Practical Engineering - Sawing a Dam in Half (on Purpose)

Short Summary

Main recommendation: when concrete structures develop alkali-silica reaction (ASR), an alternative to stopping the chemical reaction is to cut relief slots through the structure so it can expand without building up damaging stress, as the Tennessee Valley Authority did at Fontana Dam. The slots are cut with diamond wire and must be repeated periodically as the concrete keeps growing. The video frames this as a proactive way to extend the life of massive concrete dams.

Key Points

• ASR occurs when reactive silica in aggregates dissolves in the alkaline pore water of concrete and forms a gel that swells, causing internal stress and cracking.
• Fontana Dam, completed in 1944, began showing cracks in 1972, and instruments revealed a continuous permanent growth, not just seasonal thermal expansion.
• The usual prevention options (testing aggregates, using low-alkali cement or supplementary materials like fly ash, and keeping concrete dry) are hard or impossible to apply to structures already built.
• Slot cutting lets the dam expand into the cut instead of building axial stress against the rock abutments, and it works because these are gravity dams that are stable without lateral support, unlike arch dams.
• The slots close over time, so TVA monitors hundreds of instruments and uses finite element models roughly every five years to decide when to cut again.

Details Worth Noting

• Concrete instructions: The slot-cutting method uses diamond wire about 15 mm (roughly half an inch) thick, looped over pulleys mounted on the dam to advance the slot downward while maintaining constant tension. Water is handled either with a steel half-pipe cofferdam on the upstream face (used at Fontana) or by cutting the slot in three sections separated by boreholes sealed with long rubber sock seals (used at Chickamauga). Cutting produces a concrete slurry that must be captured, settled in holding tanks, and the clarified water recirculated to control dust and lubricate the wire. Cutting is continuous to prevent the slot from closing mid-cut, and parts of the dam may need to be shut down to avoid a broken wire entering a hydro unit or spillway gate. Monitor slot movement, spillway piers, and expansion joints, and reassess about every five years.
• Fontana Dam is about 150 m (nearly 500 ft) tall, contains over 2.1 million cubic meters of concrete, and is the tallest dam east of the Mississippi River.
• The slot is kept narrow (about half an inch at Chickamauga) because a wider cut is harder to seal against water and more expensive.
• Avoiding reactive local aggregates can raise costs because transporting non-local aggregates increases project expense.

Bottom Line

When a concrete dam is damaged by a reaction that cannot be stopped, cutting narrow, repeatable relief slots can manage the stress and preserve the structure's life, at the cost of ongoing monitoring and periodic recutting.