Landscaping and Foundation Maintenance
General. A house with the proper foundation and drainage can still experience distress if the homeowner does not properly landscape and maintain his property. One of the most critical aspects of landscaping is the continual maintenance of properly designed slopes.
Installing flowerbeds or shrubs next to the foundation and keeping the area flooded will result in a net increase in soil expansion in the expansive soil areas. The expansion will occur at the foundation perimeter. It is recommended that initial landscaping is done on all sides and that drainage ways from the foundation should be provided and maintained. Partial landscaping on one side of the house may result in swelling on the landscaping side of the house and resulting differential swell of foundation and structural distress in the form of brick cracking, window/door sticking, and slab cracking.
Landscaping in areas where sandy, nonexpansive soils are present with flowers and shrubs should not pose a major problem next to the foundations. This condition assumes that the foundations are designed for saturated soil conditions. Major foundation problems can occur if the planter areas are saturated, as the foundations are not designed for saturated (perched water table) conditions. Then problems can occur in the form of foundation settlement, brick cracking, etc.
Sprinkler Systems. Sprinkler systems can be used in the areas where expansive soils are present, provided the sprinkler system is placed all around the house to provide a uniform moisture condition throughout the year.
The use of the sprinkler system in parts of Houston where sandy soils are present should not pose any problems, provided the foundations are designed for saturated subsoil conditions with positive drainage away from the structure.
The excavations for the sprinkler system lines, in the areas where expansive soils are present, should be backfilled with impermeable clays. Bank sands or topsoil should not be used as backfill. These soils should be properly compacted to minimize water flow into the excavation trench and seeping under the foundations, resulting in the foundation and structural distress.
The sprinkler system must be checked for leakage at least once a month. Significant foundation movements can occur if the expansive soils under the foundations are exposed to a source of free water.
The homeowner should also be aware of the damage that leaking plumbing or underground utilities can cause if they are allowed to continue leaking and providing the expansive soils with the source of water.
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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 during 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 the long-term performance of the structure.