arch had held fast against a blow ten times greater than any designer had ever imagined, even though its structural survival could not spare the valley below. Today the 262-meter double-curved concrete arch of the Vajont Dam still stands intact across the gorge, holding back a dry mountain of limestone rubble fifty meters higher than the former waterline.