Geologically unstable area sank into a hole destroying their home and taking their lives
It’s hard to imagine but the ground sank beneath the home of Richard Préfontaine, Lyne Charbonneau and their daughters, Anaïs and Amélie at 9:30 pm on Monday night wiping out their family.
It is not the first time the fine Leda clay soil of the St. Lawrence River valley has liquefied and taken homes and lives.
The family was in their newly renovated basement rec-room proudly watching the Montreal Canadians playoffs. The earth beneath their home gave way, moving the home off its foundation and killing the family.
The town of St. Jude is in mourning over the loss of the family.
While landslides happen often in Quebec, they are impossible to predict with any certainty and remain a geological risk of the rolling farmland that makes up the river valley.
St. Jude is a small community of 1,000 people South East of Montreal near Rive Yamaska between the cities of Ste Hyacinthe and Drummondville. The modest and well-kept home displayed their family pride. It is now a ruins in the hole created during the mud slide. For a picture gallery, see the National Post.
The river valley the great St. Lawrence River was once an sea. Fine Leda clay settled on the floor as the last ice age receded leaving a flat and rich soil of dense, fine clay. When you drive through Quebec along the south side of the St. Lawrence River, the small mountains rise from the relatively flat valley. This geography dominates the Rive Sud from the river to the mountains near the US border. Leda clay is also found along the banks of the Ottawa and Saguenay Rivers.
The soil is gray Leda clay that packs hard until it gets wet enough. Then it runs like soup. New housing construction is at risk from rain when foundation holes can suddenly collapse. It happens often but risk of sink holes happens less often.
The clay can be liquefied from surface water such as rain and run-off. The more difficult problem lies in underground streams that can erode the ground above them without warning, as it happened in St. Jude. (see illustration below)
An anonymous post on Google Earth explains the problem: “I am a geographer at University of Quebec at Rimouski and one of our research professor, which happens to be one of my thesis directors, is specialised in this type of geomorphological event.”
“Basically, a long time ago, a lot of water covered the area after the last ice age, while the ground was still depressed from all the strain put upon it by hundreds of meters of ice for thousands of years, and had not sprung back up yet. So a lot of fine clays got deposited in the calm waters of what is now known as Champlain Sea, which covered a lot of area in southern Quebec.”
“Fast forward to current time. This clay has been eroded by normal processes, such as erosion from small streams crisscrossing the relatively flat plains bordering the St. Lawrence river, like the one seen in the topop map which shows a drop of elevation from about 30 meters for the plains down to 6 meters at the bottom of the small ravine etched by the stream parallel and north of the Salvail road.”
“Now clay has this interesting property: it can hold a lot of water, but the particles are so fine that under the right conditions of pressure and a quick nudge in the right direction, the clay particles can begin to float in the water accumulated, a process called liquefaction. The solid ground turns very quickly, almost instantly, in a dense but fluid sludge that has the density of cement freshly mixed with excess water. It then flows down the hill until it has lost its momentum and the clay particles have settled back and friction is restored between the soil particles. You end up caught in a dense pool of gooey clay, think quick sands but with even finer particles.” bbskeyhole
History repeats itself
I was living on that clay river bottom and quite aware of the annual mud slides during the 1970s. In 1971 a major sinkhole in St. Jean-Vianney, in the Lac St. Jean region, destroyed 41 homes and killed 31 people. Some bodies were never recovered. Quebecers were transfixed by the images of a town swept away on the television. The Quebec government relocated the town since the land was considered too unstable to re-settle.
“Saint-Jean-Vianney was a village in the Saguenay-Lac-Saint-Jean region of Quebec, which was partially destroyed in a landslide on May 4, 1971.”
“Located near the shore of the Saguenay River, Saint-Jean-Vianney was — unbeknownst to residents at the time — built atop a bed of unstable Leda clay, a type of subsoil which can liquify under stress.”
“Following unusually heavy rains in April 1971, the clay soil bed at Saint-Jean-Vianney became saturated with water that had failed to run off, causing pockets of clay to gradually dissolve. Over the few weeks leading up to the landslide, cracks were reported in some of the town’s streets and driveways, some house foundations dropped roughly six to eight inches into the soil, and some unusual noises — including underground thumps and an untraceable sound of running water — were reported.”
“At 10:45 p.m. on the evening of May 4, the earth at Saint-Jean-Vianney suddenly dropped approximately 100 feet, forming a canyon through which a river of liquefied clay flowed toward the Rivière du Petit-Bras below, swallowing houses in its path. Just before midnight, the clay finally stopped flowing and began to resolidify. By the time the landslide had ended, 41 homes had been destroyed and 31 people had been killed. The landslide created a crater of approximately 324,000 square metres in area, varying from 15 to 30 metres in depth.” Google Earthhacks