The bizarre mechanics of a Leda clay landslide

A Leda clay landslide, such as the one that killed 34 people in Notre-Dame-de-la-Salette, is a strange and frightful natural phenomenon.

Carleton University professor emeritus Kenneth Torrance, a soil scientist, says he remains in awe of Leda clay landslides after more than 30 years of studying them.

"They are utterly astonishing," he says. "The material essentially turns liquid and the debris -- the failed material -- flows away."

The mechanics of a Leda clay landslide almost defy imagination: the soil beneath the ground's weathered crust can suddenly run like water, carrying earthen slabs, rocks, trees and houses at terrific speed. The landslide can feed upon itself. Leda clay is so sensitive that the initial slide can trigger a progressive series of failures in the clay belt that can consume acres of flat land behind the original collapse.

"There's a domino effect," explains Jan Aylsworth, a research scientist and Leda clay expert with the Geological Survey of Canada.

Some landslides have eaten up more than 40 acres. Many slides stop only when enough soil has piled up to dam the flow of Leda clay.

These landslides -- they're also known as quick clay slides -- often cause secondary flooding. In Notre Dame-De-La-Salette, for instance, the slide crashed through the frozen surface of the du Lièvre, blocked the river, and flooded the town with water, debris and ice.

That disaster is one of an estimated 250 landslides that have occurred within 60 kilometres of Ottawa, according to the Geological Survey of Canada. Those landslides have occurred over the past 7,000 years within the footprint of the ancient Champlain Sea.

And in order to understand the curious mechanics of a Leda slide, it's necessary to visit that ancient history.

Leda clay deposits are a legacy of the Champlain Sea that once stretched from Pembroke to Quebec City. The clay is composed mostly of silt: fine rock particles ground from the Canadian Shield by glaciers that once covered the area. When those heavy glaciers retreated, the Atlantic Ocean flooded the resulting depression of land, and the rock particles mixed with the salt water of the new inland sea.

The powdery rock clumped together with salt particles and sank to the sea floor. The powdery particles, shaped like flakes and plates, fell on each other randomly, like so much smashed masonry. (The salt served to ensure that individual particles did not strongly repel one another by virtue of their negative electric charges.)

When the sea water drained from the Ottawa and St. Lawrence valleys about 10,000 years ago, a thick layer of sediment was left behind in many low-lying areas. But that sediment gradually was leeched of its salt content by rain and meltwater.

The resulting Leda clay sediment is dangerous because, without the salt there to suppress the natural tendency of the particles to repel one another, bonds that are broken cannot be reformed, says Professor Kenneth Torrance.

"If they're not being reformed, you get then progressive failure of bonds," says Mr. Torrance. "You will continue to break the weakest bonds until the stuff goes liquid."

Essentially, says Ms. Aylsworth, Leda clay has such a porous and watery structure that it is prone to collapse when saturated, pressured or, as in the case of an earthquake, shaken. Leda clay can derive up to 60 per cent of its weight from water.

"It is inherently unstable," she explains, "and if something disturbs it, the structure collapses: it liquifies."

Landslides can be triggered by high rivers that erode and saturate Leda clay slopes, or by meltwater and rain that destabilize a deposit. Construction projects can also destroy the integrity of a Leda clay slope. Often, the disturbances happen in concert.

The majority of quick clay landslides occur in April and May when rivers are high and the ground sodden. In the last century, at least 23 major landslides have occurred between Ottawa and Quebec City that can be blamed on the sensitive marine clay.

In May, 1971, 31 people died in Saint-Jean-Vianney, Que., when a Leda clay slide swept away a bridge and 40 homes. The rest of the town was declared uninhabitable and relocated.

In 1991, the town of Lemieux, east of Ottawa on the South Nation River, was also abandoned after testing showed it was at risk because of Leda clay. Two years later, a 17-hectare swath of land adjacent to the town slid into the river when the clay suddenly liquefied.

"Leda clay," says Ms. Aylsworth, "is a really fascinating substance -- it's quite unique -- but it can also be deadly."

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