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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 occurring, 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.
The causes of soil moisture changes, which affect the performance of house foundations, may be placed into three categories:
1. Changes in climate and vegetation
3. Post-construction landscaping and watering.
How these causes affect the soil moisture and the foundations, is described in the following sections.
Obviously, high temperatures and winds accelerate evaporation of soil moisture. The effects of vegetation are less obvious. On the one hand, vegetation tends to decrease the rate of evaporation and loss of soil moisture through a modification of temperatures and wind velocities at ground level, but on the other hand the roots extract moisture.
The National Research Council of Canada conducted a study of soil moisture changes and associated ground movements at two sites in Winnipeg. Their findings showed that in the period 1962 to 1966 soil moisture were greater in fallow areas than in grass covered areas and that large trees accelerated moisture loss from the soil. The depth to which significant soil moisture changes occurred was at least 10 feet. The maximum vertical movement of the ground surface during that period was in excess of 5 inches in an area that had a large tree and was grass covered. Corresponding vertical movements were 3 inches at a depth of 5 feet, and 1/2 inch at a depth of 10 feet in the same area.
The study also showed that substantial soil moisture changes occurred under a concrete pavement and that the pavement surface moved almost 2 inches during the study period. The significance of the National Research Council study as it relates to house foundations is that normal variations in temperature and rainfall experienced in Winnipeg cause significant changes in soil moisture to a depth of about 10 feet, and significant vertical movements of the ground to a depth of at least 5 feet.
Construction of homes usually alters the natural environment at the site, which can aggravate soil moisture changes. For example, clearing the site of trees and shrubs may reduce the rate of loss of soil moisture. Consequently, the soil moisture increases and the soil swells. This may continue for several years until a new moisture balance is reached.
Perhaps, the severest change in soil moisture is that associated with basement construction. Commonly, there is a delay of several weeks between the time the basement is excavated and the concrete floor slab is placed. In this interval there is continual evaporation without any moisture replenishment, causing the soil to shrink. After the slab is placed, further evaporation is prevented and there is a progressive increase in the soil moisture causing the soil to swell and the floor slab to heave. The swelling and heaving may continue for several years following construction.
Landscaping, which involves the changing of the type of ground cover, as well as the slope of the ground surface, is bound to precipitate soil moisture changes by changing the pattern of runoff. An undesirable condition that is quite prevalent around houses, is a depression in the ground surface adjacent to the basement walls.
Moreover, backyards are often sloped towards the house. Consequently, during spring thaw or intensive rainfalls, the runoff water collects adjacent to the foundation and makes it's way to the footing level through cracks in the soil and between the soil and the basement walls. The increase in the soil moisture beneath the footings cause the soil to swell and the footings to heave. The condition is particularly severe if an intensive rainstorm is preceded by a dry spell.
Sloping the ground downward toward the street increases runoff and consequently less water infiltrates into the soil, particularly during brief intensive rainfalls. Planting trees and shrubs increases the demand for soil moisture and if the moisture is not replenished by rainfall or watering, soil shrinkage will occur. If the trees are close to the foundation and have deep roots, they may cause foundation settlements by removing moisture from the soil, which causes the soil to shrink. Studies have shown a tree may cause moisture loss in the soil within a radial distance approximately equal to the height of the tree. Watering the lawn in a newly developed area may change the stress and soil moisture conditions in the soil causing it to swell and give a false impression of the house settling.
1. Professor Len Domaschuk, Deparment of Civil Engineering, University of Manitoba. " Is Your House Suffering " , Information for the Winnipeg Homeowner. 1986.