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Inspectors are scrambling to take soil samples along a stretch of the Salvail River where a deadly landslide killed a family of four, but experts say sensitive marine clay severely limits the level of assurance such tests can provide to residents.

Leda clay - named for a type of seashell mixed in the clay of the St. Lawrence and Ottawa River valleys - is tough to sample and control and can suddenly melt like butter in a hot pan when jostled even slightly.

"You can't even count the things that can set it off," said André Aubin, director of urban services in Nicolet, Que., a town plagued by landslides. Nicolet is about 70 kilometres north of Saint-Jude, where the family of Richard Préfontaine died Monday.

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Mr. Aubin's town and the province have spent millions in the past five years making sure a chunk of the community of 7,700 doesn't slide into the Nicolet River. Several downtown blocks slumped in 1955, killing three people, destroying homes, businesses and a school and leading to the demolition of the town's church.

Landslides may provoke an image of granite boulders rolling down a rocky western mountain, but more people in Quebec have died in landslides than in any other province. Some 330 Quebeckers have been killed in landslides since 1841, about half of them in clay slides, usually in river valleys. In mountainous British Columbia, 239 people have died in landslides.

Mr. Aubin rattles off just a few of the possible triggers of a slide: rain, saturated ground, drought, quick and slow river ice melts, minor earthquakes, vibrations from construction work, high and low water levels, and invisible phenomena far below the surface.

In other words, almost everything and its opposite can cause Leda clay to liquefy and flow down even a gentle slope. (Young friends of the dead family in Saint-Jude said a favourite childhood thrill was to roll log-style down the riverbank behind the Préfontaine home. The slope was steep enough to be fun but gentle enough to be safe, said 21-year-old Mathieu Arpin.)

Nervous residents of Saint-Jude have flooded the office of Mayor Yves de Bellefeuille. Saint-Jude was previously classified as being at low risk for a slide.

But Mr. Aubin said property owners often complicate matters. Many are fiercely protective of their right to dig a swimming pool or chop down an erosion-preventing tree. The town has experienced repeated small slides in recent years, but many owners insist their own properties are fine. "Some people will build at the foot of a volcano, if you let them," Mr. Aubin said.

Nicolet has imposed a ban on new construction along the water and reinforced the riverbank. Since 2006, the town and province have updated 30-year-old provincial risk assessment maps. The modernization effort hasn't reached Saint-Jude yet.

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Jean-Yves Chagnon, the retired provincial geologist who was in charge of the mapping effort in the 1970s, used aerial photographs to identify clay scars that could indicate vulnerable spots.

Of the 700 vulnerable areas he found, 117 of them were within a 150-kilometre radius east of Montreal along the Yamaska River. It includes Saint-Jude and Nicolet. He said it's time his work was updated.

Mr. Chagnon said it's one thing to identify areas at risk, but far more difficult to know whether a landslide will be small and affect a few metres or stretch nearly a kilometre, as in Saint-Jude.

The unpredictability makes private engineering companies reluctant to conduct tests, he said, because they could be held liable if the land gives way. The tests also cost of thousands of dollars, making property owners reluctant to order them.

Editor's Note: The landslide at Saint-Jean-Vianney, Que., in 1971 took place in the Lac-St-Jean region, not at the Catholic parish of the same name in the Lac-Mégantic region. Incorrect information appears in the above graphic, which also accompanied the version of this story in Thursday's paper.

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