Soil & Settlement

Building Settlement 13/5/05

 

'Blogging' atau cerita yang aku tulis (type) ni adalah apa yg aku paham daripada 'research' sikit sikit yang aku dapat melalui internet dan member-member dalam construction industri. Basically, apa yg aku beli ialah struktur yg berada atas satu tanah. 'Housing Construction' adalah satu proses bottom up. Kita start dari bawah sampai lah ke atas. Mula2 sekali tanah (soil), lepas tu baru struktur tapak (foundation) dan seterusnya sampai struktur bumbung.

 

Sepatutnya nak cerita pasal jenis tanah dulu. Dah jumpa 'marvelous websites' pasal soil nih. Laterlah aku blog pasal jenis tanah atau 'soil'. Sekarang ni nak cerita pasal soil dan foundation.

 

Sebelum tu disclaimer ye:

 

"Info dari blog ini bukanlah pandangan professional. Hanya research dari internet semata-mata dan sedikit interview dan borak-borak. Semua info ini mungkin 'dah obsolete' dan tidak boleh diguna pakai. Sila 'double-check dgn orang2 professional jika anda ingin menggunakan info dari blog ini utk apa-apa tujuan.”

 

Pada 16 April 2005, hari Sabtu, iaitu hari yg Developer batalkan atau tidak benarkan pembeli naik ke tapak projek, aku telah pergi jumpa En. Zul untuk bertanya pasal jenis tanah (soil type) dan foundation untuk struktur bangunan. Katanya, jenis tanah ialah laterite. (Aku tak sempat lagi selidik apa laterite tu). Dan, katanya lagi, foundation rumah ialah 'Raft Foundation'. So, dia siap lukis lagi dan 'explain' 'step' utk buat foundation. Then,aku tanya member2 dan research internet.

 

Hmmm, sekarang nak cerita pasal soil dulu. Anyway, masa aku sign S&P, aku tak nampak kerja tanah dan tak nampak tanah asal macammana. So, susah sikitlah nak cerita.

 

Tak banyak yang aku boleh cerita pasal soil. So, aku list ajelah apa2 yg mungkin berkaitan kat bawah ni yg aku dapat dari internet. Mungkin tak apply kat Malaysia. Kedua-dua article kat bawah ni ialah tentang tanah dan 'settlement' tanah. Settlement tu kira macam tanah dah settle dan tak akan bergerak ke bawah lagi.

 

    (i)  http://www.taunton.com/finehomebuilding/pages/h00008.asp

    (ii) http://irc.nrc-cnrc.gc.ca/cbd/cbd148e.html

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Artikel Satu:   http://www.taunton.com/finehomebuilding/pages/h00008.asp Soil: The Other Half of the Foundation Understanding the stuff the house sits on may prevent cost overruns, callbacks and neighborhood gossip by Robert M. Felton   Finding out what's down there. Taking samples from a series of drillings enables engineers to determine subsurface soil characteristics. Most builders and architects are familiar with the problem of building settlement: the chimney that won't stop going down, the wall cracks that keep opening up, the older home that suddenly begins to exhibit movement for no apparent reason. Everyone in the building industry has a story about a fix that "shoulda done it," but didn't. Preventing settlement problems begins with the recognition that the soil a foundation rests on is part of the foundation system; it's a building material, just like the 2x4 studs that frame the house. The fact that you can't go to a lumberyard and select this building material -- that in most cases you're simply going to use whatever you happen to have -- makes it especially important that you recognize differences among soil types, know something of the way soils respond to building loads and be able to identify potential problems. Differential settlement is the real enemy A few things need to be understood about settlement. First, all houses settle. The amount may be so small as to be undetectable or may be so uniform as to leave no signs, but it unquestionably happens. Second, because of the natural and construction-related variations in soil properties, not every point on a foundation settles the same amount. To avoid problems with entrances and utility connections, total settlement must be minimized. To avoid racking door frames and cracking walls, you must prevent differential settlement, the difference in settlement between various points on the foundation. The distinction between total and differential settlement is important. The Palace of Fine Arts in Mexico City, for instance, has settled several meters without significant distress to the structure and remains in service because the settlement has been uniform. The Leaning Tower of Pisa, on the other hand, is useless for anything but the Kodak moments of tourists. Elementary research can dig up most problems Fortunately for homebuilders, the loads involved in most residential construction are relatively light. Following time-proven procedures and steering clear of some common misconceptions will keep you out of trouble in most cases. For starters, you can learn a lot about soil conditions on your site by taking advantage of public-sector resources. The United States Department of Agriculture (www.usda.gov) has prepared 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. By studying these maps and the soil descriptions that accompany them, you can find out information such as whether your site might have a high groundwater 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 unsuspected development or manipulation of the site. An even better source for this type of information is a topographic map from the United States Geological Survey (www.usgs.gov). This map may reveal abandoned cemeteries, farm ponds, wells or the long-forgotten town dump. A topographic map may be purchased for a few dollars at outdoor-sporting-goods stores or downloaded at no cost (www.topozone.com). Two ways to look at the same piece of land (opens in new window)   Serving as snapshots in time, soil maps and topographic maps can provide valuable insight into human activity that has taken place on land (see Two ways to look at the same piece of land). Don't forget to check with the local building and engineering departments, as well; they often have a wealth of local information and experience, which they are usually happy to share. Developers who have built close by or homeowners on adjoining lots are other good sources of information. Virgin soil is not always virtuous If your research unearths potential problems, that's the time to bite the bullet and consult an expert (see When should you call in a soils engineer?). If you uncover no history of activity that may have left problems behind, that's probably good news, but there may still be things that need attention. "Virgin soil" isn't inherently problem-free. Muck (decomposed organic material), for instance, may have been undisturbed since Mother Nature deposited it, but building on it is always a bad idea. Clay can also be troublesome. The strength of clay soils varies inversely with changes in moisture content: the greater the moisture, the weaker the soil. If clay materials underlie your site, the site plan must provide for positive drainage that will direct surface water away from the structure and paved areas; otherwise, water may penetrate and weaken the supporting soil. This is, in fact, a common cause of post-construction settlement problems. If site constraints make it impossible to direct runoff away from the driveway, you should plan to provide lateral drains alongside the driveway to prevent water from accumulating beneath the pavement. Foundation drains must also be carefully designed to carry groundwater well away from the structure. These measures aren't cheap, but they cost less than repairs, ill will and neighborhood gossip. Bridging the gaps What if there's a wet, loose, soft, low or mucky spot on the site? Can you bridge it with fill? Geotechnical engineers like me have a couple of easy rules of thumb that we refer to when called to a site and asked what to do. After that, it gets complicated. Rule one: Never fill a low spot with surface strippings, shrubs or woody debris, no matter how much clean fill will be placed on top of it. It might take years, but the organic material will inevitably decompose and cause settlement. Rule two: Muck must be removed. Although it's possible to force sand and gravel fill into the muck and create a stable mixture, that's not a reliable solution. What usually happens is that pockets of weak material become scattered throughout the reworked material, and these areas slowly compress over the years. To control settlement problems, soils engineers want the foundation to rest on stable, compacted material that extends at least half of the influence depth, the distance beneath the footing that its weight is still "felt" by soil particles. The other half of the foundation (opens in new window)   Square foundations (such as those beneath a Lally column) stress the soil to a depth equal to about two times the foundation width. A 2-ft. by 2-ft. pedestal, for example, is "felt" by soil particles to a depth of about 4 ft. below the foundation. A strip foundation, one whose length is ten or more times greater than the width, e.g., a wall foundation, stresses the soil to a depth equal to about four times the foundation width. Thus, an 18-in. wide foundation is "felt" by soil particles as much as 6 ft. below the foundation. The maximum stress for either type of foundation occurs at a depth of about one-quarter of the influence depth. What all this means to the builder is that for the pedestal I just described, you must provide at least 2 ft. of competent support; for the strip, you must provide at least 3 ft. If that can't be done with the existing soil, you should plan to remove and replace the undesirable material and restore the site to grade with engineered fill. Self-compacting soil has not been invented yet One of the misconceptions I often encounter is the belief that soil will densify and strengthen if it's merely dumped on a site and left undisturbed for several months--that fill dumped in a low area in the fall will be ready to support construction in the spring without any compaction. To this opinion, I always respond: "'Fraid not." To understand why, think back to your high-school physics class: "An object at rest tends to remain at rest unless acted on by an outside force." Loosely dumped soil does not densify and strengthen by itself. Proper fill placement (opens in new window)   It is particularly important that fill be placed in thin layers so that the densification effect of the compaction equipment is felt all the way to the bottom of each layer. The maximum thickness for each layer varies depending on soil type: ordinarily about 12 in. for sand and 6 in. to 8 in. for clay. The required degree of densification is usually set forth by the local building code and specified as some percentage of maximum dry density as determined by one of several standard methods. The moisture content of the fill material must also be controlled. If the moisture is too low, it is difficult for individual soil grains to realign themselves into the densest configuration; adding moisture lubricates the grains and makes realignment easier. But if there is too much moisture, the soil becomes unstable under the influence of compaction equipment because a portion of the compactive effort will be borne by the water between the soil grains and result in a water-bed-like rolling of the soil. Granular soils such as sand are most desirable for use as fill because their moisture content can be easily tweaked. Keep heavy equipment away from foundation walls Compacting soil against below-grade walls also requires special care, lest the horizontal load imposed by the compaction equipment damage the walls. A good rule of thumb is to keep heavy equipment away from the wall a distance of at least two-thirds the unbalanced height of the fill. In other words, if the fill on the outside of the wall is 6 ft. higher than the fill on the inside, the big rollers should be kept at least 4 ft. away from the wall. Compacting soil against foundation walls takes care (opens in new window)   The strip of ground adjoining the wall should be compacted using small, hand-operated equipment. But make no mistake: It should be compacted; failure to compact is what leads to cracking sidewalks and uneven driveways. It is equally important to assure that fill placed in utility-line excavations be properly placed and compacted. Improper placement of fill by the plumbing contractor, for instance, not only causes drain problems but frequently leads to exterior-wall settlement and cracking over buried drain lines. Troubleshooting In spite of your best efforts, what if the new homeowners call your office and demand that you come over right away to inspect a crack they've just noticed? You have not only a public-relations problem, because previously unnoticed hairline cracks suddenly become subjects of concern to the buyer, but a real technical problem, too: You've got to figure out what has happened. Most cracks are minor and insignificant, a consequence of the settlement and shifting that all houses undergo. In these cases, you need to explain a few things to the homeowners. Ideally, you would have prepared them for these possibilities beforehand : The act of building the house changed the local groundwater conditions. In particular, the shallow soil zone beneath the house is drying. The minor settlement that results can be enough to cause hairline cracking. When you painted the house, it was empty. When the owner took possession and brought in the home gym, water bed and baby grand, this stress inevitably caused flexing of virtually all the structural members. This, too, can cause hairline cracking.   A crack monitor is used to track crack movement over a period of time. The pattern of movement enables an engineer to determine when and what remedial action may be necessary. Of course, there may be times when simple explanations don't suffice, when something strange seems to be happening. The best way to identify the cause of the problem is to install a crack monitor (Avongard Products; 800-244-7241; www.avongard.com), a two-piece, specialized ruler that is semipermanently mounted over a crack. Using a crack monitor spares you the work of trying to decide by eye and memory whether a crack has grown or changed. If it appears you've got a real problem, the data will be invaluable to whoever is trying to figure out what's causing the crack and what ought to be done because different settlement mechanisms leave distinctive signatures, which become apparent when the data are graphed. The correction of foundation problems requires thorough investigation by experts. You should not hire a grouting contractor to "mud-jack" (pump concrete beneath a distressed area) until the cause of the distress has been positively identified. If a house corner is settling due to the presence of buried, compressible organic material, for example, pumping a yard or two of concrete into the soil immediately beneath the foundation will increase the load over the soft material and thus increase settlement. The wall may look fine when the contractor cleans up the job site and leaves, but more irate-homeowner phone calls are inevitable. When making repairs, just as when beginning a project, you should be guided by a single rule: The best defense against future problems is doing it right the first time. Robert M. Felton is a consulting geotechnical engineer and free-lance writer in Wake Forest, North Carolina. He may be contacted via e-mail at RMFelton@aol.com.     The other half of the foundation To prevent settlement problems, soil that is stable and compacted must extend at least half the distance from the base of the footing to the influence depth (the farthest distance beneath the footing that its weight is felt by soil particles). Strip foundations (such as the wall on the right) stress the soil more than square foundations (such as the pedestal on the left) and thus have greater influence depths.               Proper fill placement To minimize settlement and to ensure that the foundation is properly supported, fill must be placed in thin layers, and each layer must be individually compacted.       Compacting soil against foundation walls takes care To avoid damage from the horizontal loads imposed by compaction, heavy equipment should be kept away from the wall a distance of at least two-thirds the unbalanced height of the fill.

 

 

Artikel Dua   http://irc.nrc-cnrc.gc.ca/cbd/cbd148e.html CBD-148.  Foundation Movements C.B. Crawford In designing a structure it is commonly assumed that the foundation will not move. Correspondingly, if cracks appear in the structure it is assumed that the foundation did move and that this is the sole cause of cracking. Neither assumption is correct. To assess the influence of the foundation on cracking it is necessary to take into account the nature and magnitude of foundation movements and to understand why and how they occur. Within reason, there is less concern for the total settlement of a building than for differential settlement. The Palace of Fine Arts in Mexico City, for example, has sunk more than 10 feet into the ground since it was built 60 years ago and the most noticeable effect is that the grand stone stairway has disappeared and the entrance is now at street level. Classical failures like this seldom occur today. Foundation failures are now quite rare, due largely to improved understanding of the properties of soil and rock materials. Nevertheless, detrimental movements do occur occasionally and it is the purpose of this Digest to explore the possible causes. Soils and bedrock are similar to other building materials in that they deform under load, but unlike them they must be used as they appear in nature; they cannot be controlled by a manufacturing process. Except for special cases bedrock can be excluded from consideration because it is normally an adequate foundation material. Soils, on the other hand, are often stressed to their limit by foundation loads. Foundation Stresses The prediction of foundation movement is based on knowledge of how foundation loads are transferred to the ground and how earth or rock materials respond to resulting increases in stress. There are too many variables for these predictions to be precise, but for most situations they are adequate. Consider first how stresses are transferred to the ground under a large and a small footing, each carrying the same unit pressure (Figure 1). The curved lines under the footings are lines of equal increase in stress due to footing load. This is often called the "bulb of pressure." Note that the deepest line, indicating a stress increase equal to 10 per cent of the applied load, extends to a depth twice the width of the footing. If a series of narrow footings is installed close together the bulbs of pressure intersect and the influence on the ground is deeper than for an isolated footing. When piles are used, foundation loads are carried to deeper strata. If the piles are long in relation to the width of the building, the effect is much greater than if the piles are relatively short. The bulb of pressure concept is used to determine the depth to which foundation soils must be explored. Figure 1.  Lines of equal vertical stress caused by surface loads. Examples of Settlement As building loads are applied to the ground an "immediate" settlement occurs as a result of instantaneous compression of the soil. Most immediate settlement may be accommodated within the structure as it is built, and fortunately much of the differential movement occurs at this stage. Under certain conditions, however, fine-grained soils will continue to compress under constant load for many years. This long-term compression is called "consolidation" settlement and is caused by the squeezing out of water from the pores in the clay. Differential settlement occurs for a number of reasons: local variations in soil compressibility, variation in thickness of compressible soil, differences in footing sizes and pressures, variation in applied loads, overlapping stresses, differences in depth of embedment of footings. A classic example of consolidation settlement is occurring under the Empress Hotel in Victoria, B.C. This building is founded on 50-foot piles which rest on gravel at one end but penetrate only to the middle of a compressible clay layer at the other. Although the maximum settlement at the deep clay end is more than 30 inches, the damage is not especially serious because the building has tended to tilt on a plane. Fortunately, level observations have been taken annually since 1912, shortly after the Empress Hotel was built, so that it has been possible to reconstruct the loading and settlement history from the beginning of construction. This shows that settlement occurred rapidly during the first five years and continues slowly 65 years after construction. If foundation loads vary, the differential settlement can be more serious even when sub-soils are relatively uniform. The National Museum Building in Ottawa is an example of such a situation. This massive structure has a complex footing system on two levels, with bearing pressures varying from less than 1 ton per sq ft to more than 4 tons per sq ft. The differential settlement became so serious five years after the building was opened that the tower structure over the main entrance had to be removed in 1915 to prevent collapse. The estimated total settlement varied from zero where the bearing pressures are small to 1.6 feet under the tower. A few years ago a section of original flooring was removed, revealing that as much as ½ foot of settlement had occurred during construction so that the total differential settlement at footing level is probably more than 2 feet. This has caused considerable damage to some interior partitions, but owing to the nature of the framing the basic structure is sound. Another example will illustrate how modern foundation practice permits successful building on even very poor subsoils. An industrial plant on the south shore of the St. Lawrence River at Varennes, 20 miles downstream from Montreal, rests on a 2.5-foot thick reinforced concrete mat over 100 feet of compressible clay. The mat foundation was integrated with pile foundations for special machinery. Mat pressure varies from 700 to 1,700 psf over the 100- by 300-ft building area. Settlement varies from more than 6 inches under the heavily loaded area to about 2 inches under the lightly loaded area. Most of the settlement occurred during the one-year construction period ending in 1957, but another 2 inches occurred in the six years following construction. This degree of differential settlement was perfectly acceptable to the owners on an economic basis and an extension begun in 1961 was built in the same way. The buildings were designed with sufficient flexibility to allow the differential settlement to pass virtually unnoticed. A second example from the same general soil area has not turned out so happily. In this case a large warehouse connected to a special facility on piles was founded on a 3-foot sand fill over compressible clay. The designers did not realize that the sand fill added more load to the ground than did the structure. The result has been more than 1 ½ feet of unanticipated differential settlement, accompanied by considerable damage and operating difficulties. In most areas of Canada the differential settlement of an important building can be limited to a fraction of an inch. The 26-storey CN Tower in Edmonton is a good example of the foundation performance that can be expected when modern technology is applied. This reinforced concrete structure, founded on spread footings resting on a sandy or silty clay till at depths of 22 to 26 feet, had a maximum settlement after 6 years of just over 1 inch, with a differential of less than ½ inch; 80 per cent of the settlement occurred during construction. The risk of detrimental settlements appears to be much greater in more modest structures such as 10- or 15-storey apartment blocks where owners may often be inclined to skimp on foundation investigation and design. Types of Settlement There are three basic types of settlement: uniform settlement, tilt, and non-uniform settlement (Figure 2). Uniform settlement and tilt (within reason) do not greatly affect a structure, but resulting movements may cause serious problems with services and appendages such as water mains and connecting tunnels. Non-uniform settlement is characterized by angular distortion and may cause cracks or even structural failure. The degree of angular distortion is indicated by the ratio of differential settlement to distance between supports, /L. Laboratory tests and field experience have given reasonable correlations between angular distortion and damage for various kinds of construction ranging from 1/750, where difficulties with sensitive machinery may occur, to 1/150 where structural damage is to be feared. Figure 2.  Types of settlement. The amount of settlement that a building can tolerate, the "allowable" settlement, depends on its size, type and intended use. The allowable settlement for the plant at Varennes, for example, would not be permitted in a city hall. For practical reasons, the amount of settlement that would be tolerated under the difficult conditions of Mexico City is greater than would be tolerated in any Canadian city. Swelling and Shrinking Soils So far the discussion has been confined to foundation movements caused by compression of subsoil due to loading. Large movements can also occur from shrinking or swelling of clay subsoil resulting from stresses unrelated to foundation pressure. Fine grained clay soils may be subjected to extremely high stresses due to air drying or vegetation. Shrinkage may take place throughout the full depth of rooting and the depth of the active layer depends on both climatic conditions and vegetation. Drought-resistant vegetation growing in semi-arid climates may have roots extending deeper than 20 feet. Because tree roots are extremely efficient in extracting soil moisture, both the depth and rate of shrinkage can be greatly accelerated in soils supporting such vegetation. Some soils will swell back to their original volume when they are rewetted. The movement of a footing up and down, therefore, depends on the condition of the soil at the time of construction and on subsequent wetting and drying. Usually the worst heaving conditions develop in soils that have been previously desiccated by heavy vegetation or extreme aridity and then subjected to greatly increased moisture conditions as a result of construction and irrigation practices. Although shrinking and swelling usually affect only shallow, lightly loaded footings they can cause considerable damage even to large buildings. Swelling soils are common on the Prairies. In Winnipeg, an extension to a church was built over an area in which the trees were cut down immediately before construction. The floor slab for the addition, which rested directly on the ground, was heaved 6 inches in two years, giving an annoying discontinuity between the old and new sections. Although the main structure was supported on piles, there was severe damage to partitions and finish in the basement, with some distortion transmitted to the superstructure above grade. It is common in many of the heavy clay regions of Canada for shallow foundations to move up or down by several inches. In these regions the problem can be overcome by designing a rigid structure to reduce differential movement or by providing a deeper foundation (usually short piles) to carry the structure and allowing space under floors for soil movements. Other Causes There are several other causes of foundation movement worthy of mention. Freezing of the ground is a problem in most parts of Canada. When fine-grained soils such as silts and clays or even dirty sands and gravels freeze, water is drawn up from the water table to freeze into discrete lenses and cause a volume expansion. Usually the normal winter heating keeps frost away from footings, but the possibility of frost heaving under attached garages or depressed driveways is often forgotten and results in considerable damage to light structures. Frost action is also a hazard during construction. A large building under construction in Ottawa was badly damaged during a cold winter period. Basement floor slabs were heaved and cracked and the movements extended up through the structure, causing distortion and cracks in partition walls. In Northern Canada there are many cases of serious distortions of heated buildings due to the thawing of ice in the permafrost under them. Even bedrock is not always reliable. The basement floor of a downtown building in Ottawa began to heave mysteriously several years after construction. Research revealed that the pyrite in the shale bedrock was being converted by oxidation and bacterial action to gypsum and other sulphate materials to cause swelling. Parts of the floor heaved as much as 4 inches in five or six years, although only minor structural distortion was caused before the swelling was arrested by chemical treatment. It may be concluded from this review that foundation movements always occur. The important point to remember is that foundation performance can be satisfactorily predicted. For simple foundations on good sites this can be done with a modest expenditure. At poor sites, or where foundations are complex, an extensive and possibly expensive investigation is required. The designer has to compromise between his desire for zero movement and the owner's desire for the cheapest foundation. On the one hand it may be advantageous to accept rather large differential movements and to design joints to accommodate them. On the other hand he may be able to demand a foundation sufficiently stable to allow deflections to be ignored for purposes of joint design, permitting joints provided for other purposes to accommodate small deflections due to foundation movement. Because of the interaction of the soil and the structure, the advantages of having the foundation engineer and structural engineer work together are very clear. Originally published April 1972.