The Dam That Skipped Its Joints

Sound mass concrete barely leaks. Under a reservoir the water does not travel through the concrete; it travels around it, along the joints between blocks, the horizontal lift lines between placements, and any crack that opens. That is why a concrete dam is really a set of joints held watertight, and why the humblest item on the drawing, a strip of waterstop across each contraction joint, does more to keep the reservoir out of the gallery than the concrete on either side of it.

When the U.S. Bureau of Reclamation built Upper Stillwater Dam in Utah, its first roller-compacted-concrete gravity dam and, at its completion in 1987, then the largest RCC dam in the world at about 1.24 million cubic metres of concrete, it decided to test that idea. To keep the RCC placement continuous and cheap, the designers built the dam with no contraction joints and no crack inducers through the RCC mass, relying on a lean mix with about 70 percent fly ash, then the highest fly-ash content of any concrete dam in the United States, to hold the heat down, and planning simply to grout any cracks that appeared.

The RCC dam was designed without formed contraction joints, allowing transverse cracking instead, with the intent to grout cracks after they formed. However, filling the cracks with a durable material has proved to be very difficult in practice.

U.S. Bureau of Reclamation, Dam Safety Office report DSO-07-10.

The dam made its own joints anyway. Through the first winter the cold exterior contracted against a still-warm interior and cracked vertically, at about 35 metre (115 ft) average spacing rather than the 15 metres (50 ft) the design had assumed, so each opening was wide. By 1989 water was running from fifteen separate cracks on the downstream face, about 255 litres per second (9 cubic feet per second) in total, with a single crack over a deforming foundation block carrying most of it. This was not the lift-joint seepage that RCC dams are usually built to prevent. The water was coming through the vertical thermal cracks, the contraction joints the dam had never been given permission to make in a controlled place.

And grout would not hold them. The cracks kept moving with the seasons, widening each winter as the concrete cooled; an inclinometer measured about 25 millimetres (an inch) of offset at the worst one, and each cycle worked the injected seal loose. Reclamation grouted the cracks with polyurethane resin in 1988 and 1989, and again in 1992 and 1993, and each round held only for a while. In the end it took a contract awarded in 2004, and on the order of six million dollars, to retrofit what the design had left out: internal corrugated stainless-steel waterstops set into slots drilled down the three worst cracks and backfilled with heated asphalt grout, and an impermeable membrane bonded over all fourteen of the treated cracks on the upstream face.

The Upper Stillwater case history affirmed the need to incorporate contraction joints or crack inducers with leakage control measures in RCC dams as a standard practice.

U.S. Bureau of Reclamation, Roller-Compacted Concrete design and construction manual.

The lesson: Contraction joints and their waterstops are not detailing you can value-engineer away. They are the seepage-control system, the one place you get to decide where the dam will crack and put a maintainable seal there, before the reservoir decides for you. A dam will find its joints with or without you. Design them where you can reach them, seal them with a waterstop you can trust, and you will never have to drill one back in under full head.

Read more: Waterstops, Joints and Sealants: Keeping Water Out of a Concrete Dam →