Why House Foundations Shift
The Shifting to a house foundation is related to the soil on which the foundation rests. Winnipeg is situated on an old glacial lake bottom. The lake, known as Lake Agassiz, was formed as the last continental ice sheet, which covered most of North America, melted and retreated northward. The ice sheet prevented natural drainage from occuring, consequently a lake was formed behind the sheet. The lake approximately 180,000 square miles in area, covered parts of Ontario, Manitoba, Saskatchewan, North Dakota and Minnesota ( see Fig. 1 ). Spring flood waters flowing into the lake carried soil in suspension. The coarse particles ( sand & gravel ) were deposited near the mouth of the rivers and streams and the finer particles ( silt & clay ) were carried out into the lake where they ultimately settled. Each year a few millimeters of soil were added to the thickness of the lake bottom.
Eventually the continental ice sheet melted and most of the lake drained away. A few smaller lakes and ponds remained, the largest of which is Lake Winnipeg. The drainage took place about 7000 years ago and the lake bottom has since compressed to provide a firm competent formation, capable of supporting small to medium sized structures. In the area where Winnipeg is situated, the thickness of the silt and clay deposit varies from about 15 to 40 feet. The significance of the existence of Lake Agassiz insofar as house foundations are concerned, is that a surface deposit of silt and clay was left behind. Most clays, and certainly those of the Lake Agassiz basin, undergo volume changes as the result of changes in soil moisture. A decrease in soil moisture is accompanied by shrinkage and an increase in soil moisture is accompanied by swelling. Typically, a layer of clay one foot thick could undergo a one-inch change in thickness as it goes from a completely dry, to a completely saturated state or vice versa. Thus foundations located within the clay are susceptible to movement when the soil moisture changes.
Fig. 1 Extent of Glacial Lake Agassiz
1. Professor Len Domaschuk, Deparment of Civil Engineering, University of Manitoba. " Is Your House Suffering " , Information for the Winnipeg Homeowner. 1986.
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