Technology >>Foundation >> Movement of House Foundations on Winnipeg Clay

L. Domaschuk and S. Rizkalla, Civil Engineering Department, University of Manitoba D. Flatt, National Testing Laboratories, Winnipeg Most of the single-family dwellings in the City of Winnipeg, Canada, of which there are approximately 160,000, are supported on shallow footings resting on glaciolacustrine clay. These dwellings have had a long history of heave and subsidence associated with moisture changes in the subsoil. Commonly, rigid materials such as plaster, drywall and stucco are used in construction of the houses. As a result of the movements, many houses suffer from unsightly cracks in walls, ceilings and basement concrete floors, as well as uneven floors and doors that bind or require trimming in order to remain functional. While in some instances the damage is barely noticeable, in others, the costs of repairs have been as high as 25% of the replacement cost of the house.
	The City of Winnipeg, is situated in the basin of former glacial Lake Agassiz. The soil profile consists mainly of laminated glaciolacustrine clay up to 20 meters thick, overlying till and limestone bedrock. A thin discontinuous layer of silt is often encountered in the upper 3 meters. The clay is generally highly plastic, has a high swell potential, and generates swelling pressures in the order of 50 to 250 kPa. Soil moisture changes brought on by climatic changes have been observed to depths of 3.7 meters (Hamilton, 1963). Because the footings and the floor slab are located within the active soil zone, they undergo vertical movement. The movements are often of sufficient magnitude to cause: cracking of floors, walls and ceilings: uneven floors: and distortion of door and window frames. The associated monetary loss includes cost of repairs, additional heating costs due to air leakage around ill-fitting doors and through cracks in exterior walls, and loss in resale value of the house. In addition to the monetary loss, there is the psychological distress associated with living in apprehension of cracks reappearing from year to year, as soil-moisture condition change. The survey revealed that over 85% of the houses surveyed had developed cracks in walls and ceilings. About 47% of the houses had water leaks through basement walls. The problems associated with shifting foundations can be eliminated of course by using piles. Unfortunately, many homebuilders have not found it advantageous to use piles from a marketing point of view and the question of the economics of using pile foundations has never been addressed before. In summary, the study of the economics of alternative house foundations for the Winnipeg area indicated that optimizing the pile foundation design resulted in a saving that changed the justifications for the use of pile foundations from 5% to 41% of the houses. This constitutes a significant change in the risk factor from a homeowners point of view, particularly in view of the fact that the added cost of the pile foundation system constitutes only a 2 to 5% increase in the total purchase price of the house.