Trees & Slabs October 1, 2015
Trees are common fixtures around the constructed environment and greatly enhance the livability and value of homes, offices, shopping centers, and other facilities used by man. If you have a shallow soil supported foundation or pavement supported on a high plasticity clay, trees can be trouble. This trouble can take the form of unexpected bulges in the interior slabs or loss of support around edges, if trees are pre-existing and removed during construction or left in place near the edge of the construction. They can also cause trouble if they are planted after construction.
Trees are living organisms and require a great deal of water for their growth and viability. This water is taken out of the ground through the root system, which can be vast and deep, depending on the tree and the geology. Some trees have an affinity for finding small breaks in sewer lines and tapping the moisture inside. If a tree has been in existence for a long time at a site and is removed during construction, it is possible that a zone of reduced soil water content has been left behind within the root zone. After construction the suction or drawing of water from the zone has been stopped because the tree is no longer there and natural forces attempt to rebuild the moisture to the previous levels or the levels of the moisture around this zone. As moisture rebuilds in the expansive clays the volume increases thereby possibly producing unpleasant results for structures established over it.
Trees existing near the edge of shallow foundations or pavements can draw water from under the slab in times of drought and cause a loss of support. Alternatively, the tree root system may maintain a lowered water content (or heightened soil suction condition) while the soil around it is inflating with increased water content due to evaporation cut off caused by constructing a slab or pavement around the area. The result is the same. There is a depression around the tree root zone and raised soil other places.
If a tree is planted near a structure or a pavement after construction in an expansive clay the moisture can be removed and a depressed or dimpled zone occurs that looks very similar to the one described above. The result is the same. Differential support conditions can lead to differential movements, cracking and distress, and failure in the foundation and structure system.
The soil suction levels produced by tree roots can be as much as 4.4 pF to 4.7 pF. At these levels the suction generated in the soil is sufficient to resist and stop the flow of water into the root system. This is also termed the wilting point by agronomists. It has been found by study that 4.5 is a typical number for the soil suction values in the vicinity of a tree. This value extends to the deepest root penetration plus about 2 feet deeper. For this reason, the depth of roots should be logged in all geotechnical investigations where high plasticity clay is involved. In the writer’s experience suction values on the order 4.5 to 4.6 have been found as deep as 17 feet in some parts of Texas and root fibers were found around the same depth.
The treatment for the tree condition in expansive clay is as follows:
Pre-existing trees removed during construction which are under a slab or pavement or within a reasonably close distance, say 15 feet, should have a considerable depth of the tree zone reworked with moisture conditioned soil or water injection. This treatment is to bring the moisture condition up to that of the surrounding soil which was not affected by the root system.
If a tree is to remain near the edge of a structure on high plasticity clay special precautions should be taken to bridge the probable loss of support zone. This zone could be taken approximately equal to the mature crown radius of the tree. Additional support can be provided by more and stronger stiffener beams in a shallow, stiffened foundation. An alternate approach could be to use a root barrier between the tree and the foundation.