RESIDENTIAL FOUNDATION MOVEMENT
Many homeowners are faced with problems associated with foundations shifting. Their houses suffer from uneven floors, doors that won't close because of distorted door frames, and unsightly cracks in walls, ceilings, and basement floors. In some instances, the cracking is so severe that remedial measures are required. The information on this page aims to provide a good understanding of the underlying cause of these problems so that appropriate corrective action can be applied.
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Why House Foundations Shift?
The shifting of house foundations is related to the soil on which the foundations rest. Winnipeg, for example, 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, and consequently, a lake was formed behind the ice sheet. The lake, approximately 180,000 square miles (465,000 sq. km's) in area, covered parts of Ontario, Manitoba, Saskatchewan, North Dakota and Minnesota.
Why Soil Moisture Changes?
The causes of soil moisture changes, which affect the performance of house foundations, may be placed into three categories. They are:
- Changes in climate and vegetation
- Post construction landscaping and watering
How these causes affect the soil moisture and the foundations, is described in the following sections.
Climate and Vegetation
Obviously, high temperatures and high 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 the loss of soil moisture through a modification of temperatures and wind velocities at ground level, but on the other hand the roots extract moisture from the ground thus increasing the loss of 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 show that in the period 1962 to 1966 soil moisture changes were greater in fallow areas than in grass-covered areas and that large trees accelerated moisture loss from the soil.
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.
Clay soils also undergo volume change due to "soil rebound". This movement results due to the elastic nature of the soil. When a basement is excavated for a house, the weight of the soil removed is much greater than the weight of the completed house. As a result the soils which were originally compressed by the weight of the excavated soil are free to swell, much like a compressed spring suddenly released.
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 after construction.
Landscaping and Watering
Landscaping which involved changing 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 sloping toward the house. Consequently during spring thaw or intensive rainfalls, the runoff water collects adjacently to the foundation and makes its way to the footing level through cracks in the soil or between the soil and basement walls. This increase in the soil moisture beneath the footing causes the soil to swell and the footing 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 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 that 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.
Common House Foundations and Related Problems
Most houses (in Winnipeg) are supported by one of the following foundation systems.
- Spread footings beneath basement walls and columns
- Slab foundation beneath the entire house
- Pile Foundations
This foundation system is the most common in Winnipeg. See diagram 1. A long strip footing supports the basement walls, individual concrete pads are used to support interior columns such as teleposts, and the basement floor rests directly on the ground. The foundation system is place at a depth of 4 to 5 feet below a ground surface which means that it is located within the depth of seasonal moisture changes and ground movements, and is therefore susceptible to movements. Moreover, different parts of the foundation system may move independently which can result in appreciable differential movement between several areas of the house.
A very common problem associated with this foundation system, is heaving of the floor slab. This can usually be attributed to soil moisture changes brought on by construction as previously described. The heave of the floor slab is generally sufficient to cause it to crack. This in itself is not destructive to the rest of the house, providing the heave is not transferred to the main structure. The transfer can take place through the central columns if they are not properly isolated from the floor slabs or through any interior partitions that have not been designed to accommodate the movement of the floor slab. If this transfer occurs, it can cause cracking of walls and ceilings, distortion of door and window frames, and uneven floors as illustrated. In some instances it may also lift the floor joists off the basement walls creating a horizontal crack opening along the top of the basement wall.
The strip footing may also heave as the result of runoff water collecting adjacently to the foundation wall, then making its way to the footing level and causing the soil to swell. Weeping tile, which is placed around the foundations, drains most of the excess water away but part of it soaks in to the soil.
Settlement of the foundations can occur as the result of the supporting soil losing its moisture and shrinking. This occurs during dry spells and can be aggravated by deep-rooted trees planted near the foundations. Foundation settlement particularly if it is uneven will cause cracking of floors, walls and ceilings.
What Preventive Measures Can Be Taken?
How About Corrective Measures?
Many homeowners are faced with the problem of knowing when a corrective measure is necessary, what type of corrective measure is required and if the measure will perform satisfactorily. As stressed throughout this article, if a home is supported on spread footings or a slab foundation, or if it is on piles but has a soil-supported floor slab, movements are likely to occur which can result in cracking of floors, walls and ceilings. These cracks do not mean that the structure is in any danger of collapsing and patching and filling of the cracks are only required for aesthetic purposes. Large cracks in the basement walls can be a source of water infiltration and therefore should be repaired.
How Can Foundation Movements Be Accommodated?
- Make sure eaves troughs and down spouts are in good working order.
- Extend down spouts away from a house at least 3 feet and preferably to splash pads.
- Slope soil away from a house.
- Water soil around a house a least twice a week in dry years.
- Water trees, especially in dry years.
- Use caution in removal of trees.
- Do not flood flower beds.
- Make sure the window well drainage system is working properly.
- If you have an underground sprinkler system, or inground pool, underground valves
and joints can leak. Check them frequently.
- Call Abalon Construction for information or advice.
This article is intended to provide general information only and in no way is it to be considered a replacement for foundation engineering consulting services. Moreover, the author disclaims all responsibility for any damages resulting from measures taken that are purportedly based on information contained here-in. Recommendations for a specific house should only be made after a proper investigation by a registered professional engineer, or consult your Abalon representative.
Residential Foundation Movement In Winnipeg was written by Leo Mulligan.
Abalon Construction acknowledges with thanks the assistance of Mr. C. Preston, P. Eng. and Mr. G. Bonham, Manager in preparation of this article. Thanks to the University of Manitoba, Department of Civil Engineering and Professor L. Domaschuk, P. Eng. No part of this article may be reproduced in any form without prior written permission of Abalon Construction.
The Influence of Trees on House Foundations in Clay Soils
Soil shrinkage caused by the removal of water by live trees can result in foundation subsidence; soil swelling caused by the recovery of moisture following tree removal can result in foundation heave or excessive horizontal pressures causing lateral cracking in the foundation walls.
Removal of a fully developed large tree near a foundation wall can cause serious damage to the wall. Before removing a tree consult with your Abalon Construction representative.
Trees, Clays and Climate
It has long been known that trees cause clay soils to shrink by withdrawing water through their roots in the summer. The amount of shrinkage depends on the type of clay soil, the type and size of tree and on weather conditions. Deciduous, broad-leaved trees extract more moisture than evergreen, coniferous trees; warm, sunny, windy weather causes greater moisture extraction than cool, cloudy, still weather. The amount of water available in the soil also plays a part in determining how much shrinkage will occur.
Proximity Of Trees To Buildings
One way to avoid root problems is to maintain a 'safe' distance between the tree and the building so that the tree cannot influence the soil beneath the building. A rule-of-thumb, which was based mainly on poplar, elm and oak trees in very highly shrinkable clay soil is that a safe distance is the expected maximum height of the tree.