Landscaping & Foundation Maintenance 2
Effect of Trees. The presence of trees near a residence is considered to be a potential contributing factor to the foundation distress. Our experience shows that the presence or removal of large trees in close proximity to residential structures can cause foundation distress. This problem is aggravated by cyclic wet and dry seasons in the area. Foundation damage of residential structures caused by the adjacent trees indicates that foundation movements of as much as 3 to 5 inches can be experienced in close proximity to residential foundations.
This condition will be more severe in the periods of extreme drought. Sometimes the root system to trees such as willow, elm, or oak can physically move foundations and walls and cause considerable structural damage. Root barriers can be installed near the exterior grade beams to a minimum depth of 36 inches, if trees are left in place in close proximity to foundations. It is recommended that trees not be planted closer than half the canopy diameter of the mature tree, typically 20 feet from foundations. Any trees in closer proximity should be thoroughly soaked at least twice a week during hot summer months, and once a week in periods of low rainfall. More frequent tree watering may be required.
Tree roots tend to desiccate the soils. In the event that the tree has been removed prior to house construction, during the useful life of the house, or if the tree dies, subsoil swelling can occur for several years. Studies have shown that this process can last as much as 20 years in the area where highly expansive clays are present. In the areas where sandy soils are present, this process does not occur.
In this case the foundation for the house should be designed for the anticipated maximum heave. Alternatively, the site should be left alone for several years so that the moisture regime in the desiccated area of the soils (where roots used to be) becomes equal/stabilized to the surrounding subsoil conditions.
Tree removal can be safe provided the tree is no older that any part of the house, since the subsequent heave can only return the foundation to its original level. In most cases there is no advantage to a staged reduction in the size of the tree and the tree should be completely removed at the earliest opportunity. The areas where expansive soils exist and where the tree is older than the house, or there are more recent extensions to the house, it is not advisable to remove the tree because the danger of inducing damaging heave; unless the foundation is designed for the total computed expected heave.
In the areas where non-expansive soils are present, no significant foundation distress will occur as a result of the tree removal.
In the areas where too much heave can occur with tree removal, some kind of pruning, such as crown thinning, crown reduction or pollarding should be considered. Pollarding which is where most of the branches are removed and the height of the main truck is reduced, is often mistakenly specified, because most published advice links the height of the tree to the likelihood of damage. In fact the leaf area is the important factor. Crown thinning or crown reduction, in which some branches are removed or shortened, is therefore generally preferable to pollarding. The pruning should be done in such a way as to minimize the future growth of the tree, without leaving it vulnerable to disease (as pollarding often does) while maintaining its shape. This should be done only be a reputable tree surgeon or qualified contractor working under the instructions of an arboriculturist.
You may find there is opposition to the removal or reduction of the offending tree; for example, it may belong to a neighbor or the local authority, or have a Tree Preservation Order on it. In such cases there are other techniques that can be used from within your own property.
One option is root pruning, which is usually performed by excavating a trench between the tree and the damaged property deep enough to cut most of the roots. The trench should not be so close to the tree that it jeopardized its stability. In time, the tree will grow new roots to replace those that are cut; however, in the short term there will be some recovery as the degree of desiccation in the soil under the foundations reduces.
Where the damage has only appeared in a period of dry weather, a return to a normal weather pattern may prevent further damage from occurring. Permission from the local authority is required before pruning the roots of a tree with a preservation order on it.
Root barriers are a variant of root pruning. However, instead of simply filling the trench with soil after cutting the roots, the trench is either filled with concrete or lined with and impermeable layer to form a “permanent” barrier to the roots. Whether the barrier will be truly permanent is questionable, because the roots may be able to grow around or under the trench. However, the barrier should at least increase the time it takes for the roots to grow back. Root barriers serve as a bio barrier root control system and appear to perform satisfactorily. The design of the root barrier system should be developed in construction with the geotechnical engineer to assume long-term performance of the structure.
Foundation Maintenance:
