TREES, FOUNDATIONS, AND SOILS

Foundation problems, or structural deficiencies ofhomes and buildings are a common concern in NorthTexas. Structural engineers and foundation expertsstate that there are two types of foundation owners,those who have had foundation problems and thosewho will. With an increasing number of areafoundations being poured and the increasing age ofexisting area foundations, a rise in future deficiencieswill likely occur. There are many potential reasons forfoundation problems, and the causes may varygreatly from site to site. The soil in your neighborhoodmay be one type, but area-wide the soil types couldinclude clay, sand, sandy loam, rock, or a combina-tion of these materials. Soil diversity can result in alarge local plant palette, which is great for plantenthusiasts but not always great for foundations. Dueto the variance of soil types and conditions at anysingle site, it is not possible to offer any one response toa problem. The subject of foundation deficiencies isbroad, and it requires different areas of expertise.Experts can include Soil Scientists, foundation repair

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When there is physical contact between a tree and afoundation, the damage, if any, may vary greatlyaccording to the species of tree, the site conditions,proximity to the foundation, the type of foundation, theportion of the tree contacting the foundation, and thetree’s size. The tree species is important because alarger, older, longer-lived, shade tree is a greaterconcern than a smaller, younger, shorter-livedornamental tree or shrub.

This is due in part to the extent of the root systeminvolved with shade trees. Most trees have root systemscontained within the top 4 to 5 feet of soil, with theabsorbing roots in the top 12 inches. The roots canextend outward beyond three times the height of thetree. Thus, a tree with a height of 50 feet, could haveroots that grow outward 150 feet from the trunk. If yourtree has limbs near your house, you are likely to haveroots growing under or along your foundation. If youdo not water around the foundation, research showsthat roots will have a tendency to congregate anddevelop more rapidly under the foundation simplybecause of the increased amount of availablemoisture underneath.

The type of foundation is important because, as ageneral rule, a pier-and-beam foundation can toleratenearby trees better than a slab. To understand why,consider the portions of the tree that contact thefoundation and the forces exerted. A root, root crown,or root flare running under a foundation can exert anupward pressure as the contacting portion expands indiameter. The root, root crown, root flare, or trunk, canalso exert an inward pressure if they contact the outerperimeter of the foundation as they expand indiameter. Since a pier-and-beam foundation gains itssupport from piers rather than just concrete on top ofsoil, they can more readily sustain minor pressure fromnearby trees. If a large shade tree is 5 feet from afoundation, physical contact and pressure exertedagainst the foundation is likely. If the tree is 15 feetaway, physical contact is likely and pressure exertedagainst the foundation is possible. If the tree is 30 feetaway, physical contact is still possible, but not as likelyto exert much pressure against the foundation.

Trees can affect the moisture content of soil under andaround foundations. A large shade tree can transpireas much as 200 gallons of water per day. Trees“transpire” or release water into the air in the form of

vapor. The water for transpiration must come from thesoil by way of the roots. This is why trees are known to“pull” water from under a foundation, but only whenwater in the surrounding soil is inadequate orunavailable. Since water naturally moves through thesoil from high moisture areas to low moisture areas,simply not watering at all can cause foundationproblems regardless of nearby trees. Trees can add tothe problem but are usually not the primary cause ofmoisture loss from soil.

When trees have died or have been removed, the rootsthat remain under or near a foundation can createproblems. As the root system begins to rot and decay,air spaces and gaps occur in the soil to replace theroots. This results in a soil that can shift to fill the void.Many years ago, foundation professionals would see alarger shade tree that is close to the house andautomatically recommend removal. Today, researchshows that removal of the tree is not always the answer.Since many trees and foundations have existedtogether for many years, any changes that occur doso over time and they can be minor, compared to theremoval of the tree. If a large shade tree, that is closeto a foundation is removed, more dramatic changescan occur to the soil and foundation than if the treewas left alone, in most cases. Consideration must begiven to the full, longterm effect of removing anexisting tree near a foundation.

In ConclusionWhen a foundation has a deficiency,many issues mustbe considered. Seldom is a tree solely at fault.Cautiously consider any recommendation to removea tree as the answer to foundation failure. In somecases, a tree will offer benefits that help offset anyfoundation concerns such as the energy savingsobtained or any of the many other benefits of trees.Obtain other opinions from numerous consultants inrelative fields.

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companies, Hydrologists, Structural Engineers andConsulting Arborists. Gathering input from eachspecialist is wise but it can lead to differences ofopinion and confusion. In some cases, the professionalopinions may be based on conflicting or outdatedresearch. Deciding which remedy is best suited to aspecific circumstance is sometimes a difficult task.

When trees are growing close to a foundation theopinion of a Consulting Arborist is often requested.Many times, trees are identified as the cause offoundation deficiencies by one or more consultingprofessionals, and removal of the plant isrecommended. Consulting Arborists usually have abasic knowledge of other professional fields, and viceversa. A Consulting Arborist will offer what he knows tobe a fact, but at some point, advice orrecommendations become a professional opinionbased on experience, common sense, and research.The Consulting Arborist’s objective is to teach theproperty owner how the trees might affectfoundations. Sometimes this requires a discussiondirected more toward the “whole picture”. Byexamining all of the related issues, arborists,foundation experts, and others can determine if treesare a problem or a contributor to a problem. Theissues are not always black and white. However,being informed and knowing what to ask, can leadto the correct solution. A list of questions to ask orissues to consider would include the following:

Site Planning and DevelopmentConsider the planning and development and thehistory of the site beginning as far back in time asrecords will allow. Determine what existed on the sitebefore it was developed. Check for records availablefrom city hall, the builder/developer or others that mayhave been involved with the site. In many housingdevelopments, the site is graded and leveled,removing or redistributing the topsoil (the good stuff forgrowing plants!). Determine if your site has any topsoilremaining. If so, measure the depth. Determinewhether the soil type and soil profile were taken intoaccount in planning and developing the area, or inthe design of the foundation. Determine if the buildermade adjustments to the soil profile before pouring thefoundation. and if so, was the soil compacted toindustry standards? Experts agree that these practicesdirectly affect the integrity and life expectancy of a

foundation BEFORE being installed. North Texasproperty owners who have researched their property’sdevelopment have sometimes found that the propertywas a construction debris dumpsite, a filled floodplain,or a covered natural spring. Poor site planning anddevelopment can lead to foundation problems.

SoilsSoils are often noted by the experts as the primaryfactor in foundation problems. Do you know the soiltype that exists under the foundation? Is there rockunderneath? If so, what type of rock? Is it fractured andmixed with soil, or is it a solid layer and to what depth?If the soil is black clay, the high shrink/swell potentialcan be a factor. The black clay soil in our area shrinkswhen it is dry, and expands when it is wet. Sandy soilsshrink and swell a smaller amount and do not retainwater like clay soils. When a foundation site contains amixture of clay, sand, sandy loam, or rock, it becomesmore difficult to predict how the soil will react. Most soiltypes, and soil mixes can adequately support afoundation if they are properly planned, designed,built, and MAINTAINED! Even the best foundation, builtunder ideal conditions, can have difficulty if it is notproperly maintained.

Lot CompletionConsider the final lot grading. Does your lot draincorrectly? Does water stand near your foundation, ordoes it drain away? Is the soil level below the bottom ofthe foundation or above the brick or exterior material?Proper drainage and final grading near a foundationare important in avoiding future problems.

Life ExpectancyConsider the type of foundation, its design and the siteconditions. Was it designed to last forever? Manypeople believe that since it is concrete, a foundationshould last forever. Almost everything has a lifeexpectancy, including concrete. Our area hasfoundations that are fifty years old or more with fewproblems and some with problems only a few yearsafter construction.

Quality, Standards, and WorkmanshipQuality issues apply to every person involved in theconstruction process from the builder/developer to thestructural engineer and even the city officials thatinspect and approve of the construction practices.Poor workmanship or inattention to the circumstances

by any person or group that is involved must beinvestigated. Foundation experts, structural engineers,and others can provide their recommendations, but beaware that a conflict of interest may exist. An un-biasedopinion of the standards utilized and the quality ofconstruction on a project is crucial. Since manyexperts in different professions may also work for othersin related fields, it is wise to ask if the person orcompany has had any relationship with othersinvolved, or to hire firms from outside our area.

Proper WateringThe goal of watering a foundation is to provideadequate and consistent soil moisture under andaround a foundation at all times throughout the year.Under-watering or over-watering for any length of timecan cause problems, regardless of the soil type. SinceNorth Central Texas has experienced droughtconditions in previous years, many homeowners whodid not adequately water may now have foundationissues.

Foundation TypesConceptually, a slab foundation is basically a “slab” ofreinforced concrete poured on top of the soil. A pier-and-beam foundation contains “piers”, or pillars, ofreinforced concrete with beams of various materialsstretched across the top of the piers (normally a beamof iron and/or concrete). A slab foundation is morereadily affected by the soil under and around it as wellas the soil moisture content. A pier-and-beamfoundation is slightly less affected by soil type andmoisture content, especially if the piers are resting ontop of a rock layer. There are other foundation types,but the important question to ask is: “What type offoundation has been installed and is it suitable to thesite conditions?”

TreesTrees can cause problems for nearby foundations inthree primary ways.

1. By physical contact with the foundation

2. By affecting the moisture content of the soilunder or near a foundation

3. By causing air gaps and shifting soil due todecaying roots under or near a foundation(when a tree dies or is removed)