Engineering and Construction

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Wood-plastic composite (WPCs) products are made from wood lumbers and  plastics of various types, mixed with  other ingredients, such as ultraviolet stabilizers, pigments, lubricants,  and biocides, which are then heated  and pressed, extruded, or injection-molded into final form. In comparison to their solid wood counterparts,

WPC materials offer more consistent  material quality, freedom from defects and distortion, and, depending  on their formulation, superior resistance to moisture. They are used  most notably for exterior decking,  as well as for exterior railing systems  and finish trim, both interior and  exterior. Like structural composites,  WPC products make productive use  of rapidly renewable or waste materials. Some WPCs also have high recycled materials content.

WPC decking, most commonly  made from blends of polyethylene  or polypropylene and wood lumber,  is  available in sizes matching conven-tional solid wood decking in lengths  up to 20 feet (6.1 m). It may be fastened with corrosion-resistant nails or  screws, or with concealed hardware  that engages the edges of the boards.

A variety of maintenance-free colors  and textured finishes are available,  some remarkably similar in appearance to genuine hardwoods.

Ingredients and manufacturing processes used in the making of  composite wood trim vary widely, as do  the workability, surface qualities, and  durability of the finished  products.

Blends of plastic and wood similar  to those used for the manufacture  of composite decking may be used.

Alternatively, formulations with a  higher proportion of wood lumberÑ more similar in composition to traditional wood panel products, such as plywood, OSB, and fiberboard,  discussed later in this part may be  used. Products may be preÞ  nished, factory primed, encapsulated within a dense plastic outer shell, or coated with a resin-impregnated paper that  improves the quality of field-applied finishes.

In comparison to solid lumber, nonstructural composite lumber expands and contracts more with changes in temperature, so greater allowance for thermal movement must be made during installation.

And in the case of spanning members such as decking, the lesser stiffness of composites necessitates closer spacing of joists or other members on which the members are supported.

Wood trim made up from shorter lengths of finger-jointed  and  glued solid wood material is also available as an alternative to conventional finish lumber. In comparison to solid pieces, finger-joint stock is more stable and more consistently free of defects. It also makes use of short-length scraps that might otherwise be treated as waste.

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Structural composite lumber, also called engineered lumber, products are substitutes for solid lumber made from wood veneers or wood fiber  strands and glue.  Laminated strand lum-ber (LSL) and  oriented strand lumber (OSL) are made from shredded wood strands, coated with adhesive, pressed into a rectangular cross section, and cured under heat and pressure (the wood strands used in the manufacture of LSL are longer than those used in OSL). LSL and OSL are the least strong and least expensive of the composite lumber products. They are used mainly for rim boards and short-span headers.  Laminated veneer lumber (LVL) is made from thin wood veneer sheets, as wide as the member is deep, that are glued and laminated into thicker members. LVL is similar in appearance to plywood except without crossbands (Figure 3.26).

Figure 3.26 An LVL beam, made of veneers similar to those used in the manufacture of plywood, resting on its side in preparation for installation on a concrete foundation wall. When placed in its fi nal position, the beam will be rotated so that the laminations are oriented vertically in the beam. Also visible in the photograph are a preservative- treated wood sill (left) and an LSL rim board (right), both of which are discussed later

Parallel strand lumber (PSL) is made from long, thin strips of wood veneer glued and pressed in a process similar to that for LSL and OSL, but with the veneer strips arranged more uniformly parallel than the strands in those other products. PSL is the heaviest, strongest, and most expensive of the composite lumber products (Figure 3.27). LVL and PSL are most commonly used for longer-span headers and oor beams.

Figure 3.27 A view of corner framing for a  residential garage. The lower beam is a PSL. A high-strength member such as this  is needed to span the roughly 9-foot (2.7-m)-wide garage opening while  supporting the fl oor load above. There are two solid wood top plates above  the PSL. Above the top plates is an LSL rim board that encloses and steadies  the fl oor framing behind it. This lower-strength member is adequate for  this nonstructural function. Several manufactured wood products discussed  later in this chapter are also in view: Above the rim board, the laminated edge  of the plywood subfl oor is visible, and to the left of the rim board, an I-joist is  exposed where the fl oor framing projects beyond the plane of the wall below.

Studs and posts may also be made of any of the composite lumber types described in the previous paragraph, as well as from structural finger-jointedlumber in which short lengths of solid  lumber scrap are finger-jointed  and glued end-to-end into longer lengths.

Composite studs and posts are well  suited for use in the framing of walls that are especially tall or that have very large openings, or wherever else long, straight, and especially strong, stiff members are needed.

Structural composite lumber products make productive use of wood materials that are rapidly renewable or that might otherwise be treated as waste, and they offer many of the benefits of glue-laminated wood: dimensional stability, structural strength up to three times that of con-ventional solid material, availability of large sizes and long lengths, and consistent quality. Particularly where these products are used in exposed interior applications, however, attention should also be paid to the types of adhesives and binders used in the products and the potential for offgassing of volatile organic compounds or formaldehyde.

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Lumber sizes in the United States are given as nominal dimensions in inches,  such as 1 x 2 (one by two), 2 x 10, and so on. At one time, sawn lumber may have approached these actual dimensions. Today however, subsequent to sawing, seasoning, and surfacing,  true sizes are less. By the time a kilndried 2 10 reaches the lumberyard, its actual dimensions are close to 1½ by 9¼ inches (38 by 235 mm). The relationship between nominal lumber  dimensions (which are always written without inch marks) and actual dimensions (which are written with inch marks) is given in simplified form in Figure 3.22 and in more complete  form in Figure 3.23. Anyone who designs or constructs wooden buildings soon commits the simpler of these  relationships to memory. Because of  changing moisture content and manufacturing tolerances, however, it is  never wise to assume that a piece of  lumber will conform precisely to its  expected dimensions. Wood members vary in size seasonally with changes in  humidity and temperature. In hot, dry  locations such as attics, wood framing  may shrink to dimensions substantially below its original measurements.

Figure 3.22 The relationship between nominal and actual dimensions for the most common sizes of kiln-dried lumber is given in this simplifi ed chart, which is extracted from the complete chart in Figure 3.23.

Figure 3.23 A complete chart of nominal and actual dimensions for both framing lumber and fi  nish lumber.

Members in older buildings may have been manufactured to full nominal  dimensions or to earlier standards of  actual dimensions such as 15/ 8 inches or 1¾ inches (41 or 44 mm) for a  nominal 2-inch member.

Pieces of lumber less than 2 inches in nominal thickness (38 mm  actual thickness) are called  boards.
Pieces ranging from 2 to 4 inches  in nominal thickness (38 to 89 mm  actual thickness) are referred to collectively as  dimension lumber. Pieces  nominally 5 inches (actual size 114  mm) and more in thickness are  termed timbers.  Dimension lumber is usually supplied in 2-foot (610-mm) increments  of length. The most commonly used  lengths are 8, 10, 12, 14, and 16 feet  (2.44, 3.05, 3.66, 4.27, and 4.88 m),  but retailers frequently stock rafter  material in lengths to 24 feet (7.32 m).

Actual lengths are usually a fraction of  an inch longer than nominal lengths.

Lumber in the United States is  priced by the  board foot. Board foot measurement is based on nominal  dimensions, not actual dimensions.

A board foot of lumber is deÞ  ned as  a solid volume 12 square inches in  nominal cross-sectional area and 1  foot long. A 1 x 12 or 2 x 6 10 feet long contains 10 board feet. A 2 x  4 10 feet long contains [(2 x 4)/12] 10 6.67 board feet, and so on. Prices of dimension lumber and timbers  in the United States are usually quoted on the basis of dollars per thou- sand board feet. In other parts of the world, lumber is sold by the cubic meter. Because the board foot is based on lumber nominal dimensions and  metric sizes use actual dimensions,  there is no direct conversion between  board feet and cubic meters.

The architect and engineer specify lumber for a particular construction use by designating its species, grade,
seasoning, surfacing, nominal size, and chemical treatment, if any. When ordering lumber, the contractor must additionally give the required lengths of pieces and the required number of pieces of each length.

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The strength of a piece of wood depends chießy on its species, its grade, and the direction in which the load acts with respect to its grain.

For example, in both tension and compression, wood is typically several times stronger parallel to grain  than perpendicular to grain. With its usual assortment of defects, it is  stronger in compression than in tension. Allowable strengths (structural  stresses that include factors of safety) vary tremendously with species and  grade. For example, allowable com- pressive strength parallel to grain  for commercially available grades  and species of framing lumber varies  from 325 to 1700 psi (2.24 to 11.71 MPa), a difference of more than five times. Figure 3.21 compares the average structural properties of framing lumber to those of some other  common structural materialsÑbrick  masonry, steel, and concrete. Of the  four materials, only wood and steel  have useful tensile strength. Defect- free wood is comparable to steel on  a strength-per-unit weight basis, but  with the ordinary run of defects, an  average piece of lumber is somewhat  inferior to steel by this yardstick.

Figure 3.21 Comparative physical properties of four common structural materials: wood (shaded row), brick masonry, steel, and concrete. Wood has signifi cant strength in both tension and compression. The ranges of values of wood strength and stiffness refl ect differences among species and grades of lumber.

When designing a wooden struc- ture, the architect or engineer determines the maximum stresses that are  likely to occur in each of the structural  members and selects an appropriate  species and grade of lumber for each.

In a given locale, a limited number of  species and grades are usually avail- able in retail lumberyards, and it is  from these that the selection is made.

It is common practice to use a stronger  but more expensive species (Douglas  for or Southern pine, for example) for  highly stressed major members and  to use a weaker, less expensive species  (such as Eastern hemlock) or species  group for the remainder of the structure.  Within each species, the designer se- lects grades based on published tables of allowable stresses. The higher the  structural grade, the higher the allowable stress. But the lower the structural  grade, the less costly is the lumber.

There are many factors other  than species and grade that inß  uence  the useful strength of wood. These include the length of time the wood will  be subjected to its maximum load, the  temperature and moisture conditions  under which it serves, and the size  and shape of the piece. Certain fire retardant treatments also reduce the  strength of wood slightly. All these factors are taken into account when engineering a building structure of wood.

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Because of the public safety considerations that are involved, building codes contain numerous provisions relating to the design and construction of excavations and foundations. The IBC defi nes which soil types are considered satisfactory for bearing the weight of buildings and establishes a set of requirements for subsurface exploration, soil testing, and submission of soil reports to the local building inspector. It goes on to specify the methods of engineering design that may be used for the foundations. It sets forth maximum loadbearing values for soils that may be assumed in the absence of detailed test procedures (Figure 2.5). It establishes minimum dimensions for footings, caissons, piles, and foundation walls and contains lengthy discussions relating to the installation of piles and caissons and the drainage and waterproofi  ng  of substructures. The IBC also requires engineering design of retaining walls.

In all, the building code attempts to ensure that every building will rest upon secure foundations and a dry substructure.

Figure 2.5                                         Presumptive surface bearing values of various soil types,                                                                 from  symbols  the 2006 IBC. Classes 3, 4, and 5 refer to the soil group in Figure 2.2.

Figure 2.2 The Unifi ed Soil Classifi cation System, from ASTM D 2487. The group symbols are a universal set of abbreviations for soil types, as seen for example, in Figure 2.8.

Figure 2.8 A typical log from a soil test boring indicating the type of soil in each stratum and the depth in feet at which it was found. The abbreviations in parentheses refer to the Unifi ed Soil Classifi cation System, and are explained in Figure 2.2.

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It is a good idea to begin the design of the foundations of a building at  the same time as architectural design  work commences. Subsurface conditions beneath a site can strongly influence fundamental decisions about  a building—its location on the site,  its size and shape, its weight, and  the required degree of flexibility of  its construction. On a large building project, at least three designers  are involved in these decisions: the  architect, who has primary responsibility for the location and form of  the building; the structural engineer,  who has primary responsibility for its  physical integrity; and the foundation  engineer, who must decide, on the basis of site exploration and laboratory reports, how best to support it  in the earth. More often than not,  it is possible for the foundation engineer to design foundations for a  building design dictated entirely by  the architect. In some cases, however, the cost of the foundations may  consume a much larger share of the  construction budget than the architect has anticipated, unless certain compromises can be reached on the  form and location of the building. It  is safer and more productive for the  architect to work with the foundation  engineer from the outset, seeking al- ternative site locations and building configurations that will result in the fewest foundation problems and the  lowest foundation cost.

In designing a foundation, a  number of different design thresh-olds need to be kept in mind. If the  designer crosses any of these thresholds, foundation costs take a sudden  jump. Some of these thresholds are:

•  Building below the water table. If the substructure and foundations of a building are above the water table, minimal effort will be required to keep the excavation dry during construction. If the water table is penetrated, even by an inch, expensive steps will have to be taken to dewater the site, strengthen the slope support system, waterproof the foundation, and either strengthen the basement floor slab against hydrostatic uplift pressure or provide for adequate drainage to relieve this pressure. For an extra inch or foot of depth, the expense would probably not be justified; for another story or two of useful building space, it might be.

•  Building close to an existing structure. If the excavation can be kept well away from adjacent structures, the foundations of these structures can remain undisturbed and no effort and expense are required to protect them. When digging close to an existing structure, and especially when digging deeper than its foundations, the structure will have to be temporarily braced and may require permanent underpinning with new foundations. Furthermore, an excavation at a distance from an existing structure may not require sheeting, while one immediately adjacent al-most certainly will.

•  Increasing the column or wall load from a building beyond what can be supported  by a shallow foundation. Shallow foundations are far less expensive than piles or caissons under most conditions. If the building grows too tall, however, a shallow foundation may no longer be able to carry the load, and a threshold must be crossed into the realm of deep foundations. If this has happened for the sake of an extra story or two of height, the designer should consider reducing the height by broadening the building. If individual column loadings are too high for shallow foundations, perhaps they can be reduced by increasing the number of columns in the building and decreasing their spacing.

For buildings at the scale of one-and two-family dwellings, foundation design is usually much simpler than for large buildings because foundation loadings are low. The uncertainties in foundation design can be reduced with reasonable economy by adopting a large factor of safety in calculating the bearing capacity of the soil.

Unless the designer has reason to suspect poor soil conditions, the footings are usually designed using rule-of-thumb allowable soil stresses and standardized footing dimensions. The designer then examines the actual soil when the excavations have been made. If it is not of the quality that was expected, the footings can be hastily redesigned  using a revised estimate of soil-bearing  capacity before construction continues. If unexpected groundwater is encountered, better drainage may have to be provided around the foundation or  the depth of the basement decreased.

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Normally, the substructure of a building is completed before work begins on its superstructure. If the building  has several levels of basements, however, substructure work can take many months or even years.

In such a case,  up–down construction is sometimes an economical option, even if its first  cost is somewhat more than that of  the normal procedure, because it can  save considerable construction time.

As diagrammed in Figure 2.69,  up–down construction begins with  installation of a perimeter slurry  wall.

Internal steel columns for the  substructure are lowered into drilled, slurry-filled holes, and concrete footings are tremied beneath them. After  the ground floor slab is in place and  connected to the substructure columns, erection of the superstructure may begin. Construction continues  simultaneously on the substructure, largely by means of mining machinery: A story of soil is excavated from  beneath the ground floor slab and a  level mud slab of CLSM is poured.

Working on the mud slab, workers reinforce and pour a concrete structural slab for the floor of the topmost
basement level and connect this floor to the columns. When the slab is sufficiently strong, another story of soil is removed from beneath it, along with the mud slab. The process  is repeated until the substructure iscomplete, by which time the super-structure has been built many storiesinto the air.

Figure 2.69 Up–down construction.

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After the basement walls have been  waterproofed or dampproofed, insulating boards or protection boards  have been applied, drainage features  have been installed, and internal constructions that support the basement walls, such as interior walls and floors,  have been completed, the area around a substructure is  backfilled to restore the level of the ground. (A substructure built tightly against sheeted walls of an excavation needs little or no backfilling.) The backfilling  operation involves placing soil back against the outside of the basement walls  and compacting it there in layers,  taking care not to damage drainage  or waterproofing components or to exert excessive soil pressure against  the walls. An open, fast-draining soil  such as gravel or sand is preferred for backfilling because it allows the perimeter drainage system around the  basement to do its work. Compaction  must be sufficient to minimize subsequent settling of the backfilled area.

In some situations, especially in backfilling utility trenches under road- ways and floor slabs, settling can be  virtually eliminated by backfilling with  controlled low-strength material (CLSM),  which is made from portland cement  and/or fly ash (a byproduct of coal- burning power plants), sand, and water.  CLSM, sometimes called “flowable fill,”  is brought in concrete mixer trucks  and poured into the excavation, where  it compacts and levels itself, then hardens into soil-like material. The strength  of CLSM is matched to the situation:

For a utility trench, CLSM is formulated so that it is weak enough to be  excavated easily by ordinary digging  equipment when the pipe needs ser-vicing, yet as strong as a good-quality compacted backfill. CLSM has many  other uses in and around foundations.

It is often used to pour mud slabs, which  are weak concrete slabs used to create  a level, dry base in an irregular, often  wet excavation. The mud slab serves  as a working surface for the reinforcing and pouring of a foundation mat  or basement floor slab and is often the  surface to which a waterproofing mem- brane is applied. CLSM is also used to  replace pockets of unstable soil that  may be encountered beneath a sub- structure or to create a stable volume  of backfill around a basement wall.

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In  shallow frost-protected foundations, extruded polystyrene foam insulation boards can be used in cold climates to construct footings that lie above the normal frost line in the soil, resulting  in lower excavation costs. Continuous  layers of insulation board are placed  around the perimeter of the building  in such a way that heat fl owing into the  soil in winter from the interior of the  building maintains the soil beneath  the footings at a temperature above  freezing (Figure 2.68). Even beneath  unheated buildings, properly installed  thermal insulation can trap enough  geothermal heat around shallow foundations to prevent freezing.

Figure 2.68 A typical detail for a shallow frost- protected footing.

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Comfort requirements, heating fuel efficiency, and building codes often require that basement walls be thermally insulated to limit the loss of heat from basements to the soil outside. Thermal insulation may be applied either inside or outside the basement wall.

Inside the wall, mineral batt or plastic foam insulation may be installed between wood or steel furring strips, as shown in Figure 23.4. Alternatively, polystyrene foam or glass fiber insulation boards, typically 2 to 4 inches (50 to 100 mm) thick may be placed on the outside of the wall, held by either adhesive, fasteners, or the pressure of the soil. Proprietary products are available that combine insulation board and drainage mat in a single assembly.

Figure 23.4 A furred gypsum board fi nish over a concrete block wall. The Z-furring channels are attached to the masonry with powder-driven fasteners. The plastic foam insulation is tucked in behind the fl ange of the channel, and the gypsum board is screwed to the face of the fl ange. Long slots punched from the web of the channel (not visible in this drawing) help to reduce the thermal bridging effect of the Z-furring channel.

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Where building substructures enclose basements, parking garages, or other usable space, groundwater must be kept out. Concrete alone is rarely adequate for this purpose. Moisture can migrate through its microscopic pores, or through other pathways created by shrinkage cracks, form tie holes, utility penetrations, and the joints between concrete pours. To ensure a substructure’s resistance to water entry,
two approaches are used: drainage and waterproofing. Drainage draws groundwater away from a foundation, reducing the volume and pressure of water acting on the foundation’s walls and slabs.

Waterproofing acts as a barrier, stopping water that reaches the foundation from passing though to the interior.

Drainage, consisting of some combination of drainage backfill (well-sorted crushed stone or gravel), drainage mat, and perforated drain piping, is used with almost every building substructure (Figure 2.60). Drainage mat is a manufactured component that may be made of a loose mat of stiff, inert fibers, a plastic egg-crate structure, or some other very open, porous material. It is faced on the outside with a filter fabric that prevents fine soil particles from entering and clogging the drainage passages in the mat. Any subterranean water that approaches the wall descends through the porous material of the mat to the drain pipe at the footing. Perforated drain piping is frequently laid around the outside perimeter of a building foundation. The pipes are 4 or 6 inches (100 or 150 mm) in diameter and provide an open channel in the crushed stone bed through which water can flow by gravity either “to day-light” at a lower elevation on a sloping site, to a municipal storm sewer system, or to a sump pit that can be automatically pumped dry whenever it fills. The pipes are laid at least 6 inches (150 mm) below the top of the basement floor slab tomaintain the groundwater level safely below that of the slab. Perforations in the pipes face downward so that water is drained from the lowest possible level. Where groundwater conditions are severe, rows of perforated pipe may be installed under the basement slab as well (Figure 2.61).

Figure 2.60 Two methods of relieving water pressure around a building substructure by drainage. The gravel drain (left) is hard to do well because of the diffi culty of depositing the crushed stone and backfi ll soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.61 For a high degree of security against substructure fl ooding, drainage both around and under the basement is required, as seen here in a section view. Above-slab drainage is used in buildings with mat foundations.

On most foundations, some form of water-repelling barrier is also used to protect against the pas-
sage of groundwater. Dampproofing is a moisture-resistant cement plaster or asphalt compound commonly applied to residential basement walls and to other substructures where groundwater conditions are mild or waterproofing requirements are not critical. Cement plaster dampproofing, or parge coating, is light gray in color and troweled on.  Asphalt or  bituminous dampproofing is dark in color and is applied in liquid form by spray, roller, or trowel. Dampproofing is less expensive and less resistant to water passage than true waterproofing.

Waterproofing, unlike dampproofing, can prevent the passage of water even under conditions of hydrostatic pressure. It is used where groundwater conditions are severe or the need to protect subgrade space from moisture is critical. Waterproof mem-branes are most commonly formulated from plastics, asphalt compounds, or synthetic rubbers and come in a great variety of forms.

Liquid waterproofing  is applied by spray gun, roller, or squeegee and then allowed to cure in place. It is easy to install and easy to form around complex shapes. When fully cured, the finished membrane is seamless and fully bonded to the underlying substrate. However, because liquid membranes are formed in the field, they are subject to uneven application, and the surfaces to which they are applied must be clean, smooth, and dry to ensure reliable adhesion of the membrane.

Preformed sheet membrane waterproofing may be adhered or mechanically fastened to substructure walls
or laid loosely over horizontal surfaces (Figure 2.62). Fabricated under controlled factory conditions, sheet membranes are reliably uniform in material quality and thickness.

Figure 2.62 A diagrammatic representation of the  placement of sheet membrane waterproofi  ng around a basement. A mud slab of low-strength concrete was poured to serve as a base for placement of the horizontal  membrane. Notice that the vertical and horizontal membranes join to wrap the  basement completely in a waterproof  enclosure.

However, they can be more difficult to form around complex shapes, and the seams between sheets, which are sealed in the field, may be subject to lapses in quality. Sheet membranes that are loosely laid or mechanically fastened can be used over substrates that will not bond with liquid-applied or adhered sheet membranes. They are also a good choice where substrate cracking or movement may be expected, because such movement is less likely to stress or damage the membrane. An advantage of adhered membranes (both sheet and liquid) is that in the case of a defect, water cannot travel far under the membrane, limiting the extent of water damage that may occur and simplifying the tracing of leaks.

Bentonite waterproofing is made from sodium bentonite, a naturally occurring, highly expansive clay. It is most often applied as preformed sheets consisting of dry clay sand-wiched within corrugated card-board, geotextile fabric, or plastic sheets (Figure 2.63). When bentonite comes in contact with moisture, it swells to several times its dry volume and forms an impervious barrier to the further passage of water. Bentonite sheets can be placed directly on the soil under a concrete slab on grade or mechanically attached to uncured, damp concrete walls. In slurry form, bentonite can  be sprayed even onto highly irregular, rough stone walls. The swelling behavior of bentonite clay also allows it to adjust to cracking and  movement in the substrate.

Figure 2.63 Waterproofi ng in progress on a  concrete foundation. Leftmost: the bare foundation wall remains exposed.  Middle: bentonite waterproofi ng panels are fastened in place. These panels are  lined on the outer face with a black- colored, high-density plastic that adds to  the waterproofi ng qualities of the panel. Right: drainage mat has been installed  over the waterproofi ng. The mat’s outer face of fi lter fabric is lightly dimpled,  telegraphing the egg crate structure of the underlying molded plastic panel.  The top edge of the mat is secured in place with an aluminum termination bar  that holds the panel in place and keeps dirt and debris from falling behind the  panel. Lower right: white perforated drain piping can be seen, temporarily  supported on wood blocking and running alongside the footing. (Photo by  Joseph Iano)

Integral waterproofing includes cementitious plaster or crystalline admixtures for concrete or mortar
that react chemically to stop up the pores of these materials and render them watertight. It may be applied
to the surface of existing concrete or masonry or used as an admixture in new concrete. Unlike most other waterproofing materials, many integral waterproofing materials can be applied as  negative side waterproof-
ing, that is, applied to the inner side of a concrete wall acting to resist water passage from the opposite side.

Blind-side waterproofing is installed prior to the pouring of concrete walls. This occurs most commonly when a substructure wall is built close to a property’s edge, and excavation cannot be enlarged beyond the property line to permit workers access to the outer face of the wall after its construction.

Drainage matting is first applied directly to the excavation sheeting, and then any of a number of possible waterproofing membranes are applied over the drainage mat. Later, the concrete wall is poured against the membrane. The sheeting remains permanently in place (Figure 2.64).

Figure 2.64 Blind-side waterproofi ng is used where there is no working space between a sheeted excavation and the outside of the foundation wall. The drainage mat and waterproof membrane are applied to the sheeting; then the basement wall is poured against them.

Joints in construction require special attention to ensure water-tightness. Preformed waterstops made of plastic, synthetic rubber, or metal can be cast into the mating concrete edges of both moving and nonmoving joints to block the passage of water (Figures 2.65 and 2.66). Waterstops for nonmoving joints such as between concrete pours of a wall or slab can also be made of strips of bentonite or mastic that are temporarily adhered to the edge of one pour. After the adjacent pour is complete, these stops remain embedded in the joint, where they form a watertight barrier (Figure 2.67).

Figure 2.65 A synthetic rubber waterstop is used to seal against water penetration at movement joints and at joints between pours of concrete in a foundation. The type shown here is split on one side so that its halves can be placed fl at against the formwork where another wall will join the one being poured. After the concrete has been poured and cured and the formwork has been removed from the first wall, the split halves are folded back together before the next wall is poured.

Most waterproofing  systems are inaccessible once building construction is complete; they are ex-
pected to perform for the life of the building, and even small defects in installation can allow the passage of large volumes of water.

For these reasons, waterproofing membranes are inspected carefully during construction and horizontal  membranes are often  flood  tested (submerged for an extended period time while leak-checking is  performed) to detect the presence of defects while repairs can still be easily made. Once inspection and testing are complete, membranes are covered with a  protection board, insulation board, or drainage matting to shield the membrane from prolonged exposure to sunlight and  to prevent physical damage during  soil backfilling or subsequent construction operations.

Figure 2.66 A rubber waterstop ready for the next pour of a concrete wall, as diagrammed in Figure 2.65. (Courtesy of Vulcan Metal Products, Inc., Birmingham, Alabama)

Figure 2.67 A bentonite waterstop is adhered to a concrete footing prior to casting of the concrete wall above. Later, if groundwater seepage occurs, the bentonite will swell to fully seal the joint between the two pours. The waterstop is positioned to the side of the steel reinforcing bars closer to the wall’s exterior, also protecting the reinforcing from moisture and corrosion. However, because of bentonite’s expansive force, the waterstop must not be positioned too close to the surface of the wall, or when it swells, it could cause portions of concrete to split away or spall.

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Geotextiles are flexible fabrics made of chemically inert  plastics that are highly resistant to deterioration in the soil. They are used for a variety of purposes relating to site development and the foundations of buildings.

As described in the accompanying text, in earth reinforcement  or  soil reinforcement, a plastic mesh fabric or grid is used to support an earth embankment or retaining wall. The  same type of mesh may be utilized in layers to stabilize engineered fill beneath a shallow footing (Figure A), or to stabilize marginal soils under driveways, roads, and airport runways, acting very much as the roots of plants do in preventing the movement of soil particles.

Another geotextile that is introduced later in this chapter is drainage matting, an open matrix of plastic filaments with a feltlike filter fabric laminated onto one side to keep soil particles from entering the matrix. In addition to providing free drainage around foundation walls, drainage matting is often used beneath the soil in the bottoms of planter boxes and under heavy paving tiles on rooftop terraces, where it maintains free passage for the drainage of water above a waterproof membrane.



Synthetic filter fabrics are wrapped over and around subterranean crushed stone drainage layers such as the one frequently used around a foundation drain. In this position, they keep the stone and pipe from gradually
becoming clogged with fine soil particles carried by the groundwater. Similar fabrics are used at grade during construction, acting as temporary barriers to filter soil out of water that runs off a construction site, thus preventing contamination of lakes, streams, or stormwater systems.

Special geotextiles are manufactured to stake down on freshly cut slopes to prevent soil erosion and encour-age revegetation; some of these are designed to decay and disappear into the soil as plants take over the function of slope stabilization. Another type of geotextile is used for weed control in landscaped beds, where it allows rainwater to penetrate the soil but blocks sunlight, preventing weeds from sprouting.

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Underpinning is the process of  strengthening and stabilizing the  foundations of an existing building.

It may be required for any of several reasons: The existing foundations may never have been adequate to carry their loads, leading to excessive settlement of the building over time. A change in building use or
additions to the building may over-load the existing foundations. New construction near a building may disturb the soil around its foundations or require that its foundations be carried deeper. Whatever the cause, underpinning is a highly specialized task that is seldom the same for any two buildings. Three different alternatives are available when foundation capacity needs to be increased: The foundations may be enlarged; new, deep foundations can be inserted under shallow ones to carry the load to a deeper, stronger stratum of soil; or the soil itself can be strengthened by grouting or by chemical treatment. Figures 2.53 and 2.54 illustrate in diagrammatic form some selected concepts of underpinning.

Figure 2.53 Two methods of supporting a building while carrying out underpinning work beneath its foundation, each shown in both elevation  and section. (a) Trenches are dug beneath the existing foundation at intervals, leaving the majority of  the foundation supported by the soil. When portions of the new foundations have been completed in the trenches, using one of the types of underpinning shown in Figure 2.54, another set of trenches is dug between them and the remainder of the foundations is completed. (b) The foundations of an entire wall can be exposed at once by needling, in which the wall is supported temporarily on needle beams threaded through holes cut in the wall. After underpinning has been accomplished, the jacks and needle beams are removed and the trench is backfi  lled

Figure 2.54 Three types of underpinning. (a) A new foundation wall and footing are constructed beneath the existing foundation. (b) New piles or caissons are constructed on either side of the existing foundation. (c) Minipiles are inserted through the existing foundation. Minipiles do not generally require excavation or temporary support of the building.

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In areas where strong earthquakes are  common, buildings are sometimes placed on base isolators. When signifi cant ground movement occurs, the  base isolators flex or yield to absorb a  significant portion of this movement;  as a result, the building and its sub- structure move significantly less than  they would otherwise, reducing the  forces acting on the structure and  lessening the potential for damage.

A frequently used type of base isolator is a multilayered sandwich of rubber and steel plates (Figure 2.52).

The rubber layers deform in shear  to allow the rectangular isolator to  become a parallelogram in cross section in response to relative motion  between the ground and the building. A lead core deforms enough to  allow this motion to occur, provides  damping action, and keeps the layers  of the sandwich aligned.

Figure 2.52 Base isolation.

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Piles may be made of timber, steel, concrete, and various combinations of these materials (Figure 2.47). Timber piles have been used since Roman times, when they were driven by large mechanical hammers hoisted by muscle power. Their main advantage is that they are economical for lightly loaded foundations. On the minus side, they cannot be spliced during driving and are, therefore, limited to the length of available tree trunks, approximately 65 feet (20 m). Unless pressure treated with a wood pre- servative or completely submerged below the water table, they will decay (the lack of free oxygen in the water prohibits organic growth). Relatively small hammers must be used in driving timber piles to avoid splitting them. Capacities of individual timber piles lie in the range of 10 to 55 tons (9000 to 50,000 kg).

Two forms of steel piles are used, H-piles and pipe piles.  H-piles are special hot-rolled, wide-flange sections, 8 to 14 inches (200 to 355 mm) deep,  which are approximately square in cross section. They are used mostly in end bearing applications. H-piles displace relatively little soil during driving. This minimizes the upward displacement of adjacent soil, called heaving, that sometimes occurs when many piles are driven close together. Heaving can be a particular problem on urban sites, where it can lift adjacent buildings.

Figure 2.47 Cross sections of common types of piles. Precast concrete piles may be square or round instead of the octagonal section shown here and may be hollow in the larger sizes.

 H-piles can be brought to the site in any convenient lengths, welded together as driving progresses to form any necessary length of pile, and cut off with an oxyacetylene torch when the required depth is reached. The cutoff ends can then be welded onto other piles to avoid waste. Corrosion can be a problem in some soils, however, and unlike closed pipe piles and hollow precast concrete piles, H-piles cannot be inspected after driving to be sure they are straight and undamaged.
Allowable loads on H-piles run from 30 to 225 tons (27,000 to 204,000 kg).

Steel pipe piles have diameters of  8 to 16 inches (200 to 400 mm). They  may be driven with the lower end either open or closed with a heavy steel  plate. An open pile is easier to drive  than a closed one, but its interior  must be cleaned of soil and inspected before being filled with concrete,  whereas a closed pile can be inspected  and concreted immediately after driving. Pipe piles are stiff and can carry  loads from 40 to 300 tons (36,000 to 270,000 kg). They displace relatively  large amounts of soil during driving,  which can lead to upward heaving of  nearby soil and buildings. The larger  sizes of pipe piles require a very heavy  hammer for driving.

Minipiles, also called  pin piles or micropiles, are a lightweight form of  steel piles made from steel bar or  pipe 2 to 12 inches (50 to 300 mm) in  diameter. Minipiles are inserted into  holes drilled in the soil and grouted in  place. When installed within existing  buildings, they may also be forced into the soil by hydraulic jacks pushing downward on the pile and upward on the building structure. Since no hammering is required, they are a good  choice for repair or improvement of  existing foundations where vibrations from the hammering of conventional piles could damage the existing structure or disrupt ongoing activities within the building (Figure 2.54).

Where vertical space is limited, such as when working in the basement of an existing building, minipiles can
be installed in individual sections as short as 3 feet (1 m) that are threaded end-to-end as driving progresses.

Minipiles can reach depths as great as 200 feet (60 m) and have working capacities as great as 200 to 300 tons (180,000 to 270,000 kg). 

Precast concrete piles are square, octagonal, or round in section, and in large sizes often have open cores to  allow inspection (Figures 2.47–2.49).

Figure 2.48 Precast, prestressed concrete piles. Lifting loops are cast into the sides of the piles as crane attachments for hoisting them into a vertical position.

Figure 2.49 A driven cluster of six precast concrete piles, ready for cutting off and capping.

Most are prestressed, but some for smaller buildings are merely rein-forced (for an explanation of prestressing, see pages 544–548). Typical cross-sectional dimensions range from 10 to 16 inches (250 to 400 mm) and bearing capacities from 45 to 500 tons (40,000 to 450,000 kg).

Advantages of precast piles include  high load capacity, an absence of corrosion or decay problems, and, in
most situations, a relative economy of cost. Precast piles must be handled carefully to avoid bending and cracking before installation. Splices be-tween lengths of precast piling can be made effectively with mechanical fastening devices that are cast into the ends of the sections.

A  sitecast concrete pile is made by driving a hollow steel shell into the ground and filling it with concrete. The shell is sometimes corru-gated to increase its stiffness; if the corrugations are circumferential, a  heavy steel mandrel (a stiff, tight-fitting liner) is inserted in the shell  during driving to protect the shell  from collapse, then withdrawn before concreting. Some shells with  longitudinal corrugations are stiff  enough that they do not require  mandrels. Some types of mandrel- driven piles are limited in length,  and the larger diameters of sitecast piles (up to 16 inches, or 400 mm)  can cause ground heaving. Load ca- pacities range from 45 to 150 tons  (40,000 to 136,000 kg). The primary  reason to use sitecast concrete piles  is their economy.

There is a variety of proprietary  sitecast concrete pile systems, each  with various advantages and disad- vantages (Figure 2.50). Concrete  pressure-injected footings (Figure 2.51)  share characteristics of piles, piers,  and footings. They are highly resistant to uplift forces, a property that  is useful for tall, slender buildings in  which there is a potential for overturning of the building, and for ten- sile anchors for tent and pneumatic  structures. Rammed aggregate piers and  stone columns  are similar to pressure- injected footings, but are constructed  of crushed rock that has been densely compacted into holes created by  drilling or the action of proprietary  vibrating probes.

Figure 2.50 Some proprietary types of sitecast concrete piles. All are cast into steel casings that have been driven into the ground; the uncased piles are made by withdrawing the casing as the concrete is poured and saving it for subsequent reuse. The numbers refer to the methods of driving that may be used with each: 1. Mandrel driven. 2. Driven from the top of the tube. 3. Driven from the bottom of the tube to avoid buckling it. 4. Jetted. Jetting is accomplished by advancing a high-pressure water nozzle ahead of the pile to wash the soil back alongside the pile to the surface. Jetting has a tendency to disrupt the soil around the pile, so it is not a favored method of driving under most circumstances

Figure 2.51 Steps in the construction of a proprietary pressure-injected, bottom-driven concrete pile footing. (a) A charge of a very low-moisture concrete mix is inserted into the bottom of the steel drive tube at the surface of the ground and compacted into a sealing plug with repeated blows of a drop hammer. (b) As the drop hammer drives the sealing plug into the ground, the drive tube is pulled along by the friction between the plug and the tube. (c) When the desired depth is reached, the tube is held and a bulb of concrete is formed by adding small charges of concrete and driving the concrete out into the soil with the drop hammer. The bulb provides an increased bearing area for the pile and strengthens the bearing stratum by compaction. (d, e) The shaft is formed of additional compacted concrete as the tube is withdrawn. (f) Charges of concrete are dropped into the tube from a special bucket supported on the leads of the driving equipment.

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A caisson, or drilled pier (Figure 2.37), is similar to a column footing in that it spreads the load from a column over a large enough area of soil that the allowable stress in the soil is not exceeded. It differs from a column footing in that it extends through strata of unsatisfactory soil beneath the substructure of a building until it reaches a more suitable stratum. A caisson is constructed by drilling or hand-digging a hole, belling (flaring) the hole out at the bottom as neces- sary to achieve the required bearing area, and fi lling the hole with concrete. Large auger drills (Figures 2.38 and 2.39) are used for drilling caissons; hand excavation is used only if the soil is too full of boulders for the drill. A temporary cylindrical steel casing is usually lowered around the drill as it progresses to support the soil around the hole. When a fi  rm bearing stratum is reached, the bell, if required, is created at the bottom of the shaft either by hand excavation  or by a special  belling bucket on the drill (Figure 2.40). The bearing sur- face of the soil at the bottom of the  hole is then inspected to be sure it  is of the anticipated quality, and the  hole is fi lled with concrete, withdrawing the casing as the concrete rises.

Reinforcing is seldom used in the  concrete except near the top of the  caisson, where it joins the columns of  the superstructure.

Figure 2.37 Deep foundations. Caissons are concrete  cylinders poured into drilled holes. They reach through weaker soil (light shading)  to bear on competent soil beneath. The end bearing caisson at the left is belled as  shown when additional bearing capacity is required. The socketed caisson is drilled  into a hard stratum and transfers its load primarily by friction between the soil or  rock and the sides of the caisson. Piles are driven into the earth. End bearing  piles act in the same way as caissons. The friction pile derives its load-carrying  capacity from friction between the soil and the sides of the pile.

Figure 2.38 A 6-foot- (1828-mm)-diameter auger on a telescoping 70-foot (21-m) bar brings up a load of soil from a caisson hole. The auger will be rotated rapidly to spin off the soil before being reinserted in the  hole.

Figure 2.39 For cutting through hard material, the caisson drill is equipped with a carbide-toothed coring bucket.

Figure 2.40 The bell is formed at the bottom of the  caisson shaft by a belling bucket with retractable cutters. The example shown  here is for an 8-foot- (2.44-m)-diameter shaft and makes a bell 21 feet (6.40 m) in  diameter.

Caissons are large, heavy-duty foundation components. Their shaft diameters range from 18 inches  (460 mm) up to 8 feet (2.4 m) or  more.  Belled caissons are practical  only where the bell can be excavated  in a cohesive soil (such as clay) that  can retain its shape until concrete is  poured. Where groundwater is present, the temporary steel casing can  prevent flooding of the caisson hole  during its construction. But where  the bearing stratum is permeable, water may be able to fi ll the hole from  below and caisson construction may  not be practical.

A  socketed caisson (Figure 2.37) is  drilled into rock at the bottom rather than belled. Its bearing capacity
comes not only from its end bearing, but from the frictional forces between  the sides of the caisson and the rock  as well. Figure 2.41 shows the installation of a rock caisson or drilled-in caisson, a special type of socketed  caisson with a steel H-section core.

Figure 2.41 Installing a rock caisson. (a) The shaft of the caisson has been drilled through softer soil to the rock beneath and cased with a steel pipe. A churn drill is being lowered into the casing to begin advancing the hole into the rock. (b) When the hole has penetrated the required distance into the rock stratum, a heavy steel H-section is lowered into the hole and suspended on steel channels across the mouth of the casing. The space between the casing and the H-section is then fi lled with concrete, producing a caisson with a very high load-carrying capacity because of the composite structural action of the steel and the concrete.