Soil: The Other Half ofthe FoundationUnderstanding the stuff the house sits on mayprevent cost overruns, callbacks and neighborhood gossip

BY ROBERT M. FELTON

Most builders and architects are fa-miliar with the problem of build-ing settlement: the chimney that

won't stop going down, the wall cracks thatkeep opening up, the older home that sud-denly begins to exhibit movement for no ap-parent reason. Everyone in the building in-dustry has a story about a fix that "shouldadone it," but didn't.

Preventing settlement problems beginswith the recognition that the soil a founda-tion rests on is part of the foundation sys-tem; it's a building material, just like the 2x4studs that frame the house. The fact that youcan't go to a lumberyard and select this

building material—that in most cases you'resimply going to use whatever you happento have—makes it especially important thatyou recognize differences among soil types,know something of the way soils respond tobuilding loads and be able to identify poten-tial problems.

Differential settlement is thereal enemyA few things need to be understood aboutsettlement. First, all houses settle. Theamount may be so small as to be unde-tectable or may be so uniform as to leave nosigns, but it unquestionably happens. Sec-

ond, because of the natural and construc-tion-related variations in soil properties,not every point on a foundation settles thesame amount.

To avoid problems with entrances and util-ity connections, total settlement must beminimized. To avoid racking door framesand cracking walls, you must prevent differ-ential settlement, the difference in settle-ment between various points on the founda-tion. The distinction between total anddifferential settlement is important. ThePalace of Fine Arts in Mexico City, for in-stance, has settled several meters withoutsignificant distress to the structure and re-

USDA soil map USGS topographic map

Two ways to look at the same piece of landSoil maps from the U. S. Department of Agriculture (left) are actually aerial photographs with soil types superimposed on them. They can alertbuilders to problem soil conditions before they start digging. Topographic maps from the U. S. Geological Survey (right) can reveal wetlands orintermittent streams. Serving as snapshots in time, both maps can provide valuable insight into human activity that has taken place on land.

City dump?The soil descriptions thataccompany this map indicatethat except for the suspiciousarea in the center (Masignifies man-made soil), allthe soils delineated in thisclose-up are builder-friendlysand. The two straight, boldlines to the left of the dumprepresent dammed lakes.

What city dump?Although a topographic mapis generally the betterindicator of previous devel-opment, a builder who reliedon only a topo map in thiscase would not be able to tellthat the lakes were man-madeand, more important, wouldhave no idea that his dreamland was once the city dump.

The other half of the foundationTo prevent settlement problems, soil that is stable and compacted must extend at least half the distance from the base of the footing to theinfluence depth (the farthest distance beneath the footing that its weight is felt by soil particles). Strip foundations (such as the wall on theright) stress the soil more than square foundations (such as the pedestal on the left) and thus have greater influence depths.

Square foundation Strip foundation

mains in service because the settlement hasbeen uniform. The Leaning Tower of Pisa,on the other hand, is useless for anything butthe Kodak moments of tourists.

Elementary research can dig upmost problemsFortunately for homebuilders, the loads in-volved in most residential construction arerelatively light. Following time-proven pro-cedures and steering clear of some commonmisconceptions will keep you out of troublein most cases.

For starters, you can learn a lot about soilconditions on your site by taking advantageof public-sector resources. The United StatesDepartment of Agriculture (USDA) hasprepared soil maps for most of the country.Available at no cost at any local USDA

branch office, these maps superimpose soil-type delineations over aerial photographs(drawing left, p. 69). By studying these mapsand the soil descriptions that accompanythem, you can find out information such aswhether your site might have a high ground-water table or whether problematic soils—such as shrink/swell-susceptible clays—might lurk beneath the surface.

Having been taken 30 years ago or more,aerial photos often reveal evidence of unsus-pected development or manipulation of thesite. An even better source for this type ofinformation is a topographic map from theUnited States Geological Survey (drawingright, p. 69). This map may reveal aban-doned cemeteries, farm ponds, wells or thelong-forgotten town dump. A topographicmap may be purchased for a few dollars at

outdoor-sporting-goods stores or down-loaded at no cost (www.topozone.com).

Don't forget to check with the local build-ing and engineering departments, either;they often have a wealth of local informationand experience, which they are usually hap-py to share. Developers who have built closeby or homeowners on adjoining lots are oth-er good sources of information.

Virgin soil is not always virtuousIf your research unearths potential problems,that's the time to bite the bullet and consultan expert (sidebar facing page). If you uncov-er no history of activity that may have leftproblems behind, that's probably good news,but there may still be things that need atten-tion. "Virgin soil" isn't inherently problem-free. Muck (decomposed organic material),

for instance, may have been undisturbedsince Mother Nature deposited it, but build-ing on it is always a bad idea.

Clay can also be troublesome. The strengthof clay soils varies inversely with changes inmoisture content: the greater the moisture,the weaker the soil. If clay materials underlieyour site, the site plan must provide for pos-itive drainage that will direct surface wateraway from the structure and paved areas;otherwise, water may penetrate and weakenthe supporting soil. This is, in fact, a com-mon cause of post-construction settlementproblems. If site constraints make it impos-sible to direct runoff away from the drive-way, you should plan to provide lateral drainsalongside the driveway to prevent waterfrom accumulating beneath the pavement.Foundation drains must also be carefully de-signed to carry groundwater well away fromthe structure. These measures aren't cheap,but they cost less than repairs, ill will andneighborhood gossip.

Bridging the gapsWhat if there's a wet, loose, soft, low ormucky spot on the site? Can you bridge itwith fill? Geotechnical engineers like mehave a couple of easy rules of thumb that werefer to when called to a site and asked whatto do. After that, it gets complicated.

Rule one: Never fill a low spot with surfacestoppings, shrubs or woody debris, no mat-ter how much clean fill will be placed on topof it. It might take years, but the organic ma-terial will inevitably decompose and causesettlement.

Rule two: Muck must be removed. Al-though it's possible to force sand and gravelfill into the muck and create a stable mixture,that's not a reliable solution. What usuallyhappens is that pockets of weak material be-come scattered throughout the reworkedmaterial, and these areas slowly compressover the years.

To control settlement problems, soils engi-neers want the foundation to rest on stable,compacted material that extends at least halfof the influence depth, the distance beneaththe footing that its weight is still "felt" by soilparticles (drawings facing page).

Square foundations (such as those beneatha Lally column) stress the soil to a depthequal to about two times the foundationwidth. A 2-ft. by 2-ft. pedestal, for example,is "felt" by soil particles to a depth of about4 ft. below the foundation.

A strip foundation, one whose length is tenor more times greater than the width, e.g., awall foundation, stresses the soil to a depthequal to about four times the foundation

When should you call in a soils engineer?Many builders and architects are reluc-tant to hire engineers to perform sub-surface investigations [photo below),and that's easy to understand: The costof a house can go up a few thousanddollars. Despite the cost, there are cir-cumstances where consulting a soilsengineer is a wise investment.• If you're wondering how on earth (tobelabor the point) you're supposed tobuild a house on a lot with a steepslope, then you need an engineer todetermine the soil properties and toevaluate the stability of the slope.• Evidence of previous earthwork atthe site, especially filling, requirescareful investigation. The site may hostburied organic materials, e.g., "muck";debris such as demolition rubble orabandoned vehicles; or simply dumpedfill. Any of these things can cause se-vere settlement problems.

• A local history of the presence ofclay soils that are susceptible toshrinking and swelling with changes inmoisture content also requires carefulinvestigation and, usually, specializeddesign services and use of an out-of-the-ordinary foundation.• Encountering groundwater or weaksoil while excavating for foundationsmay indicate a potential for settlementor instability in the future. The prob-lem area should be carefully delineatedby an expert and remedied afterclose consultation.

Consult your experience: If you'recrossing your fingers and hoping thesoil conditions won't cause a problem,you're probably right to be uneasy. Callin a specialist. To locate an engineer inyour area, contact the American Soci-ety of Civil Engineers at (800) 548-2723.—R. M. F.

Not exactly wine tasting. Soils engineers inspect core samples they've taken us-ing the split-barrel sampler on the left.

Photo this page: Courtesy of S&ME, Inc., Raleigh, NC.

width. Thus, an 18-in. wide foundation is"felt" by soil particles as much as 6 ft. belowthe foundation. The maximum stress for ei-ther type of foundation occurs at a depth ofabout one-quarter of the influence depth.

What all this means to the builder is thatfor the pedestal I just described, you mustprovide at least 2 ft. of competent support;for the strip, you must provide at least 3 ft. Ifthat can't be done with the existing soil, youshould plan to remove and replace the unde-sirable material and restore the site to gradewith engineered fill.

Self-compacting soil has not beeninvented yetOne of the misconceptions I often encounteris the belief that soil will densify andstrengthen if it's merely dumped on a siteand left undisturbed for several months—that fill dumped in a low area in the fall willbe ready to support construction in thespring without any compaction. To thisopinion, I always respond: "'Fraid not." Tounderstand why, think back to your high-school physics class: "An object at rest tendsto remain at rest unless acted on by an out-side force." Loosely dumped soil does notdensify and strengthen by itself.

It is particularly important that fill beplaced in thin layers so that the densificationeffect of the compaction equipment is felt allthe way to the bottom of each layer (drawingright). The maximum thickness for each lay-er varies depending on soil type: ordinarilyabout 12 in. for sand and 6 in. to 8 in. forclay. The required degree of densification isusually set forth by the local building codeand specified as some percentage of maxi-mum dry density as determined by one ofseveral standard methods.

The moisture content of the fill materialmust also be controlled. If the moisture istoo low, it is difficult for individual soilgrains to realign themselves into the densestconfiguration; adding moisture lubricatesthe grains and makes realignment easier.But if there is too much moisture, the soilbecomes unstable under the influence ofcompaction equipment because a portion ofthe compactive effort will be borne by thewater between the soil grains and result in awater-bed-like rolling of the soil. Granularsoils such as sand are most desirable for useas fill because their moisture content can beeasily tweaked.

Keep heavy equipment away fromfoundation wallsCompacting soil against below-grade wallsalso requires special care, lest the horizontal

load imposed by the compaction equipmentdamage the walls. A good rule of thumb is tokeep heavy equipment away from the wall adistance of at least two-thirds the unbal-anced height of the fill (drawing facingpage). In other words, if the fill on the out-side of the wall is 6 ft. higher than the fill onthe inside, the big rollers should be kept atleast 4 ft. away from the wall.

The strip of ground adjoining the wallshould be compacted using small, hand-operated equipment. But make no mistake:It should be compacted; failure to compact iswhat leads to cracking sidewalks and unevendriveways. It is equally important to assure

that fill placed in utility-line excavations beproperly placed and compacted. Improperplacement of fill by the plumbing contractor,for instance, not only causes drain problemsbut frequently leads to exterior-wall settle-ment and cracking over buried drain lines.

TroubleshootingIn spite of your best efforts, what if the newhomeowners call your office and demandthat you come over right away to inspect acrack they've just noticed? You have notonly a public-relations problem, because pre-viously unnoticed hairline cracks suddenlybecome subjects of concern to the buyer, but

Proper fill placementTo minimize settlement and to ensure that the foundation is properlysupported, fill must be placed in thin layers, and each layer must beindividually compacted.

Good compaction

Bad compaction("dump and roll")

a real technical problem, too: You've got tofigure out what has happened.

Most cracks are minor and insignificant, aconsequence of the settlement and shiftingthat all houses undergo. In these cases, youneed to explain a few things to the home-owners. Ideally, you would have preparedthem for these possibilities beforehand(FHB #126, pp. 112-113):• The act of building the house changed thelocal groundwater conditions. In particular,the shallow soil zone beneath the house isdrying. The minor settlement that resultscan be enough to cause hairline cracking.• When you painted the house, it was emp-ty. When the owner took possession andbrought in the home gym, water bed and ba-by grand, this stress inevitably caused flexingof virtually all the structural members. This,too, can cause hairline cracking.

Of course, there maybe times when simpleexplanations don't suffice, when somethingstrange seems to be happening. The best wayto identify the cause of the problem is to in-stall a crack monitor (Avongard Products;800-244-7241), a two-piece, specializedruler that is semipermanently mounted overa crack (photo below). Using a crack monitorspares you the work of trying to decide byeye and memory whether a crack has grownor changed. If it appears you've got a realproblem, the data will be invaluable to who-ever is trying to figure out what's causing thecrack and what ought to be done becausedifferent settlement mechanisms leave dis-tinctive signatures, which become apparentwhen the data are graphed.

The correction of foundation problems re-quires thorough investigation by experts.You should not hire a grouting contractor to"mud-jack" (pump concrete beneath a dis-tressed area) until the cause of the distresshas been positively identified. If a house cor-ner is settling due to the presence of buried,compressible organic material, for example,pumping a yard or two of concrete into thesoil immediately beneath the foundation willincrease the load over the soft material andthus increase settlement. The wall may lookfine when the contractor cleans up the jobsite and leaves, but more irate-homeownerphone calls are inevitable.

When making repairs, just as when begin-ning a project, you should be guided by asingle rule: The best defense against futureproblems is doing it right the first time.

Compacting soil againstfoundation walls takes careTo avoid damage from the horizontal loadsimposed by compaction, heavy equipmentshould be kept away from the wall adistance of at least two-thirds theunbalanced height of the fill.

Robert M. Felton is a consulting geotechnical engi-neer and free-lance writer in Wake Forest, NorthCarolina. He may be contacted via e-mail atRMFelton@aol.com.

Hairline crack or structural disaster? A crack monitor is used to track crack move-ment over a period of time. The pattern of movement enables an engineer to deter-mine when and what remedial action may be necessary.