When the Grout Curtain Isn't Enough

A concrete dam is only as watertight as the rock it stands on. The concrete can be flawless and the dam can still lose the reservoir beneath itself, because water under head does not need to go through the dam. It goes through the foundation, along the joints and bedding planes and, in limestone, through cavities the water has spent millennia dissolving. The defence is a grout curtain: a line of holes drilled into the rock near the heel and injected with cement grout until the rock is tight, backed by a downstream line of drains that bleeds off whatever pressure gets past. Together they hold the uplift down and keep the foundation dry.

Wolf Creek Dam in Kentucky, a concrete gravity dam joined to an earth embankment and holding back the largest reservoir by volume in the eastern United States, was built in the 1940s and early 1950s on Ordovician limestone riddled with karst. Its original grout curtain took 20,387 bags of cement. The karst won anyway. By the late 1960s the foundation was failing, water dissolving and eroding the clay-filled cavities under the embankment and carrying the foundation out grain by grain.

Muddy water was seen in the river adjacent to the powerhouse, suggesting piping. Two sinkholes appeared at the downstream toe of the dam ... Water could reportedly be heard running in the bottom of the sinkholes.

Bruce, Brosi, Dreese and Terry, "The History of Grouting at Wolf Creek Dam, Kentucky."

An emergency grouting campaign from 1968 to 1975 bought time, enough to build one of the first concrete cutoff walls ever installed through a working embankment dam. But grout in karst is temporary, because dissolution reopens what cement fills. By 2007 the U.S. Army Corps of Engineers had rated Wolf Creek DSAC I, "Critically Near Failure," lowered the reservoir well below its normal pool, and begun a 594-million-dollar remediation.

What made the second attempt hold was not more grout. It was closing the curtain on a measured number. Every stage was drilled, split-spaced and grouted until water-pressure testing proved the rock below a set permeability, ten Lugeons in the zone where the new wall would go and three Lugeons in the rock beneath it, so that a concrete barrier wall roughly 275 feet (84 metres) deep could be cut down through the treated ground without bleeding its slurry into open cavities. The grout curtain, in the end, was not the barrier. It was the pre-treatment that let the real barrier be built.

The fact remains that grouting has secured the safety of Wolf Creek Dam on at least two occasions, and has facilitated the construction of permanent solutions, involving concrete diaphragm walls: none of the five successive grouting interventions has proved detrimental to the dam.

Bruce, Brosi, Dreese and Terry, "The History of Grouting at Wolf Creek Dam, Kentucky."

The lesson: A grout curtain is not a wall you build once and trust forever, and in soluble rock it is not a wall at all. It is a treatment you verify. Close it on permeability, not on grout take, because a hole can stop taking grout and still leak. Drill, split the spacing, water-test, and repeat until the rock meets a Lugeon target you set in advance. And where the geology keeps reopening the paths, treat the curtain as what it really is, the pre-treatment that makes a positive cutoff possible, not the last line of defence. The foundation, not the concrete, is where most dams are won or lost.

Read more: Cement Grouting of Dam Foundations →