Authors: Robert D. Krebs, geotechnical engineer, and Carl E. Zipper, soils researcher, Virginia Center for Coal and Energy Research, Virginia Tech
Publication Number 460-115, posted January 1997
Ground conditions in coal-mining regions require special attention when residential development is considered. Today, the large scale of surface mining operations makes it practical to alter landforms through mining and reclamation, thus increasing land suitability for housing and related development. Such "made land," however, is typically composed of deep soil and rock fills. Such fills undergo long-term settlement under their own heavy weight. Subsurface mining operations can also affect surface structures by causing land subsidence.
This bulletin summarizes current understanding of problems associated with housing built on recontoured, reclaimed land (see references 4, 5). Special attention is given to the behavior of valley fill and to precautions that should be taken when building in reclaimed areas. Various types of foundations for light structures are considered in relation to their suitability for land that may settle.
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Two basic types of settlement occur; both result, primarily, from soil particle and rock grain crushing at points of contact. Creep settlement refers to the routine settlement of loose earth materials over time, as these materials consolidate under forces imposed by the weight of materials above. Water saturation reduces rock strength and increases contact crushing, causing collapse settlement, known, technically, as hydroconsolidation. because of its association with fill saturation by rising groundwater or percolating surface water. Fill placed in a relatively dry state, as is often the case with shaley or rocky mine spoils, induces post-construction settlement likely to consist less of the "creep" and more of the "collapse" variety. Conversely, fill placed wet will undergo considerable "creep," but less "collapse." Thus, for a particular fill, the amount, type, and even the rate of settlement will depend on a variety of factors, including fill depth, moisture and compaction conditions during placement, and groundwater conditions after placement.
Although the amount and rate of creep settlement within a deep fill is difficult to predict without careful soil testing, geotechnical engineers have formulated a few general rules. First, the amount of settlement varies directly with fill depth, so that a 100-foot fill will settle twice as much as a 50-foot fill constructed with similar materials under similar conditions. Second, the rate of creep settlement decreases "exponentially" with time; for example, if it takes one year for one inch of settlement, it will take on the order of ten years for a second inch and 100 for a third. Clearly, early settlement is rapid, but the process slows quickly. A third rule is that the settlement amount varies greatly with the degree of compaction during placement. For example, if a deep fill with little compaction were expected to undergo 10 inches of settlement, one would expect this same fill, well compacted, to settle only one or two inches. Partial compaction would give results between these extremes. Obviously, one would like to restrict housing to well-compacted, stable fills. These are called "engineered" fills because they are carefully constructed to stringent engineering specifications.
Specifications can be written to build engineered fills, but it is often difficult to achieve these specifications in the field. Running a twenty-ton vibratory roller at least six times over a one-foot layer ("lift") of soil gives good compaction, but this will be costly in terms of time, fuel, and equipment. On surface mining operations, one-foot lifts will not generally be economically feasible. Compaction can be achieved economically, however, through the action of haul trucks and other machinery. Four-foot lifts give only partial compaction, which may lead to two or three times as much settlement as "good compaction." With two-foot lifts, the nature of the material, vehicle weights and speeds, moisture content, and other variables determine whether or not compaction is adequate to minimize settlement.
The fact that creep settlement of deep fills can be held to an inch or two using compaction sounds hopeful. Light structures can easily tolerate this, especially if much of the settlement is "uniform" rather than "differential" across the length of the foundation. Extensive damage has occurred in expensive homes built on carefully engineered valley fill when "hydroconsolidation" caused severe settlement soon after construction. In many of these cases the settlement was induced by rising groundwater within the fill, but there are also cases where houses that had performed well for several years suddenly settled because of percolating water from large plumbing leaks or sudden concentrations of rainwater. Engineers are not entirely surprised by this; they are familiar with the fact that crests of large earth and rock-fill dams, carefully engineered and well compacted to hold back large volumes of water, commonly settle several inches with the first filling of the reservoir. Clearly, for deep valley fills, "collapse settlement" or "hydro- consolidation" must be expected.
The results of field observations and laboratory work suggest a few general rules for hydroconsolidation. If compaction is good, only deep fills will settle; hydroconsolidation is unlikely to occur in fills less than twenty feet deep. Also, it is unlikely to occur in fills that were placed very wet or became saturated as they were built. But, for deep soil and rock fills placed at optimum moisture content (the moisture content at which compaction is most effective), hydroconsolidation may be both substantial and rapid even when the fills have been adequately compacted during construction.
There is no easy solution for hydroconsolidation in deep fills. Surface drainage combined with rock underdrain systems may intercept and control rainwater and groundwater, but deep fills generally become wet anyway, no matter how well drained. Thus, hydroconsolidation must be anticipated. Indeed, the process may be accelerated by water brought by the development itself: for instance, effluent from septic tanks; runoff from gutters, leaders, and paved areas; and flow from plumbing leaks, common in areas of unstable ground. Such water may accelerate the softening of point-to-point rock contacts and collapse settlement of the fill. On the positive side, settlement from hydroconsolidation is short-term. It ceases in a short period of time so that once a fill has become saturated by groundwater, for example, further settlement is unlikely to occur. Thus, old fills can be expected to be relatively stable.
To summarize: Fills in reclaimed mined land can be placed to minimize post-construction creep settlement; but this requires engineered fill with good compaction, which is uneconomical in most surface mining situations. Even if good compaction is achieved, settlement from another mechanism, hydroconsolidation, may cause unacceptable surface settlement as the deep, heavily-loaded fill becomes wet from groundwater or percolating surface water. Therefore, reclaimed mined lands containing deep fills must be considered as unstable ground, subject to settlement, except where the fills are old and it is reasonably certain that they have already "aged" and settled. Accordingly, one should keep large settlements of fill in mind when designing residential and other light structures to be placed on recently-reclaimed mined land.
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Structural distortion from differential settlement is often described in terms of angular distortion. If the downward movement of a portion of a building is not shared by the rest, angular distortion shows up as "sag" or "hog" in joists or beams and "lean" or "twist" in studs or columns. As a result, walls are distorted away from a strictly rectangular shape, cracks appear in plaster and masonry (Figure 2), door and window openings become distorted and, ultimately, the integrity of the structure can be threatened. Damaging distortion also occurs as simple lateral strain in which the horizontal distance between a building's corners changes, generally increasing, leading to the opening of joints and cracks. Lateral strain is also associated with settlement, especially deep-seated settlement, and may comprise a significant part of the total structural distortion.
Damage from structural distortion may range from merely cosmetic to severe, depending on the amount of distortion and the type of structure. Many older homes have uneven floors, door frames out of plumb, and cracks in plaster walls; yet, they remain perfectly livable despite the cosmetic damage. However, when distortion prevents doors and windows from properly closing, when masonry walls develop severe cracks and fractures so that maintaining comfortable temperatures within the home becomes a problem, or when the supporting power of load-bearing walls is compromised, damage is serious. Severe structural distortion can pull apart plumbing connections, crack chimney flues, and stress electrical connections, leading to fire hazard. Generally speaking, one inch of lateral strain or two inches of differential settlement, or some of each, leads to the beginnings of severe damage in a home.
Some structures tolerate distortion better than others. Most homes can absorb slight amounts of distortion with a minimum of damage, but the more rigid a structure or structural component, the less its tolerance. Plaster, for example, can tolerate little distortion without cracking. Stud frames, panelling, and gypsum board can resist distortion better than brick or masonry block. As shown in Table 1, frame construction, open floor plans, and truss roofs can be expected to have a high tolerance to differential settlement. Also, mobile and manufactured homes are constructed to withstand distortions beyond those expected for conventional on-site construction, since they must tolerate unusual strains during transportation and placement. Nevertheless, even these homes, if rigidly anchored to settling ground, can be severely damaged.
Table 1: Building structural systems as they tolerate differential settlement. Rigid structures are least able to tolerate structural distortion; split structures are most tolerant. (Adapted from reference 2.)
| Structure | Description
| |
| Rigid | Precast concrete, concrete block, or unreinforced
brick exterior walls; masonry or plaster interior
walls; slab-on-grade acts as combined flooring and
building support.
| |
| Semirigid | Reinforced masonry or brick reinforced with steel
tie bars exterior; window and door openings
reinforced to resist angular distortion;
slab-on-grade isolated from walls.
| |
| Flexible | Steel or wood framing; exterior siding of brick
veneer with articulated joints, or panels of
metal, vinyl, or wood; interior walls of gypsum
board or wood-base panels; vertically-oriented
construction joints; strip windows or metal panels
separating rigid wall sections with 25 foot
spacing or less, to allow differential movement;
all water pipes and drains into structure with
flexible connections; suspended floor or slab
isolated from wall.
| |
| Split | Walls or rectangular sections move as units (flexible joints at 25-foot spacing or less between units); suspended floor or slab-on-grade isolated from walls (probable cracking of slab accounted for in design); all water pipes and drains into structure with flexible connections. |
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Horizontal movements of as much as 50% of the maximum vertical movement may occur near the sides of a valley fill. Also, maximum differential movement and structural distortion should be expected closer to the sides than to the centerline of a filled valley. In contrast, little horizontal movement, but maximum vertical movement, can be expected near the fill centerline. Accordingly, for structures on valley fill, one might expect the least damage in a narrow house near the middle of the fill, with the long axis of the structure parallel to and directly above the former valley's base, and the greatest damage to houses near the outer edges of the fill, above the former valley's sideslopes, where the greatest surface distortion and horizontal movement can be expected.
If one chooses to build over valley fill, site development and building construction should follow guidelines established by the National Bureau of Standards for housing in mine subsidence areas (see reference 7). Buildings should be only one or, at most, two stories in height and should not have a basements. The plan dimension should not exceed 80 ft. in length and the first floor should be on one plane only. The long axis of the building should be parallel to the contour of the original land surface beneath the fill. Separate buildings should be at least six feet apart, with breezeways and link structures less than 12 feet in width. Flexible structural systems and elements should be employed, with provision for added clearance in windows and doors to increase tolerance to distortion. For utility connections, flexible joints that can accommodate both rotation and translation should be used.
In general, a valley fill will not constitute an ideal housing site. If one has the choice of placing a building over a valley fill or a reclaimed level bench area, the level bench area will be a better choice. The total depth of fill, the vertical settlement, and the associated horizontal movements are all likely to be less over the bench area.
In geotechnical terms, some of the most favorable mined land construction sites are located on the level benches produced by the "shoot and shove" mining of the 1950s, 1960s, and early 1970s. However, strong precautions are in order for anyone choosing to build on these sites (Figure 3).
The favorable qualities of the older benches come from the fact that depth of fill to solid rock is, in many cases, less than 20 feet, while the filled material has had in excess of ten years to settle. Construction difficulties can result from the presence of a filled "outslope" (steeply-sloping area below the mining bench filled with spoil) adjacent to the housing site. The structure must be firmly and totally supported by the solid bench, and water outflow from the home must be diverted away from the outslope. Just as water can induce collapse failure of recently-filled lands, the introduction of large quantities of water to an apparently stable outslope can cause movement of the outslope materials. Water-induced slides can result from severe storm events as well as from the activities of a nearby homeowner. If such movement is severe, material that is vertically above the outer edge of the bench area can be within the slide zone. Thus, if housing is to be constructed on an older bench, it is critical to keep the home, and any water outflow from the home, away from steep outslope areas.
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The purpose of a foundation is to provide stable support for the structure, and to anchor that structure to resist the force of winds, including uplift. Because cost is a major element of concern to most homebuilders, economy is essential both in site preparation and in substructure (foundation) construction. There are three basic approaches to substructure design for unstable ground:
For filled areas of any substantial depth, it is not economically practical to carry the foundation to firm ground except for expensive, heavy structures such as multistory buildings. This suggests the second approach, which provides a substructure that is strong, stiff, and resistant to distortion, with a superstructure that is relatively flexible so as to allow for large movements without serious damage. This basic approach is commonly applied to areas of expansive soils, where large movements occur upon expansion of swelling clay as it becomes moist. The third approach, flexible or adjustable connections, has been far less well researched and developed. The design and use of adjustable connections has been employed in areas of permafrost soils, compressible fills, and subsidence from underground mining.
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The perimeter wall foundation is economical both in materials and in construction, but it is poorly suited to unstable ground. There is a great potential for damage from uneven movement in the subsoil. Fractures may appear in walls after only minimal distortion, or if differential movement exceeds 0.5 inch. Moreover, in the event of fracture, repair or adjustment is difficult. This foundation meets none of the criteria for success on unstable ground; that is, it lacks strength and stiffness and does not allow for flexible or adjustable connections with the superstructure. Field studies have shown that homes with basements are especially prone to expensive damage in areas of severe settlement. Accordingly, the elimination of basements from construction in settlement-prone areas is strongly recommended.
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The philosophy for stiffened slabs is that design must allow for the possibility of only partial ground support for the slab, at times along the edges and at other times in the interior. The slab must be able to resist the resulting stresses without excessive deformation. The Department of the Army Technical Manual Foundations in Expansive Soils (see reference 2) is close to the current state of the art, and in many places successful local practice has been established, including design innovations that economize on use of reinforcing steel and concrete.
Despite such innovations, stiffened slabs cannot be recommended at this time as the answer to problems of residential construction on filled lands. Because of the expense of conventional stiffened-slab technologies, they are best suited to high-cost homes in areas where land is costly. They are difficult to repair, so if difficulties do arise, repair can be justified only for high-cost homes. Stiffened slab design has been directed toward the problem of heaving soils; their effectiveness under settlement conditions has not been investigated or demonstrated.
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In contrast to perimeter wall footings or slab foundations, piers tend to concentrate the structural load. Thus, soil with a relatively high bearing capacity (such as those soils commonly produced by consolidated mine spoils) must be at hand. Alternatively, piers may be built deep to reach firm bearing and mobilize shaft friction, but cost increases quickly with pier depth. Piers deeper than eight feet below grade are seldom practical for residential structures. Except in areas of settlement, piers have no particular advantage over the more common perimeter wall foundation.
A major advantage of pier foundations in areas of settlement lies in the accessibility of the foundation elements. If the pier undergoes settlement, an appropriately designed foundation- superstructure connection can be adjusted. Normally, this "design for adjustment" is not the case. Foundations having built-in provision for releveling are called adjustable foundations and are discussed below.
An important cost element in construction using pier or post-and-pad foundations is the need to provide support members (generally beams or girders) spanning the distance between piers.
In the case of perimeter wall foundations, the walls serve this purpose around the exterior of the structure. Interior beams do not have to support large loads; for one and two-story houses they usually consist of economical built-up wooden beams supported by columns at 8 to 12 foot intervals. Perimeter beams, however, must be designed for the larger loads imposed by the roof and exterior walls. If this requires steel girders, material and material handling costs greatly increase. Thus, if pier foundations are to be used, economy may dictate single-story frame construction with lightweight veneer such as aluminum siding. For settlement-prone areas, an additional benefit of such construction is flexibility.
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A five-story precast concrete building in Pennsylvania was designed by GAI Consultants, Inc., using an adjustable pier foundation on deep engineered fill (see reference 3). The total settlements accommodated were as much as 8 inches with up to 6 inches differential settlement between columns. The engineer provided approximately six to eight inches of adjustment capability into each column by providing four permanent, adjustable column bolts near its base. The columns were then adjusted upward with removable hydraulic flat jacks as needed to prevent excessive differential settlement of the structure. Unfortunately, this innovative design is too expensive for modest structures such as single-family residences but it does illustrate the principles of successful design against large amounts of settlement in unstable ground.
Begley, Gray, and Zickefoose (see reference 1) have presented a detailed design for a flexible, single-story residence using pier foundations and an expansion joint system (Figure 4).Basically, the house rests on two 30' by 30' stiff-frame platforms supported by piers symmetrically placed at 15 foot intervals. A flexible-joint system allows for considerable horizontal and vertical movement between sections. The pier-frame connections allow for loosening the anchor bolts and adding or removing shims, as required. Three inches of metal shims of varying thicknesses are built-in for this purpose, and a six-inch rubber bearing pad above the shims provides additional flexibility and helps accommodate horizontal movements. Wide-flange steel beams (18 inch) are recommended to provide the stiff frame and bridge between piers. This pier-frame system adds nearly 10% to the cost of the structure over more conventional construction, but the design clearly shows that it is practical to design flexible residential structures on adjustable foundations.
It is possible that elements of both the GAI adjustable-bolt design and the Begley pier-frame design can be combined, with some additional innovation, to provide an economical, easily adjusted foundation system. A method of adjustment could be to jack the beam above a settled pier back to level, and to wedge shims between the pier cap and beam. If the pier cap-beam connections, which must provide anchorage against both uplift and lateral forces as well as support from below, are designed to allow shimming, the procedure should be relatively inexpensive. In reclaimed mine areas, the structure must also be able to respond to potential horizontal movements of the supporting piers. Such ability might be obtained by using slotted plates at the pier-beam connections. Additional flexibility might be built into the pier cap-beam connection by incorporating rubberized items, such as heavy-machinery engine mounts, into the connector design. Construction economies might be obtained by spanning piers with pressure-treated laminated wood beams rather than steel. Another design innovation would be to develop exterior wall sections with adequate stiffness to be able to span between the posts without receiving support from below.
An important element of any adjustable foundation structural design will be to incorporate a mechanism to accomodate periodic checks of the structure's bearing and consequent need for re-leveling. Conceivably, in areas where delivered concrete costs are high, a well-designed and executed adjustable pier-and-post foundation could be less expensive than a perimeter wall foundation due to reduced use of concrete.
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Although the mechanisms by which subsidence damages buildings are, in many cases, more complex, dramatic, and intense than those mechanisms associated with filled lands, settlement is an element common to both situations. Housing in subsidence-prone areas must be able to tolerate some settlement and distortion without becoming unliveable. The ability to make periodic adjustments in a home's foundation support during a subsidence episode would help the home survive intact.
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Two major types of settlement occur in deep fills. Creep settlement occurs routinely, due to the sheer weight of loose earth materials; the rate of creep settlement decreases with time. Hydroconsolidation is settlement that is induced by water saturation of the fill materials. If the fill becomes saturated, this type of settlement occurs rapidly. Once creep settlement has slowed to an acceptable rate and hydroconsolidation has occurred, filled land may be treated as essentially stable ground.
Recently constructed, deep fills, however, will be prone to settlement for many years and are generally unsuited for the types of foundations commonly used to support housing. This study suggests adjustable foundations as one means for adapting housing to settlement prone sites. Currently, adjustable foundations are not employed to any considerable extent in Appalachian areas. One cannot purchase ready-made adjustable foundation pier connectors from local sources. However, adjustable foundation elements could be fabricated from commonly-available materials such as steel plates and heavy threaded rods, using equipment such as the torch, welder, and heavy drill. Ultimately, in years to come, innovative design by local residents and industries could lead to housing systems adapted to the unique conditions of settlement-prone sites in the Appalachian coalfields.
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Appropriate engineering, based upon an understanding of the geotechnical properties of deep fills, can lead to the successful utilization of reclaimed mined lands for building construction and development. Anyone considering construction on fill materials, or in areas over mineable coal reserves, should consult a professional engineer who has a geotechnical background.
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