Wednesday, April 13, 2011

Shrinking Clay

A problem that I see frequently in Edmonton is clay shrinkage below foundation causing substantial damage to houses, sometimes destroying the value of the house.

The typical situation is as follows:
1.      The house was built on wet high plastic clay. Much of Edmonton has high plastic clay at and below foundation level. 10 to 20% of the land was wet before development into housing.
2.      The home owners (Or the city, developers) plant trees that become large, and suck water from the soil. In dry years, there roots go to the wetter soil, often below houses. The roots extend under the exterior foundations and desiccate an amount of soil. Typically values may be 3 meters of soil shrink 3% causing a 90 millimetres of settlement. If the interior foundations are unaffected, there is a noticeable slope toward the tree, usually with cracking.
3.      Now you have a problem, without a good solution. The first part of the solution is to remove the trees. Next is to underpin, and thirdly, to level the house and repair the other damage.  As the desiccated material thickens, the depth required for underpinning increases. A geotechnical investigation is required to answer the question of how deep, and what about downdrag.
4.      We geotechnical engineers think trees are nice, but in high plastic soils, they should be at least twice the height of the tree away from the houses. That makes a city treeless.
5.      The city likes to plant elm trees along the roadways because they dry out the subgrade and improve the subgrade strength, reducing the pavement failure due to loadings. The down side is the houses on the other side of the trees suffer. Is the City of Edmonton responsible for the damage there trees cause to the private properties? How about your trees and the neighbor’s house? What about the neighbor’s trees and your house?
6.      Shrinkage is only reversible after the virgin cycle; the first time there is some nonreversible shrinkage. Most of Edmonton upper clays have been desiccated to some depth, the desiccated crust, typically 2 to 3 metres; most house foundations are in or below this. Often the desiccation results in thickening of the desiccated crust.  
7.      Watering of the trees and downspouts helps but not much. Many of the large trees will use over a cubic metre of water per day. Some 2 or 3 cubic metres.
8.      Downspouts create the other side, swelling of that clay which have been desiccated. Downspouts at trees may cause a yo-yo foundation.
9.    Now one more complication, add water after shrinkage has occurred, and we get swelling. Now swelling is pressure sensitive. The exterior foundations, interior foundation and floor slab all produce different contact pressure on the soil, so different amounts of swelling causes differential elevations across the floor. What fun the kids have chasing their marbles to the walls usually. Time   to adjust your tela-posts to keep your main floor level. If you have bearing walls in the basement, perhaps you will need to install adjustable posts. Allow for future movements by providing access to the post and slip joints in the walls or above the ceiling. 

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