Trees Vs House Foundations

In Edmonton, and elsewhere, there are issues of tree roots causing foundation settlements via the mechanism of high plastic clay shrinkage. In most cases, we have no testholes, no real data, just typical of the area, testholes from other jobs. Opinions may change with additional soil information.


We are located in what was a glacial lake, which became a mud flat after the lake drained. Much of this area has high plastic soils. We have had a number of dryer years, the big tree now have roots through the desiccated crust, into the moister soils, drawing water from them, and in effect thickening the desiccated crust, with accompanied movements. Recent rains have created some surface swelling, with associated lateral pressures.


This is exclusively a clay dessication issue, not a tree root growth issue. Trees draw water. The amount varies greatly with season, species, heat, tree lief area, wind speed, time of day, and other factors. Several water balance studies have examined, and typically result in the trees will draw all available water in period of drought. Roots that found water will continue to grow: roots that fail to reach water growth slow or stops. 


This applies only to house foundation complying with part 9 of the ABC, 1.2 metres of cover, as it was since 1967 building code and remains in the current code. Slabs on grade and those exposed to freezing, may be underlain by fill and or organic materials, and garages on mud slabs are expected to move and the owners must just accept the movements. Consideration of the claimed damage to such slabs and landscaping is excluded from consideration. Only the house is considered. It is commonly recommended that residential foundations have at least 1.5 or 1.8 metres here, due to frost penetration and the new basement wall insulation standards, but the building codes have not caught up.


Foundation strength has little to do with movements, but does have with cracking and crack spacing. It is movement of the supporting soil that is the real issue. When the soil shrinks below the foundation, the foundation has little choice but to follow. As far as building codes go, the codes have allowed and still allow unreinforced concrete. Sure, we know how to build a foundation, and building that would not be effected by City trees, but the trees were placed after the structure was there. The cost of such a foundation and structure would cause houses to be constructed elsewhere. It is not tree roots that is causing the problem, but the dessication of the clay by the trees that is the issue. 


If the City thinks that deeper cover is required, it should be put into the building code that the City enforces. 


For movements to occur, the clays below foundation level must be wet before house construction, and near normal consolidation. In this area, we have a desiccation crust over near normally consolidated materials. In areas, the desiccation crust is thick, other areas less so. This area is old glacial lake bed. Depending on the area and reference, between 10 and 20 percent of the area was sloughs, and never dried out before man came along, trapped the beaver, drained the swamps, and made this into farm land. Development of housing, streets, sewer modified it more again. The slough areas have higher thickness's of high plastic clay, and less dessicated crust. Moisture contents are frequently 35 percent, and these clay will shrink to moistures of about 20 percent. 


Tree root spread is only one of the issues. As the tree dry's the clay, moisture also migrates toward the dry areas through the soil mass, and through the  fissures and cracks develop as part of the dessication.

The moisture a tree can take in is limited by the available moisture. In changing form 35% to 20% will give up about 0.25 cubic metres of water. The depth of dessication can be considerable, and ultimately results in thickening of the dessicated crust in areas which were wet to start with. The wet clay, and house before the trees were performing adequately. After the trees were added, the dessication occurred. This has been demonstrated repeatedly to have have occurred at other locations. 


Testholes are expected demonstrate the difference in moisture profile, if conducted adjacent to the four corners of the house. 


Note that root spread is dependent on water availability. Roots will expand and extend in the direction that the roots are encountering more water. They extend down the fissures between the soil peds,  The typical numbers are averages at best, not limits of spread. In addition, beyond the root, the moisture moves toward the dry soils. In shrinking, normal soil mass permeability fat underestimate the actual value due to moisture moving through the texture cracks.


Movements that result from frozen ground below the foundation during construction are often apparent during construction process, and first year of life. Fill, burred topsoil, construction issues or other causes usually shows up in the first five to ten years. Testholes and a soils log is required to eliminate some of these things.

The next logical step is a series of testholes to define the soil and moisture profiles to considerable depth at numerous locations. In addition, some testing could be done to define the swelling characteristics at this particular site.


The last statement "Historically the City... etc must mean that all the complaints to other city departments are not getting to their department. The City Geotechnical department is aware of the issue, and has been for thirty or forty years at least.