Engineering and Construction

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The foundation sill plate, usually made of preservative-treated wood for added resistance to insects and moisture, is bolted to the foundation as a base for the wood framing (Figure 5.18,  A, B, and C). A single sill, as shown in the details here, is all that is required by code, but in better-quality  work, the sill may be doubled or made from thicker stock for greater stiffness. A thicker sill plate may also  be required in areas of high seismic risk, to provide a stronger connection between the wood framing and  the foundation anchor bolts. Because the top of a foundation wall is usually somewhat uneven, the sill may  also be shimmed up at low spots with wood shingle wedges or plastic shims to provide a more level, even base  for subsequent framing. A  sill sealer, made of compressible or resilient  material, should be inserted between  the sill and the foundation to reduce air infiltration through the gap. Some  sill sealer materials also discourage  moisture wicking up from the foundation into the wood framing (Figure 5.22). Conventional anchor bolts  are sufficient to hold most buildings on their foundations, but tall frames or those subject to high winds  or strong earthquakes may require more elaborate attachments (Figures 5.39-5.41).

Figure 5.22 Carpenters apply a preservative-treated wood sill to a sitecast concrete foundation. A strip of compressible glass fiber sill sealer has been placed on the top of the concrete wall, and the sill has been drilled to fit over the projecting anchor bolts. Before each section of sill is bolted tightly, it is leveled as necessary with wood shingle or plastic shims between the concrete and the wood. As the anchor bolts are tightened, the sill sealer squeezes down to a negligible thickness. A length of bolted-down sill is visible at the upper right. The basement windows were clamped into reusable steel form inserts and placed in the formwork before the concrete was cast. After the concrete was cast and the formwork was stripped, the steel inserts were removed, leaving a neatly formed concrete frame around each window. A pocket in the top of the wall for a steel beam can be seen at the upper left.

Figure 5.39 Strap tie hold-downs are made of galvanized steel straps with hooked or deformed ends cast into the concrete foundation wall (top). After the wall is framed, the exposed length of strap may be nailed directly to framing or, as seen here, nailed through the sheathing into the studs or posts behind (bottom). The number, size, and spacing of nails used to fasten the strap depend on the magnitude of the loads that must be resisted and the capacity of the wood member to hold nails without splitting. In the top image, anchor bolts cast into the top of the foun- dation wall that will be used to secure the sill plate in place are also visible.

Figure 5.40 Hold-downs made from threaded rod and steel plate anchors can resist much greater forces than strap tie hold-downs. They may be used at each fl oor level to securely tie the full height of the building frame to its foundation. For the type shown here, the nuts at the ends of the threaded rods may require retightening after the fi rst heating season to compensate for wood shrinkage, which can mean that access holes must be provided through the interior wall surfaces. Other models rely on spring-loaded tapered shims or other mechanical compensation so as to be self-adjusting.

Figure 5.41 A heavy-duty seismic hold-down similar to that illustrated in Figure 5.40. The anchor rod with one end cast into the concrete foundation wall protrudes through the preservative-treated wood sill where its threaded end is bolted to the anchor. The anchor in turn is bolted to a 4 4 (89 89 mm) post with fi ve bolts of substantial diameter. A conventional foundation anchor bolt with an oversized square washer is also partially visible to the right of the hold-down. Also note the thicker-than-normal, 3-inch nominal (64-mm) sill plate, a common feature in wood light framing designed for high seismic forces.

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Foundations for light framing, originally made of stone or brick, are  now made in most cases of sitecast  concrete or concrete block masonry  (Figures 5.5-5.11). These materials  are highly conductive of heat and  usually must be insulated to meet  code requirements for energy con- servation (Figures 5.8, 5.9, 5.12, and  5.13). Where concrete and masonry  construction methods are not practical, such as in extremely cold regions, foundations may be constructed entirely of preservative-treated wood  (Figures 5.14 and 5.15). Such  permanent wood foundations can be con- structed in any weather by the same crew of carpenters that will frame the building; they are readily insulated in the same manner as the frame  of the house they support; and they easily accommodate the installation of electrical wiring, plumbing, and interior finish materials.

Figure 5.5 Step One in the construction of a simple platform frame building: establishing the position, shape, and size of the building on the site. After the corners of the foundation have been staked, batter boards are erected safely beyond the limits of the excavation area. String lines crossing over the corner stakes are stretched from the batter boards, and their positions on the boards are preserved with notches or nails in the boards. Later, after excavation, the strings are stretched again, and the foundation corners can be relocated at the bottom of the excavation. This drawing begins a series of isometric drawings that will follow the erection of a wood light frame building step by step throughout the course of this chapter.

Figure 5.6 Typical details for a sitecast concrete foundation and basement for a platform frame building. Details A, B, and C are keyed to the circled portions of the drawing in Figure 5.7.

Figure 5.7 Step Two in the construction of a typical platform frame building: excavation and foundations. The letters A, B, and C indicate portions of the foundation that are detailed in Figure 5.6.

Figure 5.8 Most building codes require thermal insulation of the foundation. These are three different ways of adding insulation to a  cast-in-place concrete or concrete masonry basement and one way of insulating a crawlspace foundation. The crawlspace may  alternatively be insulated on the outside with panels of plastic foam. The interior batt insulation shown in B is commonly used but  raises questions about how to avoid possible problems arising from moisture accumulating between the insulation and the wall.

Figure 5.9 Some typical concrete slab-on-grade details with thermal insulation. In areas where termites are common, there must be a metal fl ashing (termite shield) similar to that shown in Figure 5.18 C or other interruption that passes through the foam insulation above ground level to prevent termites from tunneling undetected through the insulation to reach the wood structure. This applies to the foam-insulated basement details in Figure 5.8 as well.

Insulating  concrete formwork (ICF) foundation  systems using permanent insulating  forms are easy to construct, eliminate the need for formwork removal, and provide integral insulation  (Figure 14.12).  Proprietary  precast  concrete foundation systems relying on factory-fabricated reinforced  concrete panels are rapidly erected  on site and can be used to construct  basement structures of consistent  strength and quality. They may be  manufactured with insulation integral to the precast panel or designed  to readily accept insulation added in  the field.

All basements also need to be carefully dampproofed and drained  to avoid flooding with groundwater
and to prevent the buildup of water pressure in the surrounding soil  that could cause the walls to cave in  (Figures 5.6, 5.7, and 5.11).

Figure 5.10 Erecting formwork for a sitecast concrete foundation wall. The footing has already been cast and its formwork removed. It is visible in front of the worker at the left.

Figure 5.11 Masons construct a foundation of concrete masonry. The fi rst coat of parging, which is portland cement plaster applied to help dampproof the foundation, has already been applied to the outside of the wall, and the drainage layer of crushed stone has been backfi lled in place. The projecting pilaster in the center of the wall will support a beam under the center of the main fl oor. After a second coat of parging, the outside of the foundation will be coated with an asphaltic dampproofi ng compound. (Reprinted with permission of the Portland Cement Association from Design and Control of Concrete Mixtures, 12th edition; Photos: Portland Cement Association, Skokie, IL)

Figure 5.12 A concrete masonry foundation with polystyrene foam insulation applied to its outer surface.

Figure 5.13 Following completion of the exterior siding, a worker staples a glass fi  ber reinforcing mesh to the foam insulation on the exposed portions of the basement wall. Next, he will trowel onto the mesh two thin coats of a cementitious, stucco- like material that will form a durable fi nish coating over the foam.

Figure 5.14 Erecting a permanent wood foundation. One worker applies a bead of sealant to the edge of a panel of preservative- treated wood components as another prepares to push the next panel into position against the sealant. The panels rest on a horizontal preservative-treated plank, which, in turn, rests on a drainage layer of crushed stone. Wood founda- tions can be constructed in any weather and can be insulated in the same way as the superstructure of the building.

Figure 5.15 The difference in color makes it clear where the preservative-treated wood foundation leaves off and the untreated superstructure of the building begins.

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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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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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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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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.

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Most shallow foundations are simple concrete  footings. A  column footing is  a square block of concrete, with or  without steel reinforcing, that accepts  the concentrated load placed on it  from above by a building column and spreads this load across an area of soil large enough that the allowable bearing stress of the soil is not exceeded.

A wall footing or  strip footing is a continuous strip of concrete that serves the same function for a loadbearing wall (Figures 2.30 and 2.31).

Figure 2.30 A column footing and a wall footing of concrete. The steel reinforcing bars have  been omitted from this illustration for clarity.

Figure 2.31 These concrete foundation walls for an apartment building, with their steel formwork not yet stripped, rest on wall footings. For more extensive illustrations of wall and  column footings

To minimize settlement, footings are usually placed on undisturbed soil.  Under some circumstances, footings
may be constructed over engineered fill,  which is earth that has been deposited under the supervision of a soils engineer. The engineer, working from the results of laboratory compaction tests on samples taken from the soil used for filling, makes sure that the soil is deposited in thin layers at a controlled moisture content and compacted in accordance with detailed procedures that ensure a known loadbearing ca-pacity and long-term stability.

Footings appear in many forms in different foundation systems. In  climates with little or no ground frost, a concrete  slab on grade with thickened edges is the least expensive foundation and floor system that one can use and is applicable to one- and two-story buildings of any type of construction (see Chapter 14 for further
information on slabs on grade). Or, in colder regions the edges of a slab on grade may be supported with deeper wall footings that bear on soil below the frost line. For floors raised above the ground, either over a  crawlspaceor a  basement, support is provided by concrete or masonry foundation walls supported on concrete strip footings (Figure 2.32). When building on slopes, it is often necessary to step the footings to maintain the re-quired depth of footing at all points around the building (Figure 2.33). If soil conditions or earthquake precau-tions require it, column footings on steep slopes may be linked together with reinforced concrete  tie beams to avoid possible differential slippage between footings.

Figure 2.32 Three types of substructures with shallow foundations. The slab on grade  is the most economical under many circumstances, especially where the  water table lies near the surface of the ground. A crawlspace is often used under  a floor structure of wood or steel, and gives much better access to underfloor  piping and wiring than a slab on grade. Basements provide usable space for  building occupants.

Figure 2.33 Foundations on sloping sites, viewed in a cross section  through the building. The broken line indicates the outline of the superstructure. Wall footings are stepped to maintain the necessary distance between the bottom of the footing and the surface of the ground. Separate column foundations, whether caissons, as shown here, or column footings, are often connected with reinforced concrete tie beams to reduce differential movement between the columns. A grade beam differs from a tie beam by being reinforced to distribute the continuous load from a bearing wall to separate foundations.

Footings cannot legally extend beyond a property line, even for a building built tightly against it. If the outer
toe of the footing were simply cut off at the property line, the footing would not be symmetrically loaded by the column or wall and would tend to rotate and fail. Combined footings and cantilever footings solve this problem by tying the footings for the outside row of columns to those of the next row in such a way that any rotational tendency is neutralized (Figure 2.34).

Figure 2.34 Either a combined footing (top) or a cantilevered footing (bottom) is used when columns must abut a property line. By combining the foundation for the column against the property line, at the left, with the foundation for the next interior column to the right in a single structural unit, a balanced footing design can be achieved. The concrete reinforcing steel has been omitted from these drawings for the sake of clarity.

In situations where the allowable bearing capacity of the soil is low in relation to the weight of the build-
ing, column footings may become large enough that it is more economical to merge them into a single mat or  raft foundation that supports the entire building. Mats for very tall buildings may be 6 feet (1.8 m) thick
or more and are heavily reinforced (Figure 2.35).

Figure 2.35 Pouring a large foundation mat. Six truck-mounted pumps receive concrete from a continuous procession of transit-mix concrete trucks and deliver this concrete to the heavily reinforced mat. Concrete placement continues nonstop around the clock until the mat is finished to avoid “cold joints,” weakened planes between hardened concrete and fresh concrete. The soil around this excavation is supported with a sitecast concrete slurry wall. Most of the slurry wall is tied back, but a set of rakers is visible at the lower right.

Where the bearing capacity of the soil is low and settlement must be carefully controlled, a  floating foundation is sometimes used. A floating foundation is similar to a mat foundation, but is placed beneath a building at a depth such that the weight of the soil removed from the excavation is equal to the weight of the building above. One story of ex-cavated soil weighs about the same as five to eight stories of superstructure, depending on the density of the soil and the construction of the building  (Figure 2.36).

Figure 2.36 A cross section through a building with a floating foundation. The building weighs approximately the same as the soil excavated for the substructure, so the stress in the soil beneath the building is the same after construction as it was before.

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It is convenient to think of a building as consisting of three major parts: the superstructure, which is the above-ground portion of the building; the substructure, which is the habitable below-ground portion; and the foundations, which are the components of the building that transfer its loads into the soil (Figure 2.29).

There are two basic types of foundations: shallow and deep.  Shallow foundations are those that transfer the load to the earth at the base of the column or wall of the substructure. Deep foundations, either piles or caissons, penetrate through upper layers of incompetent soil in order to transfer the load to competent bearing soil or rock deeper within the earth. Shallow foundations are generally less expensive than deep ones and can be used where suitable soil is found at the level of the bottom of the substructure, whether this be several feet or several stories below the ground surface.

Figure 2.29 Superstructure, substructure, and foundation. The substructure in  this example contains two levels of basements, and the foundation consists  of bell caissons. (In some buildings, the substructure and foundation may be partly  or wholly the same.)

The primary factors that affect the choice of a foundation type for a building are:

•  Subsurface soil and groundwater conditions
•  Structural requirements, including foundation loads, building configurations, and depth

Secondary factors that may be important include:

•  Construction methods, including access and working space
•  Environmental factors, including noise, traffic, and disposal of earth and water
•  Building codes and regulations
•  Proximity of adjacent property and potential impacts on that property
•  Time available for construction
•  Construction risks

The foundation engineer is responsible for assessing these factors and, working together with other members of the design and construction team, selecting the most suitable foundation system.

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Generally, soil groups listed toward the top of Figure 2.2 are more desirable for supporting building foundations than those listed further down. The higherlisted soils tend to have better  soil engineering properties, that is, they tend  to have greater loadbearing capacity,  to be more stable, and to react less to
changes in moisture content. Rock is  generally the best material on which  to found a building. When rock is too deep to be reached economically, the designer must  choose from the strata  of different soils that lie closer to the  surface and design a foundation to per- form satisfactorily in the selected soil.

Figure 2.5 gives some conservative values of  loadbearing capacity for  various types of soil. These values give only an approximate idea of the  relative strengths of different soils;  the strength of any particular soil is
also dependent on factors such as  the presence or absence of water, the  depth at which the soil lies beneath  the surface, and, to some extent, the manner in which the foundation acts  upon it. In practice, the designer may  also choose to reduce the pressure  of the foundations on the soil to well  below these values in order to reduce  the potential for building settlement.

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

The stability of a soil is its ability to retain its structural properties under  the varying conditions that may occur  during the lifetime of the building. In  general, rock, gravels, and sands tend  to be the most stable soils, clays the least  stable, and silts somewhere in between.

Many clays change size under chang-ing subsurface moisture conditions,  swelling considerably as they absorb  water and shrinking as they dry. In the  presence of highly expansive clay soils,  a foundation may need to be designed  with underlying void spaces into which  the clay can expand to prevent structural damage to the foundation itself.

When wet clay is put under pressure,  water can be slowly squeezed out of  it, with a corresponding gradual reduction in volume. In this circumstance, long-term settlement of a foundation bearing on such soil is a risk that must  be considered. Taken together, these properties make many clays the least predictable soils for supporting buildings. (In Figure 2.2, the fine-grained  soil groups indicated as having a liquid  limit greater than 50 are generally the  ones most affected by water content,  exhibiting higher plasticity (moldability) and greater expansion when wet  and lower strength when dry.

In regions of significant  earth- quake risk, stability of soils during seismic events is also a concern. Sands and  silts with high water content are particularly susceptible to liquefaction, that
is, a temporary change from solid to  liquid state during cyclic shaking. Soil  liquefaction can lead to loss of support  for a building foundation or excessive  pressure on foundation walls.

The drainage characteristics of a  soil are important in predicting how  water will fl ow on and under building  sites and around building substructures. Where a coarse-grained soil is composed of particles mostly of the
same size, it has the greatest possible volume of void space between particles, and water will pass through it most readily. Where coarse-grained soils are composed of particles with a diverse range of sizes, the volume of void space between particles is reduced, and such soils drain water less eficiently. Coarse-grained soils consisting of particles of all sizes are termed well graded or poorly sorted, those with a smaller range of particle sizes are termed  poorly gradedor well sorted, and those with particles mostly of one size are termed uniformly graded (Figure 2.6).

Figure 2.6 Two gravel samples, illustrating differences in range of particle sizes or grading. The left-hand sample, with a diverse range of particle sizes, comes from a well-graded sandy gravel. On the right is a sample of a uniformly graded gravel in which there is little variation in size among particles.

Because of their smaller particle size,fine-grained soils also tend to drain water less efficiently:  Water
passes slowly through very fine sands and silts and almost not at all through many clays. A building site with clayey or silty soils near the surface drains poorly and is likely to be muddy and covered with puddles during rainy periods, whereas a gravelly site is likely to remain dry. Underground, water passes quickly through strata of gravel and sand but tends to accumulate above layers of clay and fine silt. An excel-lent way to keep a basement dry is to surround it with a layer of uniformly graded gravel or crushed stone. Water passing through the soil toward the building cannot reach the basement without first falling to the bottom of
this porous layer, from where it can be drawn off in perforated pipes before it accumulates (Figures 2.60–2.62). It does little good to place perforated drainage pipes directly in clay or silt because water cannot fl  ow  through the impervious soil toward the pipes.

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 dificulty of depositing the crushed stone and backfill soil in neatly separated, alternating layers. The drainage mat (right) is easier and often more economical to install.

Figure 2.62 A diagrammatic representation of the placement of sheet membrane waterproofing 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.

Rarely is the soil beneath a building site composed of a single type. Beneath most buildings, soils of various
types are arranged in superimposed layers (strata) that were formed by various past geologic processes. Frequently, soils in any one layer are themselves also mixtures of different soil groups, bearing descriptions such as well-graded gravel with silty clay and sand, poorly graded sand with clay, lean clay with gravel, and so on. Determining the suitability of any particular site’s soils for support of a building foundation, then, depends on the be-haviors of the various soils types and how they interact with each other and  with the building foundation.

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All foundations settle to some extent as the earth materials around and beneath them adjust to the loads of the building. Foundations on bedrock settle a negligible amount. Foundations in other types of soil may settle
much more. As an extreme example, Mexico City’s Palace of Fine Arts has settled more than 15 feet (4.5 m) into the clay soil on which it is founded since it was constructed in the early 1930s. However, building foundation settlement is normally limited to amounts measured in millimeters or fractions of an inch.

Where  foundation settlement occurs at roughly the same rate through-out all portions of a building, it is termed  uniform settlement. Settlement that occurs at differing rates between different portions of a building is termed  differential settlement. When all parts of a building rest on the same kind of soil, and the loads on the building and the design of its structural system are uniform throughout, differential settlement is normally not a concern.

However, where soils, loads, or structural systems differ between parts of a building, different parts of the building structure may settle by substantially different amounts, the frame of the building may become distorted, floors may slope, walls and glass may crack, and doors and windows may not work properly (Figure 2.1). Most foundation failures are attributable to excessive differential settlement. Gross failure of a foundation, in which the soil fails completely to support the building, is extremely rare.

Figure 2.1 Uniform settlement (b) is usually of  little consequence in a building, but differential settlement (c) can cause  severe structural damage.

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A building foundation must support different kinds of loads:

•  Dead load, the combined weight of all the permanent components of the building, including its own structural frame, floors, roofs, and walls, major permanent electrical and mechanical equipment, and the foundation itself
•  Live loads, nonpermanent loads caused by the weights of the building’s occupants, furnishings, and movable equipment
•  Rain and snow loads, which act pri-marily downward on building roofs
•  Wind loads, which can act laterally (sideways), downward, or upward on a building
•  Seismic loads, horizontal and vertical forces caused by the motion of the ground relative to the building during an earthquake
•  Loads caused by soil and hydro-static pressure, including  lateral soil pressure loads, horizontal pressures of earth and groundwater against basement walls; in some instances, buoyant uplift forces from underground water, identical to the forces that cause a boat to float; and in others, lateral force flood loads that can occur in areas prone to flooding
•  In some buildings, horizontal thrustsfrom long-span structural systems  such as arches, rigid frames, domes,  vaults, or tensile structures

A satisfactory foundation for a building must meet three general requirements:

1. The foundation, including the un-derlying soil and rock, must be safe against a structural failure that could result in collapse. For example, the foundation for a skyscraper must sup-port the great weight of the building above on a relatively narrow base without danger of overturning.

2. During the life of the building, the foundation must not settle in such a way as to damage the structure or impair its function. (Foundation settlement is discussed more fully in the next section.)

3. The foundation must be feasible, both technically and economically,  and practical to build without adverse effects on surrounding property. For example, New York City’s tallest buildings tend to cluster on the central and southern portions of Manhattan Island, where the un- derlying bedrock is closest to the surface and foundations for such buildings are easiest and least expensive to construct.

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The function of a foundation is to transfer the structural loads reliably from a building into the ground. Every building needs  a foundation of some kind: A backyard toolshed will not be  damaged by slight shifting of its foundation and may need only  wooden skids to spread its load across an area of the ground  surface suffi cient to support its weight. A wood-framed house  needs greater stability than a toolshed, so its foundation reaches  through the unstable surface to underlying soil that is free of  organic matter and unreachable by winter frost. A larger building of masonry, steel, or concrete weighs many times more than  a house, and its foundations pierce the earth until they reach   soil or rock that is competent to carry its massive loads; on some  sites, this means going 100 feet (30 m) or more below the surface.

Because of the variety of soil, rock, and water conditions that  are encountered below the surface of the ground and the unique  demands that buildings make upon their foundations, foundation design is a highly specialized fi eld combining aspects of geotechnical and civil engineering that can be sketched here only in its  broad outlines.

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In the UK, two profiles, designated as U and Z, are available. U profiles are also commonly referred to as Larssen sections and Z profiles as Frodingham sections.

See Fig. 29.11. In addition, where the depth of excavation is small, trench sheet sections may be used as alternatives to sheet piles.

The essential difference between these sections and ordinary structural beams is their ability to interlock with each other to form a continuous membrane in the soil. (Sections and their properties are included in Appendices.) Steel sheet piling, including sections for box piles, is produced in accordance with BS EN 10248, the most typical grades being S270GP and S355GP with yield strengths of 270N/mm2
and 355N/mm2 respectively. In addition, higher-strength steel sheet piles can be obtained to grades S390GP and S430GP (yield strengths 390N/mm2 and 430N/mm2  respectively).

Fig. 29.11 Larssen and Frodingham sheet piles

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1 Bolt forces
The area required to transmit the compressive forces under the  baseplate is calculated at the appropriate bearing strength of the concrete.The stress block may be assumed to be rectangular with a maximum stress of 0.6 fcu where fcu is the characteristic cube strength of the concrete base or the bedding material, whichever is less. The lever arm for the design of the bolts is then from the centroid of this  stress block to the bolt position as shown in Fig. 27.12. The centroid of the stress block is often less than the edge distance from the compression edge of the plate to the holding-down bolt: it is therefore often assumed that the lever arm for  the bolts is equal to the centres of the bolts. It is also very  likely that the point of application of the compressive forces will be near to the holding-down bolts  due to the extra stiffening that is often included in the vicinity of the bolts. This is illustrated in the typical design given as the fourth worked example at the end of this part.

Fig. 27.12 Distribution of forces and equations of equilibrium

2 Bolt anchorage
Anchorage of the holding-down bolts into the concrete foundation should be sufficient to cater for any uplift forces and to provide for any shears applied to the bolts.

Attention is particularly directed to the last paragraph of clause 6.7 of BS 5950, which states that rag bolts and indented foundation bolts should not be used to resist uplift forces.The elastic elongation of indented screwed rods or bolts under tension causes the breakdown of the grout surrounding the bolt. This is even more critical in the case of resinous grouts.

The failure mode of bolts pulled from a concrete block is shown in Fig. 27.13(a); a reasonable approximation is shown in Fig. 27.13(b).The surface area of the conical pull-out (Fig. 27.14) is 4.44D2, where D  is the depth of embedment. The factored tensile capacity of an M20 (4.6) bolt is 245 x 195 x 10-3
= 47.7kN. For an M20 HD bolt 450mm long with an embedment of say 350mm


As holding-down bolts usually act in pairs the conical pull-outs often overlap depending on the depth of embedment.The BCSA have tabulated the surface areas including those which overlap (Table 27.2).

Fig. 27.13 Typical pull-out from concrete block

Fig. 27.14 Conical pull-out

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In order to assess the distribution of pressure under a foundation it is necessary to make a reasonable estimate of the weight of the foundation. In addition to distrib- uting the forces to the ground the foundation block is also required to provide stability in cases where overturning moments are present.
 
Referring to Fig. 27.2, loads P, H and M are factored as appropriate while W, the foundation mass, is factored by 1.0, being a restoring moment. Moments about A give



From this a minimum value of W for stability is produced.

The minimum value for D  for a mass concrete foundation is established by 45° dispersal from the edge of the baseplate shown in Fig. 27.3. Shallower foundations can be used if they are suitably reinforced.

The distribution of pressure under the foundation is then assessed as follows.

Fig. 27.2 Stability of foundation

Fig. 27.3 Thickness of foundation

Case 1
See Fig. 27.4(a):

It is necessary for fgmax to be less than the stipulated ground bearing capacity for the foundation to be satisfactory.

For fgmin to be zero (Fig. 27.4(b)):


Replacing the forces by the resultant acting at eccentricity x:


This is the limiting condition for the application of Case 1.

Fig. 27.4 Ground pressure – Case 1

Case 2
This occurs when  fgmin is negative. As no tension can exist between the soil and  the underside of the concrete base a compressive stress wedge is formed at the  compression side of the foundation. The summation of the stress under the block must equal the resultant of the applied loads. When  x > L/6, the length of the  triangular stress wedge is three times the edge distance (L/2  - x) in order that  the resultant acts at the centroid of the wedge. The theory proposes that  3(L/2 - x) is the length of surface contact between the foundation and the ground (Fig. 27.5).


When  fgmax exceeds the stipulated ground bearing pressure, dimensions B or L or both may be increased within the limits of economy, after which piling or ground mprovement techniques can be investigated.

Fig. 27.5 Ground pressure – Case 2